TODO-4799: Many index_merge variants made and discarded for a big OR

If a query has many OR-ed constructs which can use multiple indexes

  (key1=1 AND key2=10) OR
  (key1=2 AND key2=20) OR
  (key1=3 AND key2=30) OR
  ...

The range optimizer would construct and then discard a lot of potential
index_merge plans. This process
1. is CPU-intensive
2. can hit the @@optimizer_max_sel_args limitation after which all
   potential range or index_merge plans are discarded.

The fix is to apply a heuristic: if there is an OR clause with more than
100 branches, disallow construction of index_merge plans for the OR
branches.

mysql-test/main/range_notembedded.result

@@ -247,3 +247,70 @@ SELECT id FROM t1 WHERE id IS NULL OR id NOT BETWEEN 1 AND 4;
 id
 5
 DROP TABLE t1;
+#
+# TODO-4799: Many index_merge variants made and discarded for a big OR
+#
+CREATE TABLE t1 (
+a1 int NOT NULL,
+a2 int NOT NULL,
+filler char(100),
+KEY key1 (a1,a2),
+KEY key2 (a2,a1)
+);
+insert into t1 (a1,a2) values (1,1),(2,2),(3,3);
+set @query= concat(
+"explain select * from t1 where\n",
+(select 
+group_concat(concat("a1=", seq, " and a2=", seq, " ") separator "\nor " )
+from seq_1_to_30)
+);
+set optimizer_trace=1;
+prepare s from @query;
+execute s;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	key1,key2	NULL	NULL	NULL	3	Using where
+set @trace=json_extract((select trace from information_schema.optimizer_trace), '$**.range_analysis');
+# Observe that "key1" is a a part of several index_merge_union:
+select json_pretty(json_search(@trace, 'all', 'key1'));
+json_pretty(json_search(@trace, 'all', 'key1'))
+[
+    "$[0].potential_range_indexes[0].index",
+    "$[0].analyzing_range_alternatives.range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[0].indexes_to_merge[0].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[0].indexes_to_merge[0].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[0].indexes_to_merge[1].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[0].indexes_to_merge[1].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[0].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[0].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[1].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[1].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[2].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[1].indexes_to_merge[2].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[2].indexes_to_merge[0].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[2].indexes_to_merge[0].index_to_merge",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[2].indexes_to_merge[1].range_scan_alternatives[0].index",
+    "$[0].analyzing_range_alternatives.analyzing_index_merge_union[2].indexes_to_merge[1].index_to_merge"
+]
+#
+# Now, same as above but for a long IN-list
+#
+set @query= concat(
+"explain select * from t1 where\n",
+(select 
+group_concat(concat("a1=", seq, " and a2=", seq, " ") separator "\nor " )
+from seq_1_to_120)
+);
+set optimizer_trace=1;
+prepare s from @query;
+execute s;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	key1,key2	NULL	NULL	NULL	3	Using where
+set @trace=json_extract((select trace from information_schema.optimizer_trace), '$**.range_analysis');
+# Observe that there are NO index_merge_union candidates. Only one potential range scan:
+select json_pretty(json_search(@trace, 'all', 'key1'));
+json_pretty(json_search(@trace, 'all', 'key1'))
+[
+    "$[0].potential_range_indexes[0].index",
+    "$[0].analyzing_range_alternatives.range_scan_alternatives[0].index"
+]
+drop table t1;

mysql-test/main/range_notembedded.test

@@ -159,3 +159,51 @@ INSERT INTO t1 VALUES (1),(5);
 SELECT id FROM t1 WHERE id IS NULL OR id NOT BETWEEN 1 AND 4;
 DROP TABLE t1;
 
+--echo #
+--echo # TODO-4799: Many index_merge variants made and discarded for a big OR
+--echo #
+
+CREATE TABLE t1 (
+  a1 int NOT NULL,
+  a2 int NOT NULL,
+  filler char(100),
+  KEY key1 (a1,a2),
+  KEY key2 (a2,a1)
+);
+insert into t1 (a1,a2) values (1,1),(2,2),(3,3);
+
+
+set @query= concat(
+  "explain select * from t1 where\n",
+  (select 
+     group_concat(concat("a1=", seq, " and a2=", seq, " ") separator "\nor " )
+   from seq_1_to_30)
+  );
+
+set optimizer_trace=1;
+prepare s from @query;
+execute s;
+set @trace=json_extract((select trace from information_schema.optimizer_trace), '$**.range_analysis');
+
+--echo # Observe that "key1" is a a part of several index_merge_union:
+select json_pretty(json_search(@trace, 'all', 'key1'));
+
+--echo #
+--echo # Now, same as above but for a long IN-list
+--echo #
+set @query= concat(
+  "explain select * from t1 where\n",
+  (select 
+     group_concat(concat("a1=", seq, " and a2=", seq, " ") separator "\nor " )
+   from seq_1_to_120)
+  );
+
+set optimizer_trace=1;
+prepare s from @query;
+execute s;
+set @trace=json_extract((select trace from information_schema.optimizer_trace), '$**.range_analysis');
+
+--echo # Observe that there are NO index_merge_union candidates. Only one potential range scan:
+select json_pretty(json_search(@trace, 'all', 'key1'));
+drop table t1;
+

sql/opt_range.cc

@@ -8462,6 +8462,11 @@ SEL_TREE *Item_cond::get_mm_tree(RANGE_OPT_PARAM *param, Item **cond_ptr)
   SEL_TREE *tree= li.ref()[0]->get_mm_tree(param, li.ref());
   if (param->statement_should_be_aborted())
     DBUG_RETURN(NULL);
+  bool orig_disable_index_merge= param->disable_index_merge_plans;
+
+  if (list.elements > 100)
+    param->disable_index_merge_plans= true;
+
   if (tree)
   {
     if (tree->type == SEL_TREE::IMPOSSIBLE &&
@@ -8478,7 +8483,10 @@ SEL_TREE *Item_cond::get_mm_tree(RANGE_OPT_PARAM *param, Item **cond_ptr)
     {
       SEL_TREE *new_tree= li.ref()[0]->get_mm_tree(param, li.ref());
       if (new_tree == NULL || param->statement_should_be_aborted())
+      {
+        param->disable_index_merge_plans= orig_disable_index_merge;
         DBUG_RETURN(NULL);
+      }
       tree= tree_or(param, tree, new_tree);
       if (tree == NULL || tree->type == SEL_TREE::ALWAYS)
       {
@@ -8510,6 +8518,7 @@ SEL_TREE *Item_cond::get_mm_tree(RANGE_OPT_PARAM *param, Item **cond_ptr)
     if (replace_cond)
       *cond_ptr= replacement_item;
   }
+  param->disable_index_merge_plans= orig_disable_index_merge;
   DBUG_RETURN(tree);
 }
 
@@ -9901,6 +9910,8 @@ 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)
     {

sql/opt_range.h

@@ -713,6 +713,9 @@ class RANGE_OPT_PARAM
   */
   bool remove_false_where_parts;
 
+  /* If TRUE, do not construct index_merge plans */
+  bool disable_index_merge_plans;
+
   /*
     Which functions should give SQL notes for unusable keys.
   */