Equivalent query on InnoDB temp table not recognized as empty, DISTINCT+filesort triggers ERROR 1038 Out of sort memory on zero rows, while original MyISAM query returns empty set.

1. 1. 1. Problem Description
         Under the same data and session parameters, the original single-table query (Q1) returns an empty set directly because the optimizer recognizes the empty table; the query rewritten to use an InnoDB temporary table (Q2) does not recognize the temporary table as empty, chooses a DISTINCT + filesort plan, and triggers `ERROR 1038 (HY001): Out of sort memory` on zero-row input. After adjusting `max_sort_length`, Q2 can return an empty set, indicating that the logical result is consistent, but the query fails under the original parameters. This is not a "wrong result set" issue, but a "legal query fails under legal parameters and behaves inconsistently with an equivalent query" issue.

1. 1. 1. How to Reproduce
         Execute the following SQL:

```sql
DROP DATABASE IF EXISTS repro_mariadb813_db0_vp599_min;
CREATE DATABASE repro_mariadb813_db0_vp599_min;
USE repro_mariadb813_db0_vp599_min;

SET SESSION optimizer_switch='derived_merge=on';
SET SESSION sort_buffer_size=2097152;
SET SESSION max_sort_length=6259757;

CREATE TABLE s(id INT, c1 BOOLEAN) ENGINE=MyISAM;
CREATE TABLE l(id INT) ENGINE=MyISAM;
CREATE TABLE r(id INT, c1 BOOLEAN) ENGINE=MyISAM;

DROP TEMPORARY TABLE IF EXISTS tmp;
CREATE TEMPORARY TABLE tmp ENGINE=InnoDB AS
SELECT l.id, r.c1
FROM l JOIN r ON l.id=r.id;

– Q1: original single-table query
SELECT *
FROM (
SELECT DISTINCT REGEXP_REPLACE('1', c1, 'a') AS v
FROM s
WHERE c1
) q
ORDER BY 1;

– Q2: query after table reconstruction
SELECT *
FROM (
SELECT DISTINCT REGEXP_REPLACE('1', c1, 'a') AS v
FROM tmp
WHERE c1
) q
ORDER BY 1;
```

1. 1. 1. Expected Behavior
         Q1 and Q2 should return the same result, i.e., both should return `Empty set`.

1. 1. 1. Actual Behavior

• Q1: returns `Empty set`.
• Q2: fails with
  ```
  ERROR 1038 (HY001):
  Out of sort memory, consider increasing server sort buffer size
  ```

1. 1. 1. Error Information and/or Stack Trace
         ```
         ERROR 1038 (HY001):
         Out of sort memory, consider increasing server sort buffer size
         ```