MDEV-21184 Assertion `used_tables_cache == 0' failed in Item_func::fix_fields

           with condition_pushdown_from_having

This bug could manifest itself for queries with GROUP BY and HAVING clauses
when the HAVING clause was a conjunctive condition that depended
exclusively on grouping fields and at least one conjunct contained an
equality of the form fld=sq where fld is a grouping field and sq is a
constant subquery.
In this case the optimizer tries to perform a pushdown of the HAVING
condition into WHERE. To construct the pushable condition the optimizer
first transforms all multiple equalities in HAVING into simple equalities.
This has to be done for a proper processing of the pushed conditions
in WHERE. The multiple equalities at all AND/OR levels must be converted
to simple equalities because any multiple equality may refer to a multiple
equality at the upper level.
Before this patch the conversion was performed like this:
  multiple_equality(x,f1,...,fn) => x=f1 and ... and x=fn.
When an equality item for x=fi was constructed both the items for x and fi
were cloned. If x happened to be a constant subquery that could not be
cloned the conversion failed. If the conversions of multiple equalities
previously performed had succeeded then the whole condition became in an
inconsistent state that could cause different failures.
The solution provided by the patch is:
1. to use a different conversion rule if x is a constant
  multiple_equality(x,f1,...,fn) => f1=x and f2=f1 and ... and fn=f1
2. not to clone x if it's a constant.
Such conversions cannot fail and besides the result of the conversion
preserves the equivalence of f1,...,fn that can be used for other
optimizations.
This patch also made sure that expensive predicates are not pushed from
HAVING to WHERE.

1 files changed, 148 insertions, 0 deletions

diff --git a/mysql-test/main/having_cond_pushdown.result b/mysql-test/main/having_cond_pushdown.result
index 82a4813b156..9b124296e3d 100644
--- a/mysql-test/main/having_cond_pushdown.result
+++ b/mysql-test/main/having_cond_pushdown.result
@@ -4776,3 +4776,151 @@ WHERE t1.a = 3 AND (t1.a < 2 AND t1.b > 3) GROUP BY t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 DROP TABLE t1;
+#
+# MDEV-21184: Constant subquery in condition movable to WHERE
+#
+CREATE TABLE t1(a int, b int);
+INSERT INTO t1 VALUES
+(1,10), (2,20), (1,11), (1,15), (2,20), (1,10), (2,21);
+CREATE TABLE t2 (c INT);
+INSERT INTO t2 VALUES (2),(3);
+EXPLAIN FORMAT=JSON SELECT a FROM t1 GROUP BY a HAVING a = 8 OR a = ( SELECT MIN(c) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "filesort": {
+      "sort_key": "t1.a",
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 7,
+          "filtered": 100,
+          "attached_condition": "t1.a = 8 or t1.a = (subquery#2)"
+        },
+        "subqueries": [
+          {
+            "query_block": {
+              "select_id": 2,
+              "table": {
+                "table_name": "t2",
+                "access_type": "ALL",
+                "rows": 2,
+                "filtered": 100
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+SELECT a FROM t1 GROUP BY a HAVING a = 8 OR a = ( SELECT MIN(c) FROM t2 );
+a
+2
+EXPLAIN FORMAT=JSON SELECT a FROM t1 GROUP BY a,b
+HAVING ( a = 8 OR a = ( SELECT MIN(c) FROM t2 ) ) and b < 20;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "filesort": {
+      "sort_key": "t1.a, t1.b",
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 7,
+          "filtered": 100,
+          "attached_condition": "(t1.a = 8 or t1.a = (subquery#2)) and t1.b < 20"
+        },
+        "subqueries": [
+          {
+            "query_block": {
+              "select_id": 2,
+              "table": {
+                "table_name": "t2",
+                "access_type": "ALL",
+                "rows": 2,
+                "filtered": 100
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+SELECT a FROM t1 GROUP BY a,b
+HAVING ( a = 8 OR a = ( SELECT MIN(c) FROM t2 ) ) and b < 20;
+a
+EXPLAIN FORMAT=JSON SELECT a FROM t1 GROUP BY a
+HAVING ( a = 8 OR a = ( SELECT MIN(c) FROM t2 ) ) and SUM(b) > 20;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "having_condition": "sum(t1.b) > 20",
+    "filesort": {
+      "sort_key": "t1.a",
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 7,
+          "filtered": 100,
+          "attached_condition": "t1.a = 8 or t1.a = (subquery#2)"
+        },
+        "subqueries": [
+          {
+            "query_block": {
+              "select_id": 2,
+              "table": {
+                "table_name": "t2",
+                "access_type": "ALL",
+                "rows": 2,
+                "filtered": 100
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+SELECT a FROM t1 GROUP BY a
+HAVING ( a = 8 OR a = ( SELECT MIN(c) FROM t2 ) ) and SUM(b) > 20;
+a
+2
+EXPLAIN FORMAT=JSON SELECT a FROM t1 GROUP BY a HAVING a = ( SELECT MIN(c) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t1",
+      "access_type": "ALL",
+      "rows": 7,
+      "filtered": 100,
+      "attached_condition": "t1.a = (subquery#2)"
+    },
+    "subqueries": [
+      {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t2",
+            "access_type": "ALL",
+            "rows": 2,
+            "filtered": 100
+          }
+        }
+      }
+    ]
+  }
+}
+SELECT a FROM t1 GROUP BY a HAVING a = ( SELECT MIN(c) FROM t2 );
+a
+2
+DROP TABLE t1,t2;