MDEV-38258 No error thrown when CTE columns updated in updates set clause #4473
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…ause CTE's are read only i.e It cannot have their columns updated in updates set clause. Attempting to do so , should throw error ERROR 1288 (HY000): The target table cte of the UPDATE is not updatable with cte as (select * from t1 where c < 5) update cte set cte.a =(select a from cte); Here, conversion from a single table to a multi table update bypasses the normal check by setting the TABLE_LIST object representing cte to be merge derived. This is incorrect, so we prohibit this conversion in TABLE_LIST::init_derived.
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Description
This query
WITH cte AS ( SELECT a from t1 ) update cte set cte.a=(select a from cte);
is converted from a single table update to multi-table, bypassing the normal check.
In the conversion, the TABLE_LIST representing cte is flagged as to be merged in TABLE_LIST::init_derived
As table cte is flagged as mergeable, it is NOT populated in first_select_lex()->leaf_tables, so isn't included in the check in
Multiupdate_prelocking_strategy::handle_end() that ensures we aren't trying to update our CTE.
Setting merged in TABLE_LIST::init_derived() is incorrect, cte SHOULD NOT be flagged as mergeable as it is the table to be updated.
Release Notes
Single to multitable conversion fix for using CTE in update statement.
How can this PR be tested?
mtr main.cte_update_delete
If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.
Basing the PR against the correct MariaDB version
mainbranch.PR quality check