Hi Jorgen,
The fix is good, but there is a room for improvement. Consider the example (it
would be good if you include it into your test case):
CREATE TABLE t1 (f1 char(3) PRIMARY KEY, f2 int);
create table t2(f1 int);
insert into t2 values (1),(2),(3),(4),(5),(6),(7),(8),(9);
insert into t1(f1,f2) select A.f1 + B.f1*10 + C.f1*100, A.f1 + B.f1*10 +
C.f1*100 from t2 A,t2 B,t2 C;
mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE f1=333 and f1 >= '333' and f1
> <444;
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref |
rows | filtered | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 3 | NULL |
546 | 100.00 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------------------------------------------------
| Level | Code | Message
+---------+------+-------------------------------------------------------------------------------------
| Warning | 1708 | Can't use index 'PRIMARY' for lookups due to type conversion
on field 'f1'
[skip]
2 rows in set (0.00 sec)
And there is 2 ways to fix it:
1) (Easy) Correct warning message to be like "Type conversion on '%s' could
prevent index lookups on index '%s'".
So it could prevent, but the case when index is used is still possible.
2) (Harder) Implement more comprehensive check - first, mark keys which we
"couldn't use", after all keys are gathered compare it with possible keys,
if marked keys are absent in possible keys then type conversion actually
prevented marked keys usage.
I personally prefer #1 since it gives a clue on why an index isn't used and if
the key is still used it points to a potential problem. Also, it's much easier
and error-proof.
Regards, Evgen.
On 08/31/10 07:08, Jorgen Loland wrote:
> #At file:///export/home/jl208045/mysql/mysql-next-mr-bugfixing-53562/ based on
> revid:jorgen.loland@stripped
>
> 3264 Jorgen Loland 2010-08-31
> BUG#53562: EXPLAIN statement should hint when index is not used
> due to type conversion
>
> An index may not be used for lookup if
> a) a type conversion is needed to compare an indexed
> field to a value, or
> b) there is a collation mismatch between an indexed field
> and the value compared to
>
> Before, the index would not be used for lookup, but no hint
> was given that this was the case. With this patch,
> EXPLAIN EXTENDED will issue a warning for the cases above.
>
> Note that even if an index cannot be used for lookup, the index
> may still be scanned.
> @ mysql-test/r/explain.result
> Added test for BUG#53562
> @ mysql-test/t/explain.test
> Added test for BUG#53562
> @ sql/share/errmsg-utf8.txt
> Added ER_WARN_INDEX_NO_LOOKUP_CAST and
> ER_WARN_INDEX_NO_LOOKUP_COLLATION for use when an index cannot
> be used for lookup due to type conversion or collation mismatch.
> @ sql/sql_select.cc
> Make EXPLAIN EXTENDED issue a warning if an index cannot be used
> for lookup due to type conversion of collation mismatch.
>
> modified:
> mysql-test/r/explain.result
> mysql-test/t/explain.test
> sql/share/errmsg-utf8.txt
> sql/sql_select.cc
> === modified file 'mysql-test/r/explain.result'
> --- a/mysql-test/r/explain.result 2010-08-26 12:02:59 +0000
> +++ b/mysql-test/r/explain.result 2010-08-31 07:08:36 +0000
> @@ -324,4 +324,43 @@ id select_type table type possible_keys
> 25 UNION NULL NULL NULL NULL NULL NULL NULL No tables used
> NULL UNION
> RESULT <union1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,...,25> ALL NULL NULL NULL NULL NULL
> # End BUG#30597
> +#
> +# BUG#53562: EXPLAIN statement should hint when
> +# index is not used due to type conversion
> +#
> +CREATE TABLE t1 (url char(1) PRIMARY KEY);
> +INSERT INTO t1 VALUES ('1'),('2');
> +
> +# Normally, primary key lookup is done
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url='1';
> +id select_type table type possible_keys key key_len ref rows filtered Extra
> +1 SIMPLE t1 const PRIMARY PRIMARY 1 const 1 100.00 Using index
> +Warnings:
> +Note 1003 select '1' AS `url` from `test`.`t1` where 1
> +
> +# Test that index can't be used for lookup due to type conversion
> +# (comparing char and int)
> +SELECT * FROM t1 WHERE url=1;
> +url
> +1
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url=1;
> +id select_type table type possible_keys key key_len ref rows filtered Extra
> +1 SIMPLE t1 index PRIMARY PRIMARY 1 NULL 2 100.00 Using where; Using index
> +Warnings:
> +Warning 1708 Can't use index 'PRIMARY' for lookups due to type conversion on field
> 'url'
> +Note 1003 select `test`.`t1`.`url` AS `url` from `test`.`t1` where
> (`test`.`t1`.`url` = 1)
> +
> +# Test that index can't be used for lookup due to collation mismatch
> +SELECT * FROM t1 WHERE url='1' collate latin1_german2_ci;
> +url
> +1
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url='1' collate latin1_german2_ci;
> +id select_type table type possible_keys key key_len ref rows filtered Extra
> +1 SIMPLE t1 index PRIMARY PRIMARY 1 NULL 2 100.00 Using where; Using index
> +Warnings:
> +Warning 1709 Can't use index 'PRIMARY' for lookups due to collation mismatch on
> field 'url'
> +Note 1003 select `test`.`t1`.`url` AS `url` from `test`.`t1` where
> (`test`.`t1`.`url` =<cache>(('1' collate latin1_german2_ci)))
> +
> +DROP TABLE t1;
> +# End BUG#53562
> End of 6.0 tests.
>
> === modified file 'mysql-test/t/explain.test'
> --- a/mysql-test/t/explain.test 2010-08-26 12:02:59 +0000
> +++ b/mysql-test/t/explain.test 2010-08-31 07:08:36 +0000
> @@ -279,4 +279,30 @@ EXPLAIN
>
> --echo # End BUG#30597
>
> +--echo #
> +--echo # BUG#53562: EXPLAIN statement should hint when
> +--echo # index is not used due to type conversion
> +--echo #
> +
> +CREATE TABLE t1 (url char(1) PRIMARY KEY);
> +INSERT INTO t1 VALUES ('1'),('2');
> +
> +--echo
> +--echo # Normally, primary key lookup is done
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url='1';
> +--echo
> +--echo # Test that index can't be used for lookup due to type conversion
> +--echo # (comparing char and int)
> +SELECT * FROM t1 WHERE url=1;
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url=1;
> +--echo
> +--echo # Test that index can't be used for lookup due to collation mismatch
> +SELECT * FROM t1 WHERE url='1' collate latin1_german2_ci;
> +EXPLAIN EXTENDED SELECT * FROM t1 WHERE url='1' collate latin1_german2_ci;
> +
> +--echo
> +DROP TABLE t1;
> +
> +--echo # End BUG#53562
> +
> --echo End of 6.0 tests.
>
> === modified file 'sql/share/errmsg-utf8.txt'
> --- a/sql/share/errmsg-utf8.txt 2010-07-29 14:15:38 +0000
> +++ b/sql/share/errmsg-utf8.txt 2010-08-31 07:08:36 +0000
> @@ -6387,3 +6387,7 @@ ER_TABLES_DIFFERENT_METADATA
> ER_ROW_DOES_NOT_MATCH_PARTITION
> eng "Found row that does not match the partition"
> swe "Hittade rad som inte passar i partitionen"
> +ER_WARN_INDEX_NO_LOOKUP_CAST
> + eng "Can't use index '%-.64s' for lookups due to type conversion on field
> '%-.64s'"
> +ER_WARN_INDEX_NO_LOOKUP_COLLATION
> + eng "Can't use index '%-.64s' for lookups due to collation mismatch on field
> '%-.64s'"
>
> === modified file 'sql/sql_select.cc'
> --- a/sql/sql_select.cc 2010-08-26 13:54:59 +0000
> +++ b/sql/sql_select.cc 2010-08-31 07:08:36 +0000
> @@ -5159,6 +5159,41 @@ static uint get_semi_join_select_list_in
> }
>
> /**
> + If EXPLAIN EXTENDED, add a warning for each index that cannot be
> + used for lookup due to either type conversion or different
> + collations on the field used for comparison
> +
> + Example type conversion (char compared to int):
> +
> + CREATE TABLE t1 (url char(1) PRIMARY KEY);
> + SELECT * FROM t1 WHERE url=1;
> +
> + Example different collations (danish vs german2):
> +
> + CREATE TABLE t1 (url char(1) PRIMARY KEY) collate latin1_danish_ci;
> + SELECT * FROM t1 WHERE url='1' collate latin1_german2_ci;
> +
> + @param thd Thread for the connection that submitted the query
> + @param field Field used in comparision
> + @param cant_use_indexes Indexes that cannot be used for lookup
> + @param errorno The error message number to use in the warning
> + */
> +static void
> +warn_index_not_applicable(THD *thd, const Field *field,
> + const key_map cant_use_index, const uint errorno)
> +{
> + if (thd->lex->describe& DESCRIBE_EXTENDED)
> + for (uint j=0 ; j< field->table->s->keys ; j++)
> + if (cant_use_index.is_set(j))
> + push_warning_printf(thd,
> + MYSQL_ERROR::WARN_LEVEL_WARN,
> + errorno,
> + ER(errorno),
> + field->table->key_info[j].name,
> + field->field_name);
> +}
> +
> +/**
> Add a possible key to array of possible keys if it's usable as a key
>
> @param key_fields Pointer to add key, if usable
> @@ -5183,6 +5218,7 @@ add_key_field(KEY_FIELD **key_fields,uin
> Field *field, bool eq_func, Item **value, uint num_values,
> table_map usable_tables, SARGABLE_PARAM **sargables)
> {
> + DBUG_PRINT("info",("add_key_field for field %s",field->field_name));
> uint exists_optimize= 0;
> if (!(field->flags& PART_KEY_FLAG))
> {
> @@ -5284,7 +5320,11 @@ add_key_field(KEY_FIELD **key_fields,uin
> if ((*value)->result_type() != STRING_RESULT)
> {
> if (field->cmp_type() != (*value)->result_type())
> + {
> + warn_index_not_applicable(stat->join->thd, field, possible_keys,
> + ER_WARN_INDEX_NO_LOOKUP_CAST);
> return;
> + }
> }
> else
> {
> @@ -5294,7 +5334,11 @@ add_key_field(KEY_FIELD **key_fields,uin
> */
> if (field->cmp_type() == STRING_RESULT&&
> ((Field_str*)field)->charset() != cond->compare_collation())
> + {
> + warn_index_not_applicable(stat->join->thd, field, possible_keys,
> + ER_WARN_INDEX_NO_LOOKUP_COLLATION);
> return;
> + }
> }
> }
> }
>
>
>
>
>
