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Fixed bug#36006: Optimizer does table scan for SELECT COUNT(*)

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for ENGINE=MRG_MYISAM (should be optimized out).

Before WL#3281 MERGE engine had the HA_NOT_EXACT_COUNT flag
unset, and it worked with COUNT optimization as desired.
After the removal of the HA_NOT_EXACT_COUNT flag neither
HA_STATS_RECORDS_IS_EXACT (opposite to former HA_NOT_EXACT_COUNT
flag) nor modern HA_HAS_RECORDS flag were not added to MERGE
table flag mask.

1. The HA_HAS_RECORDS table flag has been set.
2. The ha_myisammrg::records method has been overridden to
   calculate total number of records in underlying tables.
This commit is contained in:
gshchepa/uchum@host.loc
2008-04-26 02:45:58 +05:00
parent 0b226a9299
commit 107b84f9bd
8 changed files with 70 additions and 2 deletions
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16
mysql-test/r/merge.result
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@@ -2006,3 +2006,19 @@ test.t1 optimize status OK
FLUSH TABLES m1, t1;
UNLOCK TABLES;
DROP TABLE t1, m1;
CREATE TABLE t1(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
CREATE TABLE t2(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
CREATE TABLE t3(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
CREATE TABLE t4(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2))
ENGINE=MRG_MYISAM UNION=(t1, t2, t3);
INSERT INTO t1 VALUES (1,1), (1,2),(1,3), (1,4);
INSERT INTO t2 VALUES (2,1), (2,2),(2,3), (2,4);
INSERT INTO t3 VALUES (3,1), (3,2),(3,3), (3,4);
EXPLAIN SELECT COUNT(*) FROM t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
EXPLAIN SELECT COUNT(*) FROM t4;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
DROP TABLE t1, t2, t3, t4;
End of 5.1 tests