Oracle mode has different set operator precedence and handling (not by
standard). In Oracle mode the below test case is handled as-is, in
plain order from left to right. In MariaDB default mode follows SQL
standard and makes INTERSECT prioritized, so UNION is taken from
derived table which is INTERSECT result (here and below the same
applies for EXCEPT).
Non-distinct set operator (UNION ALL/INTERSECT ALL) works via unique
key release but it can be done only once. We cannot add index to
non-empty heap table (see heap_enable_indexes()). So every UNION ALL
before rightmost UNION DISTINCT works as UNION DISTINCT. That is
common syntax, MySQL, MSSQL and Oracle work that way.
There is union_distinct property which indicates the rightmost
distinct UNION (at least, so the algorithm works simple: it releases
the unique key after union_distinct in the loop
(st_select_lex_unit::exec()).
INTERSECT ALL code (implemented by MDEV-18844 in a896beb) does not
know about Oracle mode and treats union_distinct as the last
operation, that's why it releases unique key on union_distinct
operation. INTERSECT ALL requires unique key for it to work, so before
any INTERSECT ALL unique key must not be released (see
select_unit_ext::send_data()).
The patch tweaks INTERSECT ALL code for Oracle mode. In
disable_index_if_needed() it does not allow unique key release before
the last operation and it allows unfold on the last operation. Test
case with UNION DISTINCT following INTERSECT ALL at least does not
include invalid data, but in fact the whole INTERSECT ALL code could
be refactored for better semantical triggers.
The patch fixes typo in st_select_lex_unit::prepare() where
have_except_all_or_intersect_all masked eponymous data member which
wrongly triggered unique key release in st_select_lex_unit::prepare().
The patch fixes unknown error in case ha_disable_indexes() fails.
Note: optimize_bag_operation() does some operator substitutions, but
it does not run under PS. So if there is difference in test with --ps
that means non-optimized (have_except_all_or_intersect_all == true)
code path is not good.
Note 2: VIEW is stored and executed in normal mode (see
Sql_mode_save_for_frm_handling) hence when SELECT order is different
in Oracle mode (defined by parsed_select_expr_cont()) it must be
covered by --disable_view_protocol.
