MDEV-7487 Semi-join optimization for single-table update/delete statementsbb-10.11-igor

This patch allows to use semi-join optimization at the top level of
single-table update and delete statements.
The problem of supporting such optimization became easy to resolve after
processing a single-table update/delete statement started using JOIN
structure. This allowed to use JOIN::prepare() not only for multi-table
updates/deletes but for single-table ones as well. This was done in the
patch for mdev-28883:
Re-design the upper level of handling UPDATE and DELETE statements.

Note that JOIN::prepare() detects all subqueries that can be considered
as candidates for semi-join optimization. The code added by this patch
looks for such candidates at the top level and if such candidates are found
in the processed single-table update/delete the statement is handled in
the same way as a multi-table update/delete.

    Approved by Oleksandr Byelkin <sanja@mariadb.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/sql_delete.cc b/sql/sql_delete.cc
index 42c8f7d00ce..7fc123efcc9 100644
--- a/sql/sql_delete.cc
+++ b/sql/sql_delete.cc
@@ -1662,6 +1662,10 @@ bool Sql_cmd_delete::prepare_inner(THD *thd)
     {
       goto err;
     }
+
+    if (!multitable &&
+        select_lex->sj_subselects.elements)
+      multitable= true;
   }
 
   if (multitable)