Re: proposal: schema variables - Mailing list pgsql-hackers

Hi Pavel

On 03/12/2025 05:27, Pavel Stehule wrote:
> Hi
>
> fresh rebase after a87987cafca683e9076c424f99bae117211a83a4


I'm going through the patch again and have a few initial comments.

== Memory Management ==

DROP VARIABLE seems to be leaking memory:

postgres=# CREATE TEMPORARY VARIABLE var AS text;
CREATE VARIABLE
postgres=# LET var = repeat('🐘', 100000000);
LET
postgres=# SELECT pg_size_pretty(used_bytes)
FROM pg_backend_memory_contexts
WHERE name = 'session variables';
 pg_size_pretty
----------------
 381 MB
(1 row)

postgres=# DROP VARIABLE var;
DROP VARIABLE
postgres=# SELECT pg_size_pretty(used_bytes)
FROM pg_backend_memory_contexts
WHERE name = 'session variables';
 pg_size_pretty
----------------
 381 MB
(1 row)


If we simply set the variable to NULL it works as expected:


postgres=# LET var = repeat('🐘', 100000000);
LET
postgres=# SELECT pg_size_pretty(used_bytes)
FROM pg_backend_memory_contexts
WHERE name = 'session variables';
 pg_size_pretty
----------------
 381 MB
(1 row)

postgres=# LET var = NULL;
LET
postgres=# SELECT pg_size_pretty(used_bytes)
FROM pg_backend_memory_contexts
WHERE name = 'session variables';
 pg_size_pretty
----------------
 240 bytes
(1 row)

Most likely you forgot to pfree "svar->value" at DropVariableByName(), e.g.

void
DropVariableByName(char *varname)
{

...

if (!svar->typbyval && !svar->isnull)
        pfree(DatumGetPointer(svar->value));

(void) hash_search(sessionvars,
                   varname,
                   HASH_REMOVE,
                   NULL);

}

== TAB completion ==

Why suggest CREATE VARIABLE (non-temporary) if it is not supported?

postgres=# CREATE V<TAB>
VARIABLE  VIEW
postgres=# CREATE VARIABLE var AS int;
ERROR:  only temporal session variables are supported

It would be nice to have tab completion for DROP VARIABLE and LET as well:

postgres=# DROP VARIABLE <TAB>

postgres=# LET <TAB>



== Missing IF EXISTS in DROP VARIABLE ==

DROP VARIABLE IF EXISTS var;
ERROR:  syntax error at or near "EXISTS"
LINE 1: DROP VARIABLE IF EXISTS var;

               ^

Best, Jim