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From: David G. Johnston <[email protected]>
To: Leendert Gravendeel <[email protected]>
Cc: [email protected]
Subject: Re: BUG: PL/pgSQL FOREACH misparses variable named "slice" with SLICE clause
Date: Fri, 17 Apr 2026 08:17:52 -0700
Message-ID: <CAKFQuwbjcqPV-xf6FUq4ECrwmt0woWVB78gs2sszJWVzJH5dYA@mail.gmail.com> (raw)
In-Reply-To: <CA+i9DRE8Uh_rYTcbMvQBWCmjQNgb_4N=7V2vMPOZa1iQV7HOkA@mail.gmail.com>
References: <CA+i9DRE8Uh_rYTcbMvQBWCmjQNgb_4N=7V2vMPOZa1iQV7HOkA@mail.gmail.com>

On Fri, Apr 17, 2026 at 7:33 AM Leendert Gravendeel <[email protected]>
wrote:

> I believe I have found a parser issue in PL/pgSQL involving the
> FOREACH ... SLICE syntax.
>

Thanks for the report!


>
>   CREATE FUNCTION test_slice_conflict() RETURNS text
>   LANGUAGE plpgsql AS $$
>   DECLARE
>     slice integer[];
>     arr integer[] := ARRAY[[1,2],[3,4]];
>   BEGIN
>     FOREACH slice SLICE 1 IN ARRAY arr LOOP
>     END LOOP;
>     RETURN 'ok';
>   END;
>   $$;
>
> Observed behavior:
> The function fails to compile due to incorrect parsing of `slice`
> as the SLICE keyword.
>
> Expected behavior:
> `slice` should be treated as a normal identifier (loop variable),
> and the function should compile and run successfully.
>
>
Confirmed on master.

Chat provided much more context for how/why this happened and why this
seems like a good fix; prior art is my main argument though.

If a PL/pgSQL variable is named "slice", using it in a FOREACH ... SLICE
loop produces a spurious syntax error:

  DO $$ DECLARE
      slice integer[];
      arr   integer[] := ARRAY[[1,2],[3,4]];
  BEGIN
      FOREACH slice SLICE 1 IN ARRAY arr LOOP
      END LOOP;
  END; $$;
  ERROR:  syntax error at or near "SLICE"

The one-token lookahead in the for_variable grammar action runs under
normal identifier lookup, so when "slice" is in scope the following SLICE
keyword is consumed as a T_DATUM reference rather than K_SLICE, and
foreach_slice fails.

The fix is to suppress variable lookup for that lookahead, using the same
save/restore pattern already used in pl_gram.y::read_cursor_args():

  /* Read the argument name, ignoring any matching variable */
  save_IdentifierLookup = plpgsql_IdentifierLookup;
  plpgsql_IdentifierLookup = IDENTIFIER_LOOKUP_DECLARE;
  yylex(yylvalp, yyllocp, yyscanner);
  argname = yylvalp->str;
  plpgsql_IdentifierLookup = save_IdentifierLookup;

Therefore we need:

diff --git a/src/pl/plpgsql/src/pl_gram.y b/src/pl/plpgsql/src/pl_gram.y
index 5009e59a78f..681fd3d5cff 100644
--- a/src/pl/plpgsql/src/pl_gram.y
+++ b/src/pl/plpgsql/src/pl_gram.y
@@ -1635,11 +1635,15 @@ for_variable : T_DATUM
                     else
                     {
                         int tok;
+ IdentifierLookup save_IdentifierLookup;

                         $$.scalar = $1.datum;
                         $$.row = NULL;
                         /* check for comma-separated list */
+ save_IdentifierLookup = plpgsql_IdentifierLookup;
+ plpgsql_IdentifierLookup = IDENTIFIER_LOOKUP_DECLARE;
                         tok = yylex(&yylval, &yylloc, yyscanner);
+ plpgsql_IdentifierLookup = save_IdentifierLookup;
                         plpgsql_push_back_token(tok, &yylval, &yylloc,
yyscanner);
                         if (tok == ',')
                             $$.row = (PLpgSQL_datum *)

David J.

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  Subject: Re: BUG: PL/pgSQL FOREACH misparses variable named "slice" with SLICE clause
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