ST_COLLECT can exceed group_concat_max_len without warning

With group_concat_max_len set to 10 bytes, a one-point ST_COLLECT returns a non-NULL 34-byte result without a warning, while the two-point case returns NULL with warning 1260. This was reproduced on MariaDB 12.3.3 on Linux/amd64.

Run the following statements in one session. No database objects are created, and the original session setting is restored at the end.

SET @saved_concat_limit=@@session.group_concat_max_len;

SET SESSION group_concat_max_len=10;

SELECT OCTET_LENGTH(ST_COLLECT(ST_GEOMFROMTEXT('POINT(0 0)'))) AS one_point_len;

SHOW WARNINGS;

SELECT OCTET_LENGTH(ST_COLLECT(g)) AS two_point_len

  FROM (

    SELECT ST_GEOMFROMTEXT('POINT(0 0)') AS g

    UNION ALL

    SELECT ST_GEOMFROMTEXT('POINT(1 1)') AS g

  ) AS points;

SHOW WARNINGS;

SET SESSION group_concat_max_len=@saved_concat_limit;

The recorded output is:

34

NULL

Warning	1260	Row 1 was cut by st_collect()

The one-point aggregate exceeds the configured limit without producing a warning. The two-point case reaches the existing rejection path only on the following iteration, after the accumulated content is already oversized.

The source-supported explanation is that Item_func_collect::val_str() checks the accumulated content length before appending each geometry. Consequently, the first geometry—or the final geometry appended—can cross the limit without being rejected.

ST_COLLECT must enforce group_concat_max_len when appending every input, including the first input and the input that crosses the boundary. It must not return a non-NULL 34-byte aggregate when the configured limit is 10 bytes.

This occurs when ST_COLLECT aggregates a geometry whose encoded result exceeds group_concat_max_len, including when the first or final input crosses the limit.