Fixed bug mdev-11488.

The patch for bug mdev-10882 tried to fix it by providing an
implementation of the virtual method build_clone for the class
Item_cache. It's turned out that it is not easy provide a valid
implementation for Item_cache::build_clone(). At the same time
if the condition that can be pushed into a materialized view
contains a cached item this item can be substituted for a basic
constant of the same value. In such a way we can avoid building
proper clones for Item_cache objects when constructing pushdown
conditions.

mysql-test/r/derived_cond_pushdown.result

@@ -7766,3 +7766,377 @@ EXPLAIN
 }
 DROP VIEW v;
 DROP TABLE t;
+#
+# MDEV-11488: pushdown of the predicate with cached value
+#
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(3),(2);
+CREATE TABLE t2 (j INT, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3),(4);
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+3
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 3)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = 3)"
+          }
+        }
+      }
+    }
+  }
+}
+UPDATE t2 SET j = 2 WHERE j = 3;
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+2
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i FLOAT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1.5),(3.2),(2.71);
+CREATE TABLE t2 (j FLOAT, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3.2),(2.71);
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+2.71
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 2.7100000381469727)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = 2.7100000381469727)"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i DECIMAL(10,2)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1.5),(3.21),(2.47);
+CREATE TABLE t2 (j DECIMAL(10,2), KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3.21),(4.55);
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+3.21
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 3.21)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = 3.21)"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i VARCHAR(32)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('cc'),('aa'),('ddd');
+CREATE TABLE t2 (j VARCHAR(16), KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('bbb'),('aa');
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+aa
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 'aa')",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = 'aa')"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i DATETIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES
+('2008-09-27 00:34:58'),('2007-05-28 00:00:00'), ('2009-07-25 09:21:20');
+CREATE TABLE t2 (j DATETIME, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+('2007-05-28 00:00:00'), ('2010-08-25 00:00:00');
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+2007-05-28 00:00:00
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 2007-05-28 00:00:00)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = TIMESTAMP'2007-05-28 00:00:00')"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i DATE) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('2008-09-27'),('2007-05-28'), ('2009-07-25');
+CREATE TABLE t2 (j DATE, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('2007-05-28'), ('2010-08-25');
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+2007-05-28
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 2007-05-28)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = TIMESTAMP'2007-05-28 00:00:00')"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;
+CREATE TABLE t1 (i TIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('00:34:58'),('10:00:02'), ('09:21:20');
+CREATE TABLE t2 (j TIME, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('10:00:02'), ('11:00:10');
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+i
+10:00:02
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery3>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "message": "Select tables optimized away"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 3,
+      "filtered": 100,
+      "attached_condition": "(sq.i = 10:00:02)",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 3,
+            "filtered": 100,
+            "attached_condition": "(t1.i = TIME'10:00:02')"
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1,t2;

mysql-test/t/derived_cond_pushdown.test

@@ -1166,4 +1166,110 @@ SELECT * FROM v AS v1, v AS v2
 DROP VIEW v;
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-11488: pushdown of the predicate with cached value
+--echo #
 
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(3),(2);
+
+CREATE TABLE t2 (j INT, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3),(4);
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+UPDATE t2 SET j = 2 WHERE j = 3;
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i FLOAT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1.5),(3.2),(2.71);
+
+CREATE TABLE t2 (j FLOAT, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3.2),(2.71);
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i DECIMAL(10,2)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1.5),(3.21),(2.47);
+
+CREATE TABLE t2 (j DECIMAL(10,2), KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3.21),(4.55);
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i VARCHAR(32)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('cc'),('aa'),('ddd');
+
+CREATE TABLE t2 (j VARCHAR(16), KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('bbb'),('aa');
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i DATETIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES
+ ('2008-09-27 00:34:58'),('2007-05-28 00:00:00'), ('2009-07-25 09:21:20');
+
+CREATE TABLE t2 (j DATETIME, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+ ('2007-05-28 00:00:00'), ('2010-08-25 00:00:00');
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i DATE) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('2008-09-27'),('2007-05-28'), ('2009-07-25');
+
+CREATE TABLE t2 (j DATE, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('2007-05-28'), ('2010-08-25');
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (i TIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('00:34:58'),('10:00:02'), ('09:21:20');
+
+CREATE TABLE t2 (j TIME, KEY(j)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES ('10:00:02'), ('11:00:10');
+
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM ( SELECT DISTINCT * FROM t1 ) AS sq
+  WHERE i IN ( SELECT MIN(j) FROM t2 );
+
+DROP TABLE t1,t2;

sql/item.cc

@@ -9406,6 +9406,17 @@ int Item_cache_int::save_in_field(Field *field, bool no_conversions)
 }
 
 
+Item *Item_cache_int::convert_to_basic_const_item(THD *thd)
+{
+  Item *new_item;
+  DBUG_ASSERT(value_cached || example != 0);
+  new_item= null_value ?
+            (Item*) new (thd->mem_root) Item_null(thd) :
+	    (Item*) new (thd->mem_root) Item_int(thd, val_int(), max_length);
+  return new_item;
+} 
+
+
 Item_cache_temporal::Item_cache_temporal(THD *thd,
                                          enum_field_types field_type_arg):
   Item_cache_int(thd, field_type_arg)
@@ -9561,6 +9572,23 @@ Item *Item_cache_temporal::clone_item(THD *thd)
 }
 
 
+Item *Item_cache_temporal::convert_to_basic_const_item(THD *thd)
+{
+  Item *new_item;
+  DBUG_ASSERT(value_cached || example != 0);
+  if (null_value)
+    new_item= (Item*) new (thd->mem_root) Item_null(thd);
+  else
+  {
+    MYSQL_TIME ltime;
+    unpack_time(val_datetime_packed(), &ltime);
+    new_item= (Item*) new (thd->mem_root) Item_datetime_literal(thd, &ltime,
+                                                                decimals);
+  }
+  return new_item;
+}
+
+
 bool Item_cache_real::cache_value()
 {
   if (!example)
@@ -9609,6 +9637,18 @@ my_decimal *Item_cache_real::val_decimal(my_decimal *decimal_val)
 }
 
 
+Item *Item_cache_real::convert_to_basic_const_item(THD *thd)
+{
+  Item *new_item;
+  DBUG_ASSERT(value_cached || example != 0);
+  new_item= null_value ?
+            (Item*) new (thd->mem_root) Item_null(thd) :
+	    (Item*) new (thd->mem_root) Item_float(thd, val_real(),
+                                                   decimals);
+  return new_item;
+} 
+
+
 bool Item_cache_decimal::cache_value()
 {
   if (!example)
@@ -9660,6 +9700,19 @@ my_decimal *Item_cache_decimal::val_decimal(my_decimal *val)
 }
 
 
+Item *Item_cache_decimal::convert_to_basic_const_item(THD *thd)
+{
+  Item *new_item;
+  my_decimal decimal_value;
+  my_decimal *result= val_decimal(&decimal_value);
+  DBUG_ASSERT(value_cached || example != 0);
+  new_item= null_value ?
+            (Item*) new (thd->mem_root) Item_null(thd) :
+	    (Item*) new (thd->mem_root) Item_decimal(thd, result);
+  return new_item;
+} 
+
+
 bool Item_cache_str::cache_value()
 {
   if (!example)
@@ -9739,6 +9792,26 @@ bool Item_cache_row::allocate(THD *thd, uint num)
 }
 
 
+Item *Item_cache_str::convert_to_basic_const_item(THD *thd)
+{
+  Item *new_item;
+  char buff[MAX_FIELD_WIDTH];
+  String tmp(buff, sizeof(buff), value->charset());
+  String *result= val_str(&tmp);
+  DBUG_ASSERT(value_cached || example != 0);
+  if (null_value)
+    new_item= (Item*) new (thd->mem_root) Item_null(thd);
+  else
+  {
+    uint length= result->length();
+    char *tmp_str= thd->strmake(result->ptr(), length);
+    new_item= new (thd->mem_root) Item_string(thd, tmp_str, length,
+                                              result->charset());
+  }
+  return new_item;
+}
+
+
 bool Item_cache_row::setup(THD *thd, Item *item)
 {
   example= item;

sql/item.h

@@ -5422,15 +5422,14 @@ public:
     example->split_sum_func2(thd, ref_pointer_array, fields, &example, flags);
   }
   Item *get_example() const { return example; }
-  Item* build_clone(THD *thd, MEM_ROOT *mem_root)
+
-  {
+  virtual Item *convert_to_basic_const_item(THD *thd) { return 0; };
-    Item_cache *copy= (Item_cache *) get_copy(thd, mem_root);
+  Item *derived_field_transformer_for_having(THD *thd, uchar *arg)
-    if (!copy)
+  { return convert_to_basic_const_item(thd); }
-      return 0;
+  Item *derived_field_transformer_for_where(THD *thd, uchar *arg)
-    if (!( copy->example= example->build_clone(thd, mem_root)))
+  { return convert_to_basic_const_item(thd); }
-      return 0;
+  Item *derived_grouping_field_transformer_for_where(THD *thd, uchar *arg)
-    return copy;
+  { return convert_to_basic_const_item(thd); }
-  } 
 };
 
 
@@ -5451,6 +5450,7 @@ public:
   enum Item_result result_type() const { return INT_RESULT; }
   bool cache_value();
   int save_in_field(Field *field, bool no_conversions);
+  Item *convert_to_basic_const_item(THD *thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_cache_int>(thd, mem_root, this); }
 };
@@ -5477,6 +5477,7 @@ public:
     Important when storing packed datetime values.
   */
   Item *clone_item(THD *thd);
+  Item *convert_to_basic_const_item(THD *thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_cache_temporal>(thd, mem_root, this); }
 };
@@ -5495,6 +5496,7 @@ public:
   my_decimal *val_decimal(my_decimal *);
   enum Item_result result_type() const { return REAL_RESULT; }
   bool cache_value();
+  Item *convert_to_basic_const_item(THD *thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_cache_real>(thd, mem_root, this); }
 };
@@ -5513,6 +5515,7 @@ public:
   my_decimal *val_decimal(my_decimal *);
   enum Item_result result_type() const { return DECIMAL_RESULT; }
   bool cache_value();
+  Item *convert_to_basic_const_item(THD *thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_cache_decimal>(thd, mem_root, this); }
 };
@@ -5541,6 +5544,7 @@ public:
   CHARSET_INFO *charset() const { return value->charset(); };
   int save_in_field(Field *field, bool no_conversions);
   bool cache_value();
+  Item *convert_to_basic_const_item(THD *thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_cache_str>(thd, mem_root, this); }
 };