MDEV-40565 LIMIT / OFFSET is discarded when a table-less scalar subquery is flattened, so a zero-row subquery returns a value instead of NULL - #5707
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…ery is flattened, so a zero-row subquery returns a value instead of NULL SELECT (SELECT 1 LIMIT 0) should return NULL, but flattening the table- less scalar subquery discards its row-limiting semantics and returns 1. A nonzero OFFSET can have the same effect. The scalar-subquery pull-out checks that there is one selected expression and no tables or filtering conditions, but does not establish that LIMIT/OFFSET preserves that row. Add a guard for scalar-subquery LIMIT/OFFSET before pulling the selected item into the outer query. Allow a missing limit or a positive basic constant limit, and a missing offset or a basic constant offset of zero. Keep zero limits, nonzero offsets, and limits that cannot be established statically on the normal subquery path. The regression evaluates SELECT (SELECT 1 LIMIT 0) and requires a NULL result. Bug report: https://jira.mariadb.org/browse/MDEV-40565
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SELECT (SELECT 1 LIMIT 0) should return NULL, but flattening the table-less scalar subquery discards its row-limiting semantics and returns 1. A nonzero OFFSET can have the same effect.
Bug report: https://jira.mariadb.org/browse/MDEV-40565
Root cause
The scalar-subquery pull-out checks that there is one selected expression and no tables or filtering conditions, but does not establish that LIMIT/OFFSET preserves that row.
Changes
Regression coverage
The regression evaluates SELECT (SELECT 1 LIMIT 0) and requires a NULL result.
mysql-test/main/scalar_subquery_limit_zero.testmysql-test/main/scalar_subquery_limit_zero.resultValidation
On
11.4atd10e5d726799b1cd57cc866f40aad68c720803da:main.scalar_subquery_limit_zerofailed on the unchanged target branch and passed with this fix.main.select,main.subselect.