Message-ID: <d6ac54b9-79f4-485b-a540-96c56cd0b991@uni-muenster.de>
Date: Tue, 3 Mar 2026 08:42:59 +0100
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Subject: Re: POC: PLpgSQL FOREACH IN JSON ARRAY
To: Pavel Stehule <pavel.stehule@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Jim Jones <jim.jones@uni-muenster.de>
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Hi Pavel,

I quickly tested the patch, and I also could observe a ~3x performance
improvement!

A few first impressions:

## in exec_stmt_foreach_json_a the boolean variable found is declared as
false, bit its value is never set to true until exec_set_found() is called:

/*
 * Set the FOUND variable to indicate the result of executing the loop
 * (namely, whether we looped one or more times). This must be set here so
 * that it does not interfere with the value of the FOUND variable inside
 * the loop processing itself.
 */
exec_set_found(estate, found);


Test:


DO $$
DECLARE
  x int;
BEGIN
  FOREACH x IN JSON ARRAY '[1,2,3]'
  LOOP
    RAISE NOTICE 'x: %', x;
  END LOOP;

  IF FOUND THEN
    RAISE NOTICE 'FOUND is true';
  ELSE
    RAISE NOTICE 'FOUND is false';
  END IF;
END;
$$;
NOTICE:  x: 1
NOTICE:  x: 2
NOTICE:  x: 3
NOTICE:  FOUND is false


## Suggestion in the plpgsql.sgml

The <literal>FOREACH</literal> loop is much like a
<literal>FOREACH</literal> loop,

to

"much like a regular <literal>FOREACH</literal> loop over arrays"

## Typo in comment

/*
 * We cannot to use fieldnames for tupdescentry, because
 * these names can be suffixed by name of row variable.
...

We cannot to use > We cannot use


## Nit pick

These error messages are not wrong, but IMO a errhint/errdetail could
add some value here:

ereport(ERROR,
    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
     errmsg("cannot extract elements from a scalar")));

ereport(ERROR,
    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
    errmsg("cannot extract elements from an object")));

Something like this perhaps?

ereport(ERROR,
    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
     errmsg("cannot extract elements from a scalar"),
     errhint("FOREACH IN JSON ARRAY requires an array value.")));

ereport(ERROR,
    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
     errmsg("FOREACH expression must evaluate to a JSON array"),
     errdetail("Cannot iterate over a scalar value.")));


Thanks for the patch!

Best, Jim