This happens in external_lock() whilst locking the table.
Fixes LIMIT=1 optimization for EXISTS_SUBS subqueries.
external_lock() contains if condition that gives false positive
for SH + pushed conditions.
external_lock() now resets in_subquery_conversion_threshold
variable that governs IN_INTO_SUBQUERY optimization for
queries run in table mode.
external_lock() now purges dynamicall allocated condInfo for
SH and DH execution path.
Commented out UNION check b/c if condition gives false positives
and silently enables table mode execution for queries w/o
UNION.
