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Re: [PATCH] Generic type subscripting

Artur Zakirov <a.zakirov@postgrespro.ru>

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From: Arthur Zakirov <a.zakirov@postgrespro.ru>
To: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, David Steele <david@pgmasters.net>, peter.eisentraut@2ndquadrant.com, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2017-03-30T17:36:40Z
Lists: pgsql-hackers 
On 29.03.2017 20:14, Arthur Zakirov wrote:
>
> I wanted to implement subscripting for ltree or hstore extensions.
> Subscripting for ltree looks more interesting. Especially with slicing.
> But I haven't done it yet. I hope that I will implement it tomorrow.
>

ltree
-----

I've implemented fetching ltree elements using subscripting. But haven't 
implemented assigning ltree elements yet. I'll send second patch after 
implementing assigning.

Now you can execute the following query:

SELECT ('Top.Science.Astronomy.Astrophysics'::ltree)[1:2];
     ltree
-------------
  Top.Science

Comments
--------

But I've noticed about some points.

In array_subscript_parse() passed uninitialized values of "typesource" 
and "typeneeded" variables for coerce_to_target_type().

> +			typesource = exprType(assignExpr);
> +			typesource = is_slice ? sbsref->refcontainertype : sbsref->refelemtype;

Here is the bug. Second variable should be "typeneeded". Moreover these 
assignments should be moved up to first coerce_to_target_type() execution.

> +	foreach(l, sbsref->reflowerindexpr)
> +	{
> +		List *expr_ai = (List *) lfirst(l);
> +		A_Indices *ai = (A_Indices *) lfirst(list_tail(expr_ai));
> +
> +		subexpr = (Node *) lfirst(list_head(expr_ai));

This code looks like a magic. This happens because of appending 
A_Indeces to lowerIndexpr:

> -			subexpr = NULL;
> +			lowerIndexpr = lappend(lowerIndexpr, list_make2(subexpr, ai));
>  		}

And this A_Indeces used only when slicing is not used to make a constant 
1. Maybe there are another way?

Also it would be better if "refevalfunc" and "refnestedfunc" had 
pointers to functions not Oid type. Now you need to create "..._fetch" 
and "..._assign" functions in catalog and in "..._parse" function you 
need get their Oid using to_regproc() function.

Can we use IndexAmRoutine structure method, when you use only pointers 
to necessary functions? You can see an example in blhandler() function 
in blutils.c.

The last point is about the tutorial. As Tom pointed it is not useful 
when the tutorial doesn't execute. It happens because there is not 
"custom" type in subscripting.sql. Also it contradicts the README of 
tutorials.

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

Commits

  1. Throw error when assigning jsonb scalar instead of a composite object

  2. Filling array gaps during jsonb subscripting

  3. Implementation of subscripting for jsonb

  4. Allow ALTER TYPE to update an existing type's typsubscript value.

  5. Allow subscripting of hstore values.

  6. Support subscripting of arbitrary types, not only arrays.

  7. jit: Reference function pointer types via llvmjit_types.c.

  8. Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.

  9. jit: Correct parameter type for generated expression evaluation functions.

  10. Renaming for new subscripting mechanism

  11. Fix assertion failure for SSL connections.

  12. Teach eval_const_expressions() to handle some more cases.