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  1. Ensure commands in extension scripts see the results of preceding DDL.

  2. Stamp 9.6.2.

  1. Column rename in an extension update script

    Philippe BEAUDOIN <phb.emaj@free.fr> — 2017-05-01T06:54:56Z

    Hi all,

I am coding an update script for an extension. And I am in trouble when 
trying to rename a column of an existing table.

Just after the ALTER TABLE statement, I want to access this table. But 
at this time, the altered column is not visible with its new name.

I wrote a simple test case to show this. Here is the shell script that 
can be easily adapted.

# issue in postgres extension when trying to access a column that has 
been renamed inside an extension update script
#
export EXTDIR="/tmp"
export PGDIR="/usr/local/pg962/share/postgresql/extension"
export PGHOST=localhost
export PGPORT=5496
export PGDATABASE='postgres'

echo "create files for the extension"
echo "------------------------------"
cat >$EXTDIR/myextension.control <<*END*
default_version        = '1'
directory            = '$EXTDIR'
*END*
sudo ln -s $EXTDIR/myextension.control $PGDIR/myextension.control

cat >$EXTDIR/myextension--1.sql <<*END*
CREATE TABLE mytable (col_old INT);
*END*

cat >$EXTDIR/myextension--1--2.sql <<*END*
ALTER TABLE mytable RENAME col_old TO col_new;
UPDATE mytable SET col_new = 0;
*END*

echo "psql: run the test ==> FAILS"
echo "----------------------------"
psql -a <<*END*
select version();
CREATE EXTENSION myextension VERSION '1';
ALTER EXTENSION myextension UPDATE TO '2';
DROP EXTENSION IF EXISTS myextension;
*END*

echo "psql: similar statements outside extension ==> WORKS"
echo "----------------------------------------------------"
psql -a <<*END*
CREATE TABLE mytable (col_old INT);
BEGIN;
   ALTER TABLE mytable RENAME col_old TO col_new;
   UPDATE mytable SET col_new = 0;
COMMIT;
DROP TABLE IF EXISTS mytable;
*END*

sudo rm $PGDIR/myextension.control
rm $EXTDIR/myextension*

And here is the result:

create files for the extension
------------------------------
psql: run the test ==> FAILS
----------------------------
select version();
version
-----------------------------------------------------------------------------------------------------------------
  PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 
5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, 64-bit
(1 row)

CREATE EXTENSION myextension VERSION '1';
CREATE EXTENSION
ALTER EXTENSION myextension UPDATE TO '2';
ERROR:  column "col_new" of relation "mytable" does not exist
DROP EXTENSION IF EXISTS myextension;
DROP EXTENSION
psql: similar statements outside extension ==> WORKS
----------------------------------------------------
CREATE TABLE mytable (col_old INT);
CREATE TABLE
BEGIN;
BEGIN
   ALTER TABLE mytable RENAME col_old TO col_new;
ALTER TABLE
   UPDATE mytable SET col_new = 0;
UPDATE 0
COMMIT;
COMMIT
DROP TABLE IF EXISTS mytable;
DROP TABLE

As you can see:

- the error message is "ERROR:  column "col_new" of relation "mytable" 
does not exist", while the ALTER TABLE statement doesn't return any error,

- the same statements in a simple psql script works fine,

- I reproduce this with all supported postgres versions.

As a workaround, I perform the UPDATE statement before the ALTER TABLE 
operation, using of course the old column name.

I probably do something wrong. But I can't see what.

Thanks by advance for any piece of advise.

Best regards. Philippe Beaudoin.

  2. Re: [GENERAL] Column rename in an extension update script

    Julien Rouhaud <julien.rouhaud@dalibo.com> — 2017-05-02T19:45:26Z

    moving this thread to -hackers, since this looks like a bug.

On 01/05/2017 08:54, Philippe BEAUDOIN wrote:
> Hi all,
> 
> I am coding an update script for an extension. And I am in trouble when
> trying to rename a column of an existing table.
> 
> Just after the ALTER TABLE statement, I want to access this table. But
> at this time, the altered column is not visible with its new name.
> 

I can reproduce this issue.

It looks like this is due to missing cache invalidation between the
ALTER TABLE execution and next query planning (failure happens during
pg_analyze_and_rewrite()).

AFAICT, CommandCounterIncrement() is responsible for handling
invalidation messages, but in execute_sql_string() this function is
called before executing a Stmt instead of after.  Moving the
CommandCounterIncrement() just before the PopActiveSnapshot() call (and
removing the final one) fixes this issue for me, but I have no idea if
this is the right fix.

-- 
Julien Rouhaud
http://dalibo.com - http://dalibo.org

  3. Re: [GENERAL] Column rename in an extension update script

    Tom Lane <tgl@sss.pgh.pa.us> — 2017-05-02T21:23:17Z

    Julien Rouhaud <julien.rouhaud@dalibo.com> writes:
> moving this thread to -hackers, since this looks like a bug.
> On 01/05/2017 08:54, Philippe BEAUDOIN wrote:
>> I am coding an update script for an extension. And I am in trouble when
>> trying to rename a column of an existing table.
>> 
>> Just after the ALTER TABLE statement, I want to access this table. But
>> at this time, the altered column is not visible with its new name.

> I can reproduce this issue.

> It looks like this is due to missing cache invalidation between the
> ALTER TABLE execution and next query planning (failure happens during
> pg_analyze_and_rewrite()).

Yes.  Kind of surprising nobody noticed this before --- you'd certainly
think that extension scripts would have lots of cases of statements
depending on DDL done just before them.  I think it must have been masked
by the fact that many DDL commands *end* with CommandCounterIncrement,
or at least have one pretty close to the end.  But not renameatt().

> AFAICT, CommandCounterIncrement() is responsible for handling
> invalidation messages, but in execute_sql_string() this function is
> called before executing a Stmt instead of after.  Moving the
> CommandCounterIncrement() just before the PopActiveSnapshot() call (and
> removing the final one) fixes this issue for me, but I have no idea if
> this is the right fix.

I'm inclined to add one before the parse step, rather than removing any.
The sequence of bump-the-command-counter-then-capture-a-snapshot is
pretty well established in places like spi.c, so I don't want to change
that usage.  Also, I think part of the point here was to ensure that
any DDL done *before* reaching execute_sql_string() is visible to the
first command.  Also note the assumption in ApplyExtensionUpdates that
execute_sql_string will do at least one CommandCounterIncrement, even
if the script is empty.  A CCI that has nothing to do is quite cheap,
and we're not that worried about performance here anyway IMO, so I'd
just as soon err on the side of having more than enough CCIs.

			regards, tom lane

  4. Re: Column rename in an extension update script

    Adrian Klaver <adrian.klaver@aklaver.com> — 2017-05-03T17:10:44Z

    On 04/30/2017 11:54 PM, Philippe BEAUDOIN wrote:
> Hi all,
>
> I am coding an update script for an extension. And I am in trouble when
> trying to rename a column of an existing table.
>
> Just after the ALTER TABLE statement, I want to access this table. But
> at this time, the altered column is not visible with its new name.
>
>

 From the error it looks to me like the statements are each run in a 
separate session and the UPDATE is not seeing the ALTER TABLE. A quick 
search of the source indicates this is handled in extension.c:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/commands/extension.c;h=33b0de0a7657298729ad5c3b185dc2f4aab0bb73;hb=6a18e4bc2d13d077c52cf90a4c6ec68343808ba7

In particular execute_sql_string line 684. I do not understand C well 
enough to figure out if the above is actually creating separate sessions 
or not. Maybe you understand it or someone else can chime in.


>
> # issue in postgres extension when trying to access a column that has
> been renamed inside an extension update script
> #
> export EXTDIR="/tmp"
> export PGDIR="/usr/local/pg962/share/postgresql/extension"
> export PGHOST=localhost
> export PGPORT=5496
> export PGDATABASE='postgres'
>
> echo "create files for the extension"
> echo "------------------------------"
> cat >$EXTDIR/myextension.control <<*END*
> default_version        = '1'
> directory            = '$EXTDIR'
> *END*
> sudo ln -s $EXTDIR/myextension.control $PGDIR/myextension.control
>
> cat >$EXTDIR/myextension--1.sql <<*END*
> CREATE TABLE mytable (col_old INT);
> *END*
>
> cat >$EXTDIR/myextension--1--2.sql <<*END*
> ALTER TABLE mytable RENAME col_old TO col_new;
> UPDATE mytable SET col_new = 0;
> *END*
>
> echo "psql: run the test ==> FAILS"
> echo "----------------------------"
> psql -a <<*END*
> select version();
> CREATE EXTENSION myextension VERSION '1';
> ALTER EXTENSION myextension UPDATE TO '2';
> DROP EXTENSION IF EXISTS myextension;
> *END*
>
> echo "psql: similar statements outside extension ==> WORKS"
> echo "----------------------------------------------------"
> psql -a <<*END*
> CREATE TABLE mytable (col_old INT);
> BEGIN;
>   ALTER TABLE mytable RENAME col_old TO col_new;
>   UPDATE mytable SET col_new = 0;
> COMMIT;
> DROP TABLE IF EXISTS mytable;
> *END*
>
> sudo rm $PGDIR/myextension.control
> rm $EXTDIR/myextension*
>
> And here is the result:
>
> create files for the extension
> ------------------------------
> psql: run the test ==> FAILS
> ----------------------------
> select version();
>
> version
> -----------------------------------------------------------------------------------------------------------------
>  PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
> 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, 64-bit
> (1 row)
>
> CREATE EXTENSION myextension VERSION '1';
> CREATE EXTENSION
> ALTER EXTENSION myextension UPDATE TO '2';
> ERROR:  column "col_new" of relation "mytable" does not exist
> DROP EXTENSION IF EXISTS myextension;
> DROP EXTENSION
> psql: similar statements outside extension ==> WORKS
> ----------------------------------------------------
> CREATE TABLE mytable (col_old INT);
> CREATE TABLE
> BEGIN;
> BEGIN
>   ALTER TABLE mytable RENAME col_old TO col_new;
> ALTER TABLE
>   UPDATE mytable SET col_new = 0;
> UPDATE 0
> COMMIT;
> COMMIT
> DROP TABLE IF EXISTS mytable;
> DROP TABLE
>
> As you can see:
>
> - the error message is "ERROR:  column "col_new" of relation "mytable"
> does not exist", while the ALTER TABLE statement doesn't return any error,
>
> - the same statements in a simple psql script works fine,
>
> - I reproduce this with all supported postgres versions.
>
> As a workaround, I perform the UPDATE statement before the ALTER TABLE
> operation, using of course the old column name.
>
> I probably do something wrong. But I can't see what.
>
> Thanks by advance for any piece of advise.
>
> Best regards. Philippe Beaudoin.
>
>


-- 
Adrian Klaver
adrian.klaver@aklaver.com

  5. Re: Column rename in an extension update script

    Tom Lane <tgl@sss.pgh.pa.us> — 2017-05-03T17:29:57Z

    Adrian Klaver <adrian.klaver@aklaver.com> writes:
> On 04/30/2017 11:54 PM, Philippe BEAUDOIN wrote:
>> Just after the ALTER TABLE statement, I want to access this table. But
>> at this time, the altered column is not visible with its new name.

>  From the error it looks to me like the statements are each run in a 
> separate session and the UPDATE is not seeing the ALTER TABLE.

No, it's in the same session; the problem is the lack of a
CommandCounterIncrement call between the ALTER's update and the parsing
of the next statement.  That means the update isn't visible yet,
even in its own session.  See the fix here:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=9209e07605afe0349660447f20d83ef165cdd0ae

			regards, tom lane

  6. Re: Column rename in an extension update script

    Philippe BEAUDOIN <phb.emaj@free.fr> — 2017-05-03T17:36:58Z

    Le 03/05/2017 à 19:29, Tom Lane a écrit :
> Adrian Klaver <adrian.klaver@aklaver.com> writes:
>> On 04/30/2017 11:54 PM, Philippe BEAUDOIN wrote:
>>> Just after the ALTER TABLE statement, I want to access this table. But
>>> at this time, the altered column is not visible with its new name.
>>   From the error it looks to me like the statements are each run in a
>> separate session and the UPDATE is not seeing the ALTER TABLE.
> No, it's in the same session; the problem is the lack of a
> CommandCounterIncrement call between the ALTER's update and the parsing
> of the next statement.  That means the update isn't visible yet,
> even in its own session.  See the fix here:
>
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=9209e07605afe0349660447f20d83ef165cdd0ae
>
> 			regards, tom lane
Thanks Tom for the fix. And thanks to Julien and Adrian too, for the 
time spent on this issue.

Regards.