MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs

(Variant 2)
SEL_TREE* tree_or(SEL_TREE *X, SEL_TREE *Y) tries to conserve memory by
reusing object *X for the return value when possible.

MDEV-34620 has added logic to disable construction of index_merge plans
for N-way ORs with large N. That logic interfered with object reuse logic:

for the parameters of:
X = SEL_TREE{ trees=[key1_treeA, key2_treeB]}
Y = SEL_TREE{ trees=[key1_treeC]}

we would decide to reuse object X.
For key1, we would produce key_or(key1_treeA, key1_treeC)
but key2_treeB would be just left there.
Then, we would construct a range scan from key2_treeB.

Fixed by moving the "disable building index_merge plans" logic into a
location where it would not interfere with object reuse logic.

Author: spetrunia

mysql-test/main/range.result

@@ -3736,6 +3736,27 @@ DROP TABLE t1;
 #
 # End of 10.5 tests
 #
+#
+# MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs
+#
+CREATE TABLE t1 (
+id int primary key,
+key1 int,
+index(key1)
+);
+INSERT INTO t1 VALUES
+(1, 1),
+(2, 1),
+(3, 2);
+$query;
+id	key1
+1	1
+2	1
+3	2
+drop table t1;
+#
+# End of 10.11 tests
+#
 set global innodb_stats_persistent= @innodb_stats_persistent_save;
 set global innodb_stats_persistent_sample_pages=
 @innodb_stats_persistent_sample_pages_save;

mysql-test/main/range.test

@@ -2533,6 +2533,32 @@ DROP TABLE t1;
 --echo # End of 10.5 tests
 --echo #
 
+--echo #
+--echo # MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs
+--echo #
+CREATE TABLE t1 (
+  id int primary key,
+  key1 int,
+  index(key1)
+);
+
+INSERT INTO t1 VALUES
+(1, 1),
+(2, 1),
+(3, 2);
+
+let $query=`
+  select concat('select * from t1 where (key1 = 2 AND id = 3) ',
+               REPEAT(' OR (key1 = 1)', 100))
+`;
+
+evalp $query;
+
+drop table t1;
+
+--echo #
+--echo # End of 10.11 tests
+--echo #
 set global innodb_stats_persistent= @innodb_stats_persistent_save;
 set global innodb_stats_persistent_sample_pages=
               @innodb_stats_persistent_sample_pages_save;

mysql-test/main/range_mrr_icp.result

@@ -3725,6 +3725,27 @@ DROP TABLE t1;
 #
 # End of 10.5 tests
 #
+#
+# MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs
+#
+CREATE TABLE t1 (
+id int primary key,
+key1 int,
+index(key1)
+);
+INSERT INTO t1 VALUES
+(1, 1),
+(2, 1),
+(3, 2);
+$query;
+id	key1
+1	1
+2	1
+3	2
+drop table t1;
+#
+# End of 10.11 tests
+#
 set global innodb_stats_persistent= @innodb_stats_persistent_save;
 set global innodb_stats_persistent_sample_pages=
 @innodb_stats_persistent_sample_pages_save;

sql/opt_range.cc

@@ -9974,8 +9974,6 @@ tree_or(RANGE_OPT_PARAM *param,SEL_TREE *tree1,SEL_TREE *tree2)
   {
     bool must_be_ored= sel_trees_must_be_ored(param, tree1, tree2, ored_keys);
     no_imerge_from_ranges= must_be_ored;
-    if (param->disable_index_merge_plans)
-      no_imerge_from_ranges= true;
 
     if (no_imerge_from_ranges && no_merges1 && no_merges2)
     {
@@ -10025,6 +10023,13 @@ tree_or(RANGE_OPT_PARAM *param,SEL_TREE *tree1,SEL_TREE *tree2)
     DBUG_RETURN(result);
   }
 
+  /*
+    Ok, the result now has the ranges that one gets for (RT1 OR RT2).
+    If construction of SEL_IMERGE is disabled, stop right here.
+  */
+  if (param->disable_index_merge_plans)
+    DBUG_RETURN(result);
+
   SEL_IMERGE *imerge_from_ranges;
   if (!(imerge_from_ranges= new SEL_IMERGE()))
     result= NULL;