MDEV-32868 SELECT NULL,NULL IN (SUBQUERY) returns 0 instead of NULL #4550
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spetrunia
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Review comments: 3 comments starting from https://jira.mariadb.org/browse/MDEV-32868?focusedCommentId=322008&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-322008 .
When evaluating (SELECT NULL, NULL) IN (SELECT 1, 2 FROM t), the result was incorrectly 0 instead of NULL. The IN-to-EXISTS transformation wraps comparison predicates with trigcond() guards: WHERE trigcond(NULL = 1) AND trigcond(NULL = 2) During optimization, make_join_select() evaluated this as a "constant condition". With guards ON, the condition evaluated to FALSE (NULL treated as FALSE), triggering "Impossible WHERE". At runtime, guards would be turned OFF for NULL columns, but the optimizer had already marked the subquery as returning no rows. Fix: check can_eval_in_optimize() instead of is_expensive() in make_join_select(). Unlike is_expensive(), can_eval_in_optimize() also verifies const_item(), which returns FALSE for Item_func_trig_cond
Switch branches of the `if` condition to make the code flow more naturally.
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When evaluating (SELECT NULL, NULL) IN (SELECT 1, 2 FROM t), the result was incorrectly 0 instead of NULL.
The IN-to-EXISTS transformation wraps comparison predicates with trigcond() guards:
WHERE trigcond(NULL = 1) AND trigcond(NULL = 2)
During optimization, make_join_select() extracted this as a "constant condition" (used_tables() = 0) and evaluated it. With guards ON, the condition evaluated to FALSE (NULL treated as FALSE), triggering "Impossible WHERE". At runtime, guards would be turned OFF for NULL columns, but the optimizer had already marked the subquery as returning no rows.
Fix: Override used_tables() in Item_func_trig_cond to include RAND_TABLE_BIT. This prevents trigcond() from being extracted as a constant condition, deferring evaluation to runtime when guards have correct values.