[MDEV-8679] Equal field propagation is not used for VARCHAR when it safely could Created: 2015-08-26  Updated: 2015-09-12  Resolved: 2015-08-26

Status: Closed
Project: MariaDB Server
Component/s: Optimizer
Affects Version/s: 5.5, 10.0, 10.1
Fix Version/s: 10.1.7

Type: Bug Priority: Major
Reporter: Alexander Barkov Assignee: Alexander Barkov
Resolution: Fixed Votes: 0
Labels: propagation, upstream

Issue Links:
Blocks
blocks MDEV-8728 Fix a number of problems in equal fie... Closed

 Description   

This script:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a INT);
 
INSERT INTO t1 VALUES (10),(11),(12);
 
EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=10 AND IF(a=10,1,0)=1;
 
SHOW WARNINGS;

returns

+-------+------+-----------------------------------------------------------------------------+
 
| Level | Code | Message                                                                     |
 
+-------+------+-----------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 10) |
 
+-------+------+-----------------------------------------------------------------------------+

I.e. the field equal value of 10 was propagated into IF, then the condition part with IF was evaluated as a constant and removed from the condition.

If I use VARCHAR in the same scenario:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a VARCHAR(10));
 
INSERT INTO t1 VALUES ('10'),('11'),('12');
 
EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='10' AND IF(a='10',1,0)=1;
 
SHOW WARNINGS;

it returns:

+-------+------+----------------------------------------------------------------------------------------------------------------------------+
 
| Level | Code | Message                                                                                                                    |
 
+-------+------+----------------------------------------------------------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = '10') and (if((`test`.`t1`.`a` = '10'),1,0) = 1)) |
 
+-------+------+----------------------------------------------------------------------------------------------------------------------------+

Propagation did not work.

Tracing in gdb shows the following:

(gdb) b Item_field::subst_argument_checker(unsigned char**) 
 
 
Breakpoint 3 at 0x555555c46d98: file /home/bar/maria-git/server/sql/item.cc, line 5361.
 
(gdb) c
 
Continuing.
 
 
 
Breakpoint 3, Item_field::subst_argument_checker (this=0x7fff9c007f70, 
    arg=0x7fffd9f04a00) at /home/bar/maria-git/server/sql/item.cc:5361
 
5361	  return *arg &&
 
(gdb) p (Subst_constraint) *arg
 
$3 = Item::IDENTITY_SUBST

I.e. Item_field::subst_argument_checker() is called with a wrong Subst_constaint value.
The correct value should be Item::ANY_SUBST, because the field appears in a comparison context.

 Comments   
Comment by Alexander Barkov [ 2015-08-26 ]

Another example:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a VARCHAR(10));
 
INSERT INTO t1 VALUES ('10'),('11'),('12');
 
EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='10' AND CASE WHEN a='10' THEN 1 ELSE 0 END;
 
SHOW WARNINGS;

returns:

+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------+
 
| Level | Code | Message                                                                                                                                    |
 
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = '10') and (case when (`test`.`t1`.`a` = '10') then 1 else 0 end)) |
 
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------+

Equal fields propagation did not work.
The expected result is:

+-------+------+-------------------------------------------------------------------------------+
 
| Level | Code | Message                                                                       |
 
+-------+------+-------------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = '10') |
 
+-------+------+-------------------------------------------------------------------------------+

Comment by Alexander Barkov [ 2015-09-12 ]

After the fix this script in MariaDB:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a VARCHAR(10));
 
INSERT INTO t1 VALUES ('10'),('11'),('12');
 
EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='10' AND IF(a='10',1,0)=1;
 
SHOW WARNINGS;

returns

+-------+------+-------------------------------------------------------------------------------+
 
| Level | Code | Message                                                                       |
 
+-------+------+-------------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = '10') |
 
+-------+------+-------------------------------------------------------------------------------+

MySQL-5.7.8 does not support propagation in the same scenario:

+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------+
 
| Level   | Code | Message                                                                                                                                   |
 
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------+
 
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release.                                                                         |
 
| Note    | 1003 | /* select#1 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` where ((`test`.`t1`.`a` = '10') and (if((`test`.`t1`.`a` = '10'),1,0) = 1)) |
 
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------+

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