Shift-Reduce conflicts prevented parsing some queries with subqueries that
used set operations when the subqueries occurred in expressions or in IN
predicands.
The grammar rules for query expression were transformed in order to avoid
these conflicts. New grammar rules employ an idea taken from MySQL 8.0.
Parenthesis around table names and derived tables should be allowed
in FROM clauses and some other context as it was in earlier versions.
Returned test queries that used such parenthesis in 10.3 to their
original form. Adjusted test results accordingly.
This patch corrects the patch for MDEV-19324. The latter did not
work properly in the cases when the transformation
(SELECT ... ORDER BY ...) LIMIT ... =>
SELECT ... ORDER BY ... LIMIT ...
was applied to the operands of a set operation.
If a select query was of the form (SELECT ... ORDER BY ...) LIMIT ...
then in most cases it returned incorrect result. It happened because
SELECT ... ORDER BY ... was wrapped into a select with materialized
derived table:
SELECT ... ORDER BY ... =>
SELECT * FROM (SELECT ... ORDER BY ...) dt.
Yet for any materialized derived table ORDER BY without LIMIT is ignored.
This patch resolves the problem by the conversion
(SELECT ... ORDER BY ...) LIMIT ... =>
SELECT ... ORDER BY ... LIMIT ...
at the parser stage.
Similarly
((SELECT ... UNION ...) ORDER BY ...) LIMIT ...
is converted to
(SELECT ... UNION ...) ORDER BY ... LIMIT ...
This conversion optimizes execution of the query because the result of
(SELECT ... UNION ...) ORDER BY ... is not materialized into a temporary
table anymore.
Due to an obvious bug in st_select_lex_unit::reset_distinct() a union
without duplicates of more than 2 selects could return a result set
containing duplicate rows if this union was enclosed in ().
