Horton

Thread

  1. Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-05-23T16:42:34Z

    I'd like to revive the discussion that arose during the last CommitFest over
the proper design for identifying no-op length coercions like varchar(4) ->
varchar(8).  Here is the root of the original debate:

  http://archives.postgresql.org/message-id/20110109220353.GD5777@tornado.leadboat.com

There were two proposals on the table:

1. Attach a "f(from_typmod, to_typmod, is_explicit) RETURNS boolean" function
   to the pg_cast; call it in find_coercion_pathway()
2. Attach a "f(FuncExpr) RETURNS Expr" (actually internal/internal) function
   to the pg_proc; call it in simplify_function()

I tried and failed to write a summary of the respective arguments that could
legitimately substitute for (re-)reading the original thread, so I haven't
included one.  I myself find the advantages of #2 mildly more compelling.

As previously noted, if we enrich the typmod system, only #1 will require
interface changes.  One of the suggestions that came up before was designing
the typmod enrichment before designing this.  Unless someone has short-term
plans to prepare that design, I'd like to avoid serializing on it -- it may
never happen.  One thought: if #1 proves preferable independent of this
concern, should we consider implementing it without documenting it or exposing
it at the SQL level?  External type developers could still directly update
pg_cast, so long as they'll be happy to keep both pieces if we someday change
the required function signature.

Can we reach a design that nobody dislikes excessively?

Thanks,
nm

  2. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-05-23T16:49:31Z

    On Mon, May 23, 2011 at 12:42 PM, Noah Misch <noah@leadboat.com> wrote:
> I'd like to revive the discussion that arose during the last CommitFest over
> the proper design for identifying no-op length coercions like varchar(4) ->
> varchar(8).  Here is the root of the original debate:
>
>  http://archives.postgresql.org/message-id/20110109220353.GD5777@tornado.leadboat.com
>
> There were two proposals on the table:
>
> 1. Attach a "f(from_typmod, to_typmod, is_explicit) RETURNS boolean" function
>   to the pg_cast; call it in find_coercion_pathway()
> 2. Attach a "f(FuncExpr) RETURNS Expr" (actually internal/internal) function
>   to the pg_proc; call it in simplify_function()
>
> I tried and failed to write a summary of the respective arguments that could
> legitimately substitute for (re-)reading the original thread, so I haven't
> included one.  I myself find the advantages of #2 mildly more compelling.

The main reason I preferred #1 is that it would only get invoked in
the case of casts, whereas #2 would get invoked for all function
calls.  For us to pay that overhead, there has to be some use case,
and I didn't find the examples that were offered very compelling.  How
many CPU cycles are we willing to spend trying to simplify x + 0 to
just x, or x * 0 to just 0?  I might be missing something here, but
those strikes me as textbook examples of cases where it's not worth
slowing down the whole system to fix a query that the user could have
easily written in some less-pathological way to begin with.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  3. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-05-23T17:21:10Z

    Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, May 23, 2011 at 12:42 PM, Noah Misch <noah@leadboat.com> wrote:
>> There were two proposals on the table:
>> 
>> 1. Attach a "f(from_typmod, to_typmod, is_explicit) RETURNS boolean" function
>>  to the pg_cast; call it in find_coercion_pathway()
>> 2. Attach a "f(FuncExpr) RETURNS Expr" (actually internal/internal) function
>>  to the pg_proc; call it in simplify_function()
>> 
>> I tried and failed to write a summary of the respective arguments that could
>> legitimately substitute for (re-)reading the original thread, so I haven't
>> included one. I myself find the advantages of #2 mildly more compelling.

> The main reason I preferred #1 is that it would only get invoked in
> the case of casts, whereas #2 would get invoked for all function
> calls.  For us to pay that overhead, there has to be some use case,
> and I didn't find the examples that were offered very compelling.

Well, as I pointed out in
http://archives.postgresql.org/pgsql-hackers/2011-01/msg02570.php
a hook function attached to pg_proc entries would cost nothing
measurable when not used.  You could possibly make the same claim
for attaching the hook to pg_cast entries, if you cause the optimization
to occur during initial cast lookup rather than expression
simplification.  But I remain of the opinion that that's the wrong place
to put it.

> How
> many CPU cycles are we willing to spend trying to simplify x + 0 to
> just x, or x * 0 to just 0?

I'm not sure that's worthwhile either, but it was an example of a
possible future use-case rather than something that anybody was
proposing doing right now.  Even though I tend to agree that it wouldn't
be worth looking for such cases with simple numeric datatypes, it's not
that hard to believe that someone might want the ability for complicated
datatypes with expensive operations.

In the short term, the only actual cost we'd incur is that we'd be
bloating pg_proc instead of pg_cast with an extra column, and there's
more rows in pg_proc.  But pg_proc rows are already pretty darn wide.

			regards, tom lane

  4. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-05-23T17:40:51Z

    On Mon, May 23, 2011 at 1:21 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Mon, May 23, 2011 at 12:42 PM, Noah Misch <noah@leadboat.com> wrote:
>>> There were two proposals on the table:
>>>
>>> 1. Attach a "f(from_typmod, to_typmod, is_explicit) RETURNS boolean" function
>>>   to the pg_cast; call it in find_coercion_pathway()
>>> 2. Attach a "f(FuncExpr) RETURNS Expr" (actually internal/internal) function
>>>   to the pg_proc; call it in simplify_function()
>>>
>>> I tried and failed to write a summary of the respective arguments that could
>>> legitimately substitute for (re-)reading the original thread, so I haven't
>>> included one.  I myself find the advantages of #2 mildly more compelling.
>
>> The main reason I preferred #1 is that it would only get invoked in
>> the case of casts, whereas #2 would get invoked for all function
>> calls.  For us to pay that overhead, there has to be some use case,
>> and I didn't find the examples that were offered very compelling.
>
> Well, as I pointed out in
> http://archives.postgresql.org/pgsql-hackers/2011-01/msg02570.php
> a hook function attached to pg_proc entries would cost nothing
> measurable when not used.  You could possibly make the same claim
> for attaching the hook to pg_cast entries, if you cause the optimization
> to occur during initial cast lookup rather than expression
> simplification.  But I remain of the opinion that that's the wrong place
> to put it.

So you said here:

http://archives.postgresql.org/pgsql-hackers/2011-01/msg02575.php
http://archives.postgresql.org/pgsql-hackers/2011-01/msg02585.php

The trouble is, I still can't see why type OIDs and typemods should be
handled differently.  Taking your example again:

CREATE TABLE base (f1 varchar(4));
CREATE VIEW vv AS SELECT f1::varchar(8) FROM base;
ALTER TABLE base ALTER COLUMN f1 TYPE varchar(16);

Your claim on the thread is that we want to someday allow this case.
But what if the last statement were instead:

ALTER TABLE base ALTER COLUMN f1 TYPE integer;

Should it also be our goal to handle that case?  If not, why are they different?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  5. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-05-23T17:53:36Z

    Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, May 23, 2011 at 1:21 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> ... But I remain of the opinion that that's the wrong place
>> to put it.

> So you said here:

> http://archives.postgresql.org/pgsql-hackers/2011-01/msg02575.php
> http://archives.postgresql.org/pgsql-hackers/2011-01/msg02585.php

> The trouble is, I still can't see why type OIDs and typemods should be
> handled differently.  Taking your example again:

> CREATE TABLE base (f1 varchar(4));
> CREATE VIEW vv AS SELECT f1::varchar(8) FROM base;
> ALTER TABLE base ALTER COLUMN f1 TYPE varchar(16);

> Your claim on the thread is that we want to someday allow this case.
> But what if the last statement were instead:

> ALTER TABLE base ALTER COLUMN f1 TYPE integer;

> Should it also be our goal to handle that case?

Maybe.  But casts would be the least of our concerns if we were trying
to change the column type.  Changing typmod doesn't affect the set of
operations that could be applied to a column, whereas changing type
surely does.  In any case, the fact that the current implementation can't
readily support that is a poor excuse for building entirely new features
that also can't support it, when said features could easily be designed
without the restriction.

But more generally, I don't believe that you've made any positive case
whatever for doing it from pg_cast.  It's not faster, it's not more
flexible, so why should we choose that approach?

			regards, tom lane

  6. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-05-23T18:11:39Z

    On Mon, May 23, 2011 at 1:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Maybe.  But casts would be the least of our concerns if we were trying
> to change the column type.  Changing typmod doesn't affect the set of
> operations that could be applied to a column, whereas changing type
> surely does.

OK, this is the crucial point I was missing.  Sorry for being a bit
fuzzy-headed about this.

My mental model of our type system, or of what a type system ought to
do, just doesn't match the type system we've got.

So let's do it the way you proposed.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  7. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-05-23T18:46:43Z

    On Mon, May 23, 2011 at 02:11:39PM -0400, Robert Haas wrote:
> On Mon, May 23, 2011 at 1:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Maybe. ?But casts would be the least of our concerns if we were trying
> > to change the column type. ?Changing typmod doesn't affect the set of
> > operations that could be applied to a column, whereas changing type
> > surely does.
> 
> OK, this is the crucial point I was missing.  Sorry for being a bit
> fuzzy-headed about this.
> 
> My mental model of our type system, or of what a type system ought to
> do, just doesn't match the type system we've got.
> 
> So let's do it the way you proposed.

Good deal.  Given that conclusion, the other policy decision I anticipate
affecting this particular patch is the choice of syntax.  Presumably, it will be
a new common_func_opt_item.  When I last looked at the keywords list and tried
to come up with something, these were the best I could do:

  CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
  CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)

Both feel forced, to put it generously.  Any better ideas?  Worth adding a
keyword to get something decent?

Thanks,
nm

  8. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-05-23T18:53:01Z

    On Mon, May 23, 2011 at 2:46 PM, Noah Misch <noah@leadboat.com> wrote:
> On Mon, May 23, 2011 at 02:11:39PM -0400, Robert Haas wrote:
>> On Mon, May 23, 2011 at 1:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> > Maybe. ?But casts would be the least of our concerns if we were trying
>> > to change the column type. ?Changing typmod doesn't affect the set of
>> > operations that could be applied to a column, whereas changing type
>> > surely does.
>>
>> OK, this is the crucial point I was missing.  Sorry for being a bit
>> fuzzy-headed about this.
>>
>> My mental model of our type system, or of what a type system ought to
>> do, just doesn't match the type system we've got.
>>
>> So let's do it the way you proposed.
>
> Good deal.  Given that conclusion, the other policy decision I anticipate
> affecting this particular patch is the choice of syntax.  Presumably, it will be
> a new common_func_opt_item.  When I last looked at the keywords list and tried
> to come up with something, these were the best I could do:
>
>  CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
>  CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)
>
> Both feel forced, to put it generously.  Any better ideas?  Worth adding a
> keyword to get something decent?

Do you have something specific in mind?

Just to throw out another few possibilities, how about INLINE FUNCTION
or ANALYZE FUNCTION?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  9. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-05-23T19:01:40Z

    Noah Misch <noah@leadboat.com> writes:
> Good deal.  Given that conclusion, the other policy decision I anticipate
> affecting this particular patch is the choice of syntax.  Presumably, it will be
> a new common_func_opt_item.  When I last looked at the keywords list and tried
> to come up with something, these were the best I could do:

>   CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
>   CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)

We could go with your previous idea of not bothering to expose this in
the SQL syntax.  Given that the helper function is going to have a
signature along the lines of "(internal, internal) -> internal", it's
going to be difficult for anyone to use it for non-builtin functions
anyhow.

But if you really don't like that, what about

	TRANSFORM helperfunctionname

Although TRANSFORM isn't currently a keyword for us, it is a
non-reserved keyword in SQL:2008, and it seems possible that we might
someday think about implementing the associated features.

			regards, tom lane

  10. Re: Identifying no-op length coercions

    Greg Stark <gsstark@mit.edu> — 2011-05-23T20:31:41Z

    On Mon, May 23, 2011 at 12:01 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>  Given that the helper function is going to have a
> signature along the lines of "(internal, internal) -> internal", it's
> going to be difficult for anyone to use it for non-builtin functions
> anyhow.

I hate to go around in circles on this but I didn't see the original discussion.

This was the thing that concerned me. If anyone wants to add this
feature for a new data type they're going to have to understand and
tie their code to all this internal parser node stuff. That means
their code will be much more closely tied to a specific version, will
have to be written in C, and will require much more in-depth
understanding of Postgres internal data structures.

By comparison the boolean cast predicate could be written in any
language and only required the data type implementor to understand
their data type. It seems much more likely to actually get used and be
used correctly.



-- 
greg

  11. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-05-23T20:41:06Z

    Greg Stark <gsstark@mit.edu> writes:
> This was the thing that concerned me. If anyone wants to add this
> feature for a new data type they're going to have to understand and
> tie their code to all this internal parser node stuff. That means
> their code will be much more closely tied to a specific version, will
> have to be written in C, and will require much more in-depth
> understanding of Postgres internal data structures.

> By comparison the boolean cast predicate could be written in any
> language and only required the data type implementor to understand
> their data type. It seems much more likely to actually get used and be
> used correctly.

I don't think I believe that interesting length-conversion functions are
going to be written in anything but C anyway, so I don't find that that
argument holds much water.  Generally, the low-level functions for a new
datatype have to be in C.

As for the other point, if all you want to do is examine the
expression's typmod, the API for that is pretty stable (exprTypmod()).

			regards, tom lane

  12. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-05-23T21:14:53Z

    On Mon, May 23, 2011 at 02:53:01PM -0400, Robert Haas wrote:
> On Mon, May 23, 2011 at 2:46 PM, Noah Misch <noah@leadboat.com> wrote:
> > On Mon, May 23, 2011 at 02:11:39PM -0400, Robert Haas wrote:
> >> On Mon, May 23, 2011 at 1:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> > Maybe. ?But casts would be the least of our concerns if we were trying
> >> > to change the column type. ?Changing typmod doesn't affect the set of
> >> > operations that could be applied to a column, whereas changing type
> >> > surely does.
> >>
> >> OK, this is the crucial point I was missing. ?Sorry for being a bit
> >> fuzzy-headed about this.
> >>
> >> My mental model of our type system, or of what a type system ought to
> >> do, just doesn't match the type system we've got.
> >>
> >> So let's do it the way you proposed.
> >
> > Good deal. ?Given that conclusion, the other policy decision I anticipate
> > affecting this particular patch is the choice of syntax. ?Presumably, it will be
> > a new common_func_opt_item. ?When I last looked at the keywords list and tried
> > to come up with something, these were the best I could do:
> >
> > ?CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
> > ?CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)
> >
> > Both feel forced, to put it generously. ?Any better ideas? ?Worth adding a
> > keyword to get something decent?
> 
> Do you have something specific in mind?

I had not.  Having thought about it some, maybe "PLAN TRANSFORM FUNCTION".  Do
we have a name for the pass over the tree rooted at eval_const_expressions()?

> Just to throw out another few possibilities, how about INLINE FUNCTION
> or ANALYZE FUNCTION?

INLINE FUNCTION evokes, for me, having a different version of a function that
gets substituted when it's inlined.  ANALYZE FUNCTION seems reasonable, though.
I don't think any name we'd pick will make a significant number of readers
understand it without reading the full documentation.

nm

  13. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-05-23T21:15:13Z

    On Mon, May 23, 2011 at 03:01:40PM -0400, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > Good deal.  Given that conclusion, the other policy decision I anticipate
> > affecting this particular patch is the choice of syntax.  Presumably, it will be
> > a new common_func_opt_item.  When I last looked at the keywords list and tried
> > to come up with something, these were the best I could do:
> 
> >   CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
> >   CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)
> 
> We could go with your previous idea of not bothering to expose this in
> the SQL syntax.  Given that the helper function is going to have a
> signature along the lines of "(internal, internal) -> internal", it's
> going to be difficult for anyone to use it for non-builtin functions
> anyhow.
> 
> But if you really don't like that, what about

That would be just fine with me.  We can always expose it later.

> 
> 	TRANSFORM helperfunctionname
> 
> Although TRANSFORM isn't currently a keyword for us, it is a
> non-reserved keyword in SQL:2008, and it seems possible that we might
> someday think about implementing the associated features.

I was thinking of that word too, along the lines of "PLAN TRANSFORM FUNCTION
helperfunctionname".  Then again, that wrongly sounds somewhat like it's
transforming planner nodes.  Perhaps plain TRANSFORM or TRANSFORM FUNCTION would
be the way to go.

nm

  14. Re: Identifying no-op length coercions

    Alexey Klyukin <alexk@commandprompt.com> — 2011-06-02T14:08:51Z

    On May 24, 2011, at 12:15 AM, Noah Misch wrote:

> On Mon, May 23, 2011 at 03:01:40PM -0400, Tom Lane wrote:
>> Noah Misch <noah@leadboat.com> writes:
>>> Good deal.  Given that conclusion, the other policy decision I anticipate
>>> affecting this particular patch is the choice of syntax.  Presumably, it will be
>>> a new common_func_opt_item.  When I last looked at the keywords list and tried
>>> to come up with something, these were the best I could do:
>> 
>>>  CREATE FUNCTION ... PARSER MAPPING helperfunc(args)
>>>  CREATE FUNCTION ... PLANS CONVERSION helperfunc(args)
>> 
>> We could go with your previous idea of not bothering to expose this in
>> the SQL syntax.  Given that the helper function is going to have a
>> signature along the lines of "(internal, internal) -> internal", it's
>> going to be difficult for anyone to use it for non-builtin functions
>> anyhow.
>> 
>> But if you really don't like that, what about
> 
> That would be just fine with me.  We can always expose it later.
> 
>> 
>> 	TRANSFORM helperfunctionname
>> 
>> Although TRANSFORM isn't currently a keyword for us, it is a
>> non-reserved keyword in SQL:2008, and it seems possible that we might
>> someday think about implementing the associated features.
> 
> I was thinking of that word too, along the lines of "PLAN TRANSFORM FUNCTION
> helperfunctionname".  Then again, that wrongly sounds somewhat like it's
> transforming planner nodes.  Perhaps plain TRANSFORM or TRANSFORM FUNCTION would
> be the way to go.

Looks like this thread has silently died out. Is there an agreement on the
syntax and implementation part? We (CMD) have a customer, who is interested in
pushing this through, so, if we have a patch, I'd be happy to assist in
reviewing it.


--
Alexey Klyukin
The PostgreSQL Company - Command Prompt, Inc.

  15. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-02T19:22:49Z

    Hi Alexey,

On Thu, Jun 02, 2011 at 05:08:51PM +0300, Alexey Klyukin wrote:
> Looks like this thread has silently died out. Is there an agreement on the
> syntax and implementation part? We (CMD) have a customer, who is interested in
> pushing this through, so, if we have a patch, I'd be happy to assist in
> reviewing it.

I think we have a consensus on the implementation.  We didn't totally lock down
the syntax.  Tom and I seem happy to have no SQL exposure at all, so that's what
I'm planning to submit.  However, we were pretty close to a syntax consensus in
the event that it becomes desirable to do otherwise.

Is your interest in cheap varchar(N)->varchar(N+M) conversions specifically, or
in some broader application of this facility?

Thanks for volunteering to review; that will be a big help.  Actually, I could
especially use some feedback now on a related design and implementation:
  http://archives.postgresql.org/message-id/20110524104029.GB18831@tornado.gateway.2wire.net
Note that the third and fifth sentences of that description are incorrect.  The
rest stands without them.  Even just some feedback on the mundane issue noted in
the last paragraph would help.

Thanks,
nm

  16. Re: Identifying no-op length coercions

    Alexey Klyukin <alexk@commandprompt.com> — 2011-06-03T15:11:37Z

    Hi,

On Jun 2, 2011, at 10:22 PM, Noah Misch wrote:

> Hi Alexey,
> 
...
> Is your interest in cheap varchar(N)->varchar(N+M) conversions specifically, or
> in some broader application of this facility?

Exactly varchar conversions.

> 
> Thanks for volunteering to review; that will be a big help.  Actually, I could
> especially use some feedback now on a related design and implementation:
>  http://archives.postgresql.org/message-id/20110524104029.GB18831@tornado.gateway.2wire.net
> Note that the third and fifth sentences of that description are incorrect.  The
> rest stands without them.  Even just some feedback on the mundane issue noted in
> the last paragraph would help.

Ok, I'll review it.

Thank you,
Alexey.

--
Command Prompt, Inc.                              http://www.CommandPrompt.com
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

  17. Re: Identifying no-op length coercions

    Jim Nasby <jim@nasby.net> — 2011-06-03T15:43:17Z

    On Jun 3, 2011, at 10:11 AM, Alexey Klyukin wrote:
>> Is your interest in cheap varchar(N)->varchar(N+M) conversions specifically, or
>> in some broader application of this facility?
> 
> Exactly varchar conversions.

Why limit it to varchar? Shouldn't we be able to do this for any varlena? The only challenge I see is numeric; we'd need to ensure that both size and precision are not decreasing.
--
Jim C. Nasby, Database Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

  18. Re: Identifying no-op length coercions

    Kevin Grittner <kevin.grittner@wicourts.gov> — 2011-06-03T16:12:19Z

    Jim Nasby <jim@nasby.net> wrote:
 
> The only challenge I see is numeric; we'd need to ensure that both
> size and precision are not decreasing.
 
To be picky, wouldn't that need to be "neither (precision - scale)
nor scale is decreasing"?
 
-Kevin

  19. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-06-03T16:13:44Z

    On Fri, Jun 3, 2011 at 11:43 AM, Jim Nasby <jim@nasby.net> wrote:
> On Jun 3, 2011, at 10:11 AM, Alexey Klyukin wrote:
>>> Is your interest in cheap varchar(N)->varchar(N+M) conversions specifically, or
>>> in some broader application of this facility?
>>
>> Exactly varchar conversions.
>
> Why limit it to varchar? Shouldn't we be able to do this for any varlena? The only challenge I see is numeric; we'd need to ensure that both size and precision are not decreasing.

More than that: you should also be able to make it work for things
like xml -> text.

Indeed, I believe Noah has plans to do just that.

Which, quite frankly, will be awesome.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  20. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-03T16:27:40Z

    On Fri, Jun 03, 2011 at 10:43:17AM -0500, Jim Nasby wrote:
> On Jun 3, 2011, at 10:11 AM, Alexey Klyukin wrote:
> >> Is your interest in cheap varchar(N)->varchar(N+M) conversions specifically, or
> >> in some broader application of this facility?
> > 
> > Exactly varchar conversions.
> 
> Why limit it to varchar? Shouldn't we be able to do this for any varlena? The only challenge I see is numeric; we'd need to ensure that both size and precision are not decreasing.

I've implemented support for varchar, varbit, numeric, time, timetz, timestamp,
timestamptz, and interval.  However, I'll probably submit only varchar in the
initial infrastructure patch and the rest in followup patches in a later CF.

For numeric, we store the display scale in every datum, so any change to it
rewrites the table.  You'll be able to cheaply change numeric(7,2) to
numeric(9,2) but not to numeric(9,3).

  21. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-11T17:40:54Z

    On Thu, Jun 02, 2011 at 03:22:49PM -0400, Noah Misch wrote:
> On Thu, Jun 02, 2011 at 05:08:51PM +0300, Alexey Klyukin wrote:
> > Looks like this thread has silently died out. Is there an agreement on the
> > syntax and implementation part? We (CMD) have a customer, who is interested in
> > pushing this through, so, if we have a patch, I'd be happy to assist in
> > reviewing it.
> 
> I think we have a consensus on the implementation.  We didn't totally lock down
> the syntax.  Tom and I seem happy to have no SQL exposure at all, so that's what
> I'm planning to submit.  However, we were pretty close to a syntax consensus in
> the event that it becomes desirable to do otherwise.

Here's the patch.  It contains the core support for transform functions and
applies that support to the varchar() length coercion.

We originally discussed having the transform function take and return Expr
nodes.  It turns out that simplify_function() does not have the Expr, probably
because the particular choice of node type among the many that can convey a
function call does not matter to it.  The same should be true of a transform
function -- it should do the same thing whether the subject call arrives from
a FuncExpr or an OpExpr.  Therefore, I changed the transform function
signature to "Expr *protransform(List *args)".

Previously, ALTER TABLE did not plan its expressions until ATRewriteTable.
Since we need to make a decision on whether to rewrite based on examination of
a fully simplified expression, I moved the planning earlier.  This means
planning now happens before the catalog updates associated with the ALTER
TABLE.  This is at least as safe as what we did before, because the heap
available to expressions during ATRewriteTable is the pre-rewrite heap.  I
wouldn't be surprised if there was and is some way to get into trouble with a
USING expression that refers back to the current table indirectly, but I did
not explore that in any detail.

The large pg_proc.h diff mostly just adds protransform = 0 to every function.
Due to the resulting patch size, I've compressed it.  There are new/otherwise
changed DATA lines for OIDs 669 and 3097 only.

Any transform function for a length coercion cast will need code to strip
subsidiary RelabelType nodes and apply a new one that changes only the typmod.
I've gone ahead and split that out into a function in parse_clause.c.

I verified the patch's behavior using pg_regress tests I developed previously.
The tests rely on already-rejected instrumentation, so I don't include them in
the patch.  If anyone would like an updated copy of the tests patch to aid
your review, let me know.

This will obviously require a catversion bump.

I looked for a README file that might be suitable for a brief note on how to
use the feature.  src/backend/optimizer/README seemed closest, but it's all
much higher-level.  Perhaps, like most optimization microfeatures, this
requires no particular discussion outside the code implementing it.  I would
include approximately this README text if anyone feels it's useful:

pg_proc.protransform functions
------------------------------

Some functions calls can be simplified at plan time based on properties
specific to the function.  For example, "int4mul(n, 1)" simplifies to "n", and
"varchar(s::varchar(4), 8, true)" simplifies to "s::varchar(4)".  To define
such function-specific optimizations, write a "transform function" and store
its OID in the pg_proc.protransform of the primary function.  This transform
function has the signature "protransform(internal) RETURNS internal", mapping
internally as "Expr *protransform(List *args)".  If the transform function's
study of the argument Nodes proves that a simplified Expr substitutes for
every call represented by such arguments, return that Expr.  Otherwise, return
the NULL pointer.  We make no guarantee that PostgreSQL will never call the
main function in cases that the transform function would simplify.  Ensure
rigorous equivalence between the simplified expression and an actual call to
the main function.

Thanks,
nm

  22. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-06-11T18:11:55Z

    Noah Misch <noah@leadboat.com> writes:
> We originally discussed having the transform function take and return Expr
> nodes.  It turns out that simplify_function() does not have the Expr, probably
> because the particular choice of node type among the many that can convey a
> function call does not matter to it.  The same should be true of a transform
> function -- it should do the same thing whether the subject call arrives from
> a FuncExpr or an OpExpr.  Therefore, I changed the transform function
> signature to "Expr *protransform(List *args)".

That seems like the wrong thing.  For one thing, it makes it quite
impossible to share one transform support function among multiple
functions/operators, since it won't know which one the argument list
is for.  More generally, I foresee the transform needing to know
most of the other information that might be in the FuncExpr or OpExpr.
It certainly would need access to the collation, and it would probably
like to be able to copy the parse location to whatever it outputs,
and so forth and so on.  So I really think passing the node to the
function is the most future-proof way to do it.  If that means you
need to rejigger things a bit in clauses.c, so be it.

> The large pg_proc.h diff mostly just adds protransform = 0 to every function.
> Due to the resulting patch size, I've compressed it.  There are new/otherwise
> changed DATA lines for OIDs 669 and 3097 only.

The chances that this part will still apply cleanly by the time anyone
gets around to committing it are nil.  I'd suggest you break it into two
separate patches, one that modifies the existing lines to add the
protransform = 0 column and then one to apply the remaining deltas in
the file.  I envision the eventual committer doing the first step
on-the-fly (perhaps with an emacs macro, at least that's the way I
usually do it) and then wanting a patchable diff for the rest.  Or if
you wanted to be really helpful, you could provide a throwaway perl
script to do the modifications of the existing lines ...

I haven't read the actual patch, these are just some quick reactions
to your description.

			regards, tom lane

  23. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-11T18:34:48Z

    On Sat, Jun 11, 2011 at 02:11:55PM -0400, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > We originally discussed having the transform function take and return Expr
> > nodes.  It turns out that simplify_function() does not have the Expr, probably
> > because the particular choice of node type among the many that can convey a
> > function call does not matter to it.  The same should be true of a transform
> > function -- it should do the same thing whether the subject call arrives from
> > a FuncExpr or an OpExpr.  Therefore, I changed the transform function
> > signature to "Expr *protransform(List *args)".
> 
> That seems like the wrong thing.  For one thing, it makes it quite
> impossible to share one transform support function among multiple
> functions/operators, since it won't know which one the argument list
> is for.  More generally, I foresee the transform needing to know
> most of the other information that might be in the FuncExpr or OpExpr.
> It certainly would need access to the collation, and it would probably
> like to be able to copy the parse location to whatever it outputs,
> and so forth and so on.  So I really think passing the node to the
> function is the most future-proof way to do it.  If that means you
> need to rejigger things a bit in clauses.c, so be it.

Good points.  I'm thinking, then, add an Expr argument to simplify_function()
and have the CoerceViaIO branch of eval_const_expressions_mutator() pass NULL
for both its simplify_function() calls.  If simplify_function() gets a NULL Expr
for a function that has a protransform, synthesize a FuncExpr based on its other
arguments before calling the transform function.  Seem reasonable?  Granted,
that would be dead code until someone applies a transform function to a type I/O
function, which could easily never happen.  Perhaps just ignore the transform
function when we started with a CoerceViaIO node?

> > The large pg_proc.h diff mostly just adds protransform = 0 to every function.
> > Due to the resulting patch size, I've compressed it.  There are new/otherwise
> > changed DATA lines for OIDs 669 and 3097 only.
> 
> The chances that this part will still apply cleanly by the time anyone
> gets around to committing it are nil.  I'd suggest you break it into two
> separate patches, one that modifies the existing lines to add the
> protransform = 0 column and then one to apply the remaining deltas in
> the file.  I envision the eventual committer doing the first step
> on-the-fly (perhaps with an emacs macro, at least that's the way I
> usually do it) and then wanting a patchable diff for the rest.  Or if
> you wanted to be really helpful, you could provide a throwaway perl
> script to do the modifications of the existing lines ...

That would be better; I'll do it for the next version.

> I haven't read the actual patch, these are just some quick reactions
> to your description.

Thanks,
nm

  24. Re: Identifying no-op length coercions

    Tom Lane <tgl@sss.pgh.pa.us> — 2011-06-11T19:03:18Z

    Noah Misch <noah@leadboat.com> writes:
> Good points.  I'm thinking, then, add an Expr argument to simplify_function()
> and have the CoerceViaIO branch of eval_const_expressions_mutator() pass NULL
> for both its simplify_function() calls.  If simplify_function() gets a NULL Expr
> for a function that has a protransform, synthesize a FuncExpr based on its other
> arguments before calling the transform function.  Seem reasonable?  Granted,
> that would be dead code until someone applies a transform function to a type I/O
> function, which could easily never happen.  Perhaps just ignore the transform
> function when we started with a CoerceViaIO node?

Until we actually have a use-case for simplifying I/O functions like this,
I can't see going out of our way for it ...

			regards, tom lane

  25. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-11T21:13:04Z

    On Sat, Jun 11, 2011 at 03:03:18PM -0400, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > Good points.  I'm thinking, then, add an Expr argument to simplify_function()
> > and have the CoerceViaIO branch of eval_const_expressions_mutator() pass NULL
> > for both its simplify_function() calls.  If simplify_function() gets a NULL Expr
> > for a function that has a protransform, synthesize a FuncExpr based on its other
> > arguments before calling the transform function.  Seem reasonable?  Granted,
> > that would be dead code until someone applies a transform function to a type I/O
> > function, which could easily never happen.  Perhaps just ignore the transform
> > function when we started with a CoerceViaIO node?
> 
> Until we actually have a use-case for simplifying I/O functions like this,
> I can't see going out of our way for it ...

Sounds good.  Updated patch attached, incorporating your ideas.  Before applying
it, run this command to update the uninvolved pg_proc.h DATA entries:
  perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h

Thanks,
nm

  26. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-06-19T02:57:13Z

    On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
> Sounds good.  Updated patch attached, incorporating your ideas.  Before applying
> it, run this command to update the uninvolved pg_proc.h DATA entries:
>  perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h

This doesn't quite apply any more.  I think the pgindent run broke it slightly.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  27. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-19T03:06:25Z

    On Sat, Jun 18, 2011 at 10:57:13PM -0400, Robert Haas wrote:
> On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
> > Sounds good. ?Updated patch attached, incorporating your ideas. ?Before applying
> > it, run this command to update the uninvolved pg_proc.h DATA entries:
> > ?perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h
> 
> This doesn't quite apply any more.  I think the pgindent run broke it slightly.

Hmm, I just get two one-line offsets when applying it to current master.  Note
that you need to run the perl invocation before applying the patch.  Could you
provide full output of your `patch' invocation, along with any reject files?

Thanks,
nm

  28. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-06-19T03:12:26Z

    On Sat, Jun 18, 2011 at 11:06 PM, Noah Misch <noah@leadboat.com> wrote:
> On Sat, Jun 18, 2011 at 10:57:13PM -0400, Robert Haas wrote:
>> On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
>> > Sounds good. ?Updated patch attached, incorporating your ideas. ?Before applying
>> > it, run this command to update the uninvolved pg_proc.h DATA entries:
>> > ?perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h
>>
>> This doesn't quite apply any more.  I think the pgindent run broke it slightly.
>
> Hmm, I just get two one-line offsets when applying it to current master.  Note
> that you need to run the perl invocation before applying the patch.  Could you
> provide full output of your `patch' invocation, along with any reject files?

Ah, crap.  You're right.  I didn't follow your directions for how to
apply the patch.  Sorry.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  29. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-06-19T03:32:20Z

    On Sat, Jun 18, 2011 at 11:12 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Sat, Jun 18, 2011 at 11:06 PM, Noah Misch <noah@leadboat.com> wrote:
>> On Sat, Jun 18, 2011 at 10:57:13PM -0400, Robert Haas wrote:
>>> On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
>>> > Sounds good. ?Updated patch attached, incorporating your ideas. ?Before applying
>>> > it, run this command to update the uninvolved pg_proc.h DATA entries:
>>> > ?perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h
>>>
>>> This doesn't quite apply any more.  I think the pgindent run broke it slightly.
>>
>> Hmm, I just get two one-line offsets when applying it to current master.  Note
>> that you need to run the perl invocation before applying the patch.  Could you
>> provide full output of your `patch' invocation, along with any reject files?
>
> Ah, crap.  You're right.  I didn't follow your directions for how to
> apply the patch.  Sorry.

I think you need to update the comment in simplify_function() to say
that we have three strategies, rather than two.  I think it would also
be appropriate to add a longish comment just before the test that
calls protransform, explaining what the charter of that function is
and why the mechanism exists.

Documentation issues aside, I see very little not to like about this.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

  30. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-19T11:10:42Z

    On Sat, Jun 18, 2011 at 11:32:20PM -0400, Robert Haas wrote:
> On Sat, Jun 18, 2011 at 11:12 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> > On Sat, Jun 18, 2011 at 11:06 PM, Noah Misch <noah@leadboat.com> wrote:
> >> On Sat, Jun 18, 2011 at 10:57:13PM -0400, Robert Haas wrote:
> >>> On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
> >>> > Sounds good. ?Updated patch attached, incorporating your ideas. ?Before applying
> >>> > it, run this command to update the uninvolved pg_proc.h DATA entries:
> >>> > ?perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h
> >>>
> >>> This doesn't quite apply any more. ?I think the pgindent run broke it slightly.
> >>
> >> Hmm, I just get two one-line offsets when applying it to current master. ?Note
> >> that you need to run the perl invocation before applying the patch. ?Could you
> >> provide full output of your `patch' invocation, along with any reject files?
> >
> > Ah, crap. ?You're right. ?I didn't follow your directions for how to
> > apply the patch. ?Sorry.
> 
> I think you need to update the comment in simplify_function() to say
> that we have three strategies, rather than two.  I think it would also
> be appropriate to add a longish comment just before the test that
> calls protransform, explaining what the charter of that function is
> and why the mechanism exists.

Good idea.  See attached.

> Documentation issues aside, I see very little not to like about this.

Great!  Thanks for reviewing.

  31. Re: Identifying no-op length coercions

    Alexey Klyukin <alexk@commandprompt.com> — 2011-06-21T15:31:44Z

    Hi,

On Jun 19, 2011, at 2:10 PM, Noah Misch wrote:

> On Sat, Jun 18, 2011 at 11:32:20PM -0400, Robert Haas wrote:
>> On Sat, Jun 18, 2011 at 11:12 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> On Sat, Jun 18, 2011 at 11:06 PM, Noah Misch <noah@leadboat.com> wrote:
>>>> On Sat, Jun 18, 2011 at 10:57:13PM -0400, Robert Haas wrote:
>>>>> On Sat, Jun 11, 2011 at 5:13 PM, Noah Misch <noah@leadboat.com> wrote:
>>>>>> Sounds good. ?Updated patch attached, incorporating your ideas. ?Before applying
>>>>>> it, run this command to update the uninvolved pg_proc.h DATA entries:
>>>>>> ?perl -pi -e 's/PGUID(\s+\d+){4}/$& 0/' src/include/catalog/pg_proc.h
>>>>> 
>>>>> This doesn't quite apply any more. ?I think the pgindent run broke it slightly.
>>>> 
>>>> Hmm, I just get two one-line offsets when applying it to current master. ?Note
>>>> that you need to run the perl invocation before applying the patch. ?Could you
>>>> provide full output of your `patch' invocation, along with any reject files?
>>> 
>>> Ah, crap. ?You're right. ?I didn't follow your directions for how to
>>> apply the patch. ?Sorry.
>> 
>> I think you need to update the comment in simplify_function() to say
>> that we have three strategies, rather than two.  I think it would also
>> be appropriate to add a longish comment just before the test that
>> calls protransform, explaining what the charter of that function is
>> and why the mechanism exists.
> 
> Good idea.  See attached.
> 
>> Documentation issues aside, I see very little not to like about this.
> 
> Great!  Thanks for reviewing.
> <noop-length-coercion-v3.patch>


Here is my review of this patch. 

The patch applies cleanly to the HEAD, produces no additional warnings. It
doesn't include additional regression tests. One can include a test, using the
commands like the ones included below.

Changes to the documentation are limited to the a description of a new
pg_class attribute. It would probably make sense to document all the
exceptions to the table's rewrite on ALTER TABLE documentation page, although
it could wait for more such exceptions.

The feature works as intended and I haven't noticed any crashes or assertion failures, i.e.

postgres=# create table test(id integer, name varchar);
CREATE TABLE
postgres=# insert into test values(1, 'test');
INSERT 0 1
postgres=# select relfilenode from pg_class where oid='test'::regclass;
 relfilenode
-------------
       66302
(1 row)
postgres=# alter table test alter column name type varchar(10);
ALTER TABLE
postgres=# select relfilenode from pg_class where oid='test'::regclass;
 relfilenode
-------------
       66308
(1 row)
postgres=# alter table test alter column name type varchar(65535);
ALTER TABLE
postgres=# select relfilenode from pg_class where oid='test'::regclass;
 relfilenode
-------------
       66308
(1 row)

The code looks good and takes into account all the previous suggestions.

The only nitpick code-wise is these lines  in varchar_transform:

+ 		int32		old_max = exprTypmod(source) - VARHDRSZ;
+ 		int32		new_max = new_typmod - VARHDRSZ;

I have a hard time understanding why  VARHDRSZ is subtracted here, so I'd assume that's a bug.

Other than that, I haven't noticed any issues w/ this patch.

Sincerely,
Alexey.

--
Command Prompt, Inc.                              http://www.CommandPrompt.com
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

  32. Re: Identifying no-op length coercions

    Noah Misch <noah@leadboat.com> — 2011-06-21T18:58:33Z

    On Tue, Jun 21, 2011 at 06:31:44PM +0300, Alexey Klyukin wrote:
> Here is my review of this patch. 

Thanks!

> The patch applies cleanly to the HEAD, produces no additional warnings. It
> doesn't include additional regression tests. One can include a test, using the
> commands like the ones included below.
> 
> Changes to the documentation are limited to the a description of a new
> pg_class attribute. It would probably make sense to document all the
> exceptions to the table's rewrite on ALTER TABLE documentation page, although
> it could wait for more such exceptions.

I like the current level of detail in the ALTER TABLE page.  Having EXPLAIN
ALTER TABLE would help here, but that's a bigger project than this one.

> postgres=# alter table test alter column name type varchar(10);
> ALTER TABLE
> postgres=# select relfilenode from pg_class where oid='test'::regclass;
>  relfilenode
> -------------
>        66308
> (1 row)
> postgres=# alter table test alter column name type varchar(65535);
> ALTER TABLE
> postgres=# select relfilenode from pg_class where oid='test'::regclass;
>  relfilenode
> -------------
>        66308
> (1 row)

A pg_regress test needs stable output, so we would do it roughly like this:

	CREATE TEMP TABLE relstorage AS SELECT 0::regclass AS oldnode;
	...
	UPDATE relstorage SET oldnode =
		(SELECT relfilenode FROM pg_class WHERE oid = 'test'::regclass);
	ALTER TABLE test ALTER name TYPE varchar(65535);
	SELECT oldnode <> relfilenode AS rewritten
	FROM pg_class, relstorage WHERE oid = 'test'::regclass;

I originally rejected that as too ugly to read.  Perhaps not.

> The only nitpick code-wise is these lines  in varchar_transform:
> 
> + 		int32		old_max = exprTypmod(source) - VARHDRSZ;
> + 		int32		new_max = new_typmod - VARHDRSZ;
> 
> I have a hard time understanding why  VARHDRSZ is subtracted here, so I'd assume that's a bug.

We track the varchar typmod internally as (max length) + VARHDRSZ.

  33. Re: Identifying no-op length coercions

    Alexey Klyukin <alexk@commandprompt.com> — 2011-06-21T21:50:23Z

    On Jun 21, 2011, at 9:58 PM, Noah Misch wrote:

> 
> A pg_regress test needs stable output, so we would do it roughly like this:
> 
> 	CREATE TEMP TABLE relstorage AS SELECT 0::regclass AS oldnode;
> 	...
> 	UPDATE relstorage SET oldnode =
> 		(SELECT relfilenode FROM pg_class WHERE oid = 'test'::regclass);
> 	ALTER TABLE test ALTER name TYPE varchar(65535);
> 	SELECT oldnode <> relfilenode AS rewritten
> 	FROM pg_class, relstorage WHERE oid = 'test'::regclass;
> 
> I originally rejected that as too ugly to read.  Perhaps not.

Yes, your example is more appropriate. I think you can make it more
straightforward by getting rid of the temp table:

CREATE TABLE test(oldnode oid, name varchar(5));

INSERT INTO test(oldnode) SELECT relfilenode FROM pg_class WHERE
oid='test'::regclass;

ALTER TABLE test ALTER name TYPE varchar(10);

SELECT oldnode <> relfilenode AS rewritten FROM pg_class, test WHERE
oid='test'::regclass;



> 
>> The only nitpick code-wise is these lines  in varchar_transform:
>> 
>> + 		int32		old_max = exprTypmod(source) - VARHDRSZ;
>> + 		int32		new_max = new_typmod - VARHDRSZ;
>> 
>> I have a hard time understanding why  VARHDRSZ is subtracted here, so I'd assume that's a bug.
> 
> We track the varchar typmod internally as (max length) + VARHDRSZ.

Oh, right, haven't thought that this is a varchar specific thing.

Thank you,
Alexey.

--
Command Prompt, Inc.                              http://www.CommandPrompt.com
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

  34. Re: Identifying no-op length coercions

    Robert Haas <robertmhaas@gmail.com> — 2011-06-22T02:22:46Z

    On Tue, Jun 21, 2011 at 5:50 PM, Alexey Klyukin <alexk@commandprompt.com> wrote:
> On Jun 21, 2011, at 9:58 PM, Noah Misch wrote:
>> A pg_regress test needs stable output, so we would do it roughly like this:
>>
>>       CREATE TEMP TABLE relstorage AS SELECT 0::regclass AS oldnode;
>>       ...
>>       UPDATE relstorage SET oldnode =
>>               (SELECT relfilenode FROM pg_class WHERE oid = 'test'::regclass);
>>       ALTER TABLE test ALTER name TYPE varchar(65535);
>>       SELECT oldnode <> relfilenode AS rewritten
>>       FROM pg_class, relstorage WHERE oid = 'test'::regclass;
>>
>> I originally rejected that as too ugly to read.  Perhaps not.
>
> Yes, your example is more appropriate. I think you can make it more
> straightforward by getting rid of the temp table:
>
> CREATE TABLE test(oldnode oid, name varchar(5));
>
> INSERT INTO test(oldnode) SELECT relfilenode FROM pg_class WHERE
> oid='test'::regclass;
>
> ALTER TABLE test ALTER name TYPE varchar(10);
>
> SELECT oldnode <> relfilenode AS rewritten FROM pg_class, test WHERE
> oid='test'::regclass;

Without wishing to foreclose the possibility of adding a suitable
regression test, I've committed the main patch.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company