mirror of
https://github.com/MariaDB/server.git
synced 2025-01-17 12:32:27 +01:00
f8dbbc010f
Analysis:
The optimizer distinguishes two kinds of 'constant' conditions:
expensive ones, and non-expensive ones. The non-expensive conditions
are evaluated inside make_join_select(), and if false, already the
optimizer detects empty query results.
In order to avoid arbitrarily expensive optimization, the evaluation of
expensive constant conditions is delayed until execution. These conditions
are attached to JOIN::exec_const_cond and evaluated in the beginning of
JOIN::exec. The relevant execution logic is:
JOIN::exec()
{
if (! join->exec_const_cond->val_int())
{
produce an empty result;
stop execution
}
continue execution
execute the original WHERE clause (that contains exec_const_cond)
...
}
As a result, when an expensive constant condition is
TRUE, it is evaluated twice - once through
JOIN::exec_const_cond, and once through JOIN::cond.
When the expensive constant condition is a subquery,
predicate, the subquery is evaluated twice. If we have
many levels of subqueries, this logic results in a chain
of recursive subquery executions that walk a perfect
binary tree. The result is that for subquries with depth N,
JOIN::exec is executed O(2^N) times.
Solution:
Notice that the second execution of the constant conditions
happens inside do_select(), in the branch:
if (join->table_count == join->const_tables) { ... }
In this case exec_const_cond is equivalent to the whole WHERE
clause, therefore the WHERE clause has already been checked in
the beginnig of JOIN::exec, and has been found to be true.
The bug is addressed by not evaluating the WHERE clause if there
was exec_const_conds, and it was TRUE.
130 lines
4.8 KiB
Text
130 lines
4.8 KiB
Text
select @test_compress_string:='string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ';
|
|
@test_compress_string:='string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa '
|
|
string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
select length(@test_compress_string);
|
|
length(@test_compress_string)
|
|
117
|
|
select uncompress(compress(@test_compress_string));
|
|
uncompress(compress(@test_compress_string))
|
|
string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
explain extended select uncompress(compress(@test_compress_string));
|
|
id select_type table type possible_keys key key_len ref rows filtered Extra
|
|
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL No tables used
|
|
Warnings:
|
|
Note 1003 select uncompress(compress((@test_compress_string))) AS `uncompress(compress(@test_compress_string))`
|
|
select uncompressed_length(compress(@test_compress_string))=length(@test_compress_string);
|
|
uncompressed_length(compress(@test_compress_string))=length(@test_compress_string)
|
|
1
|
|
explain extended select uncompressed_length(compress(@test_compress_string))=length(@test_compress_string);
|
|
id select_type table type possible_keys key key_len ref rows filtered Extra
|
|
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL No tables used
|
|
Warnings:
|
|
Note 1003 select (uncompressed_length(compress((@test_compress_string))) = length((@test_compress_string))) AS `uncompressed_length(compress(@test_compress_string))=length(@test_compress_string)`
|
|
select uncompressed_length(compress(@test_compress_string));
|
|
uncompressed_length(compress(@test_compress_string))
|
|
117
|
|
select length(compress(@test_compress_string))<length(@test_compress_string);
|
|
length(compress(@test_compress_string))<length(@test_compress_string)
|
|
1
|
|
create table t1 (a text, b char(255), c char(4)) engine=myisam;
|
|
insert into t1 (a,b,c) values (compress(@test_compress_string),compress(@test_compress_string),'d ');
|
|
select uncompress(a) from t1;
|
|
uncompress(a)
|
|
string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
select uncompress(b) from t1;
|
|
uncompress(b)
|
|
string for test compress function aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
|
|
select concat('|',c,'|') from t1;
|
|
concat('|',c,'|')
|
|
|d|
|
|
drop table t1;
|
|
select compress("");
|
|
compress("")
|
|
|
|
select uncompress("");
|
|
uncompress("")
|
|
|
|
select uncompress(compress(""));
|
|
uncompress(compress(""))
|
|
|
|
select uncompressed_length("");
|
|
uncompressed_length("")
|
|
0
|
|
create table t1 (a text);
|
|
insert t1 values (compress(null)), ('A\0\0\0BBBBBBBB'), (compress(space(50000))), (space(50000));
|
|
select length(a) from t1;
|
|
length(a)
|
|
NULL
|
|
12
|
|
76
|
|
50000
|
|
select length(uncompress(a)) from t1;
|
|
length(uncompress(a))
|
|
NULL
|
|
NULL
|
|
50000
|
|
NULL
|
|
Warnings:
|
|
Error 1259 ZLIB: Input data corrupted
|
|
Error 1256 Uncompressed data size too large; the maximum size is 1048576 (probably, length of uncompressed data was corrupted)
|
|
drop table t1;
|
|
set @@global.max_allowed_packet=1048576*100;
|
|
select compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null;
|
|
compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null
|
|
0
|
|
set @@global.max_allowed_packet=default;
|
|
create table t1(a blob);
|
|
insert into t1 values(NULL), (compress('a'));
|
|
select uncompress(a), uncompressed_length(a) from t1;
|
|
uncompress(a) uncompressed_length(a)
|
|
NULL NULL
|
|
a 1
|
|
drop table t1;
|
|
create table t1(a blob);
|
|
insert into t1 values ('0'), (NULL), ('0');
|
|
select compress(a), compress(a) from t1;
|
|
select compress(a) is null from t1;
|
|
compress(a) is null
|
|
0
|
|
1
|
|
0
|
|
drop table t1;
|
|
End of 4.1 tests
|
|
create table t1 (a varchar(32) not null);
|
|
insert into t1 values ('foo');
|
|
explain select * from t1 where uncompress(a) is null;
|
|
id select_type table type possible_keys key key_len ref rows Extra
|
|
1 SIMPLE t1 system NULL NULL NULL NULL 1
|
|
Warnings:
|
|
Error 1259 ZLIB: Input data corrupted
|
|
select * from t1 where uncompress(a) is null;
|
|
a
|
|
foo
|
|
Warnings:
|
|
Error 1259 ZLIB: Input data corrupted
|
|
Error 1259 ZLIB: Input data corrupted
|
|
explain select *, uncompress(a) from t1;
|
|
id select_type table type possible_keys key key_len ref rows Extra
|
|
1 SIMPLE t1 system NULL NULL NULL NULL 1
|
|
select *, uncompress(a) from t1;
|
|
a uncompress(a)
|
|
foo NULL
|
|
Warnings:
|
|
Error 1259 ZLIB: Input data corrupted
|
|
select *, uncompress(a), uncompress(a) is null from t1;
|
|
a uncompress(a) uncompress(a) is null
|
|
foo NULL 1
|
|
Warnings:
|
|
Error 1259 ZLIB: Input data corrupted
|
|
Error 1259 ZLIB: Input data corrupted
|
|
drop table t1;
|
|
CREATE TABLE t1 (c1 INT);
|
|
INSERT INTO t1 VALUES (1), (1111), (11111);
|
|
SELECT UNCOMPRESS(c1), UNCOMPRESSED_LENGTH(c1) FROM t1;
|
|
UNCOMPRESS(c1) UNCOMPRESSED_LENGTH(c1)
|
|
NULL NULL
|
|
NULL NULL
|
|
NULL 825307441
|
|
EXPLAIN EXTENDED SELECT * FROM (SELECT UNCOMPRESSED_LENGTH(c1) FROM t1) AS s;
|
|
DROP TABLE t1;
|
|
End of 5.0 tests
|