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From IRC:
<igor> elenst: could you come up with the test case where e.g. mysql.stat_table is corrupted somehow, then we 1) flush table t1; 2) set use_stat_tables='preferably' 3) run explain select * from t1;
<igor> elenst: in this case the result of explain is expected the same as we do not use ant stat tables (+ some warning).
<igor> elenst: I'm afraid now we just have rows == 0 in the explain output with this scenario, which not correct of course.
The provided test case confirms the suspicion: EXPLAIN shows 0 rows while the real number is 2.
Test case
# Run as perl ./mtr <testname>
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CREATE TABLE t1 (i INT) ENGINE=MyISAM;
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INSERT INTO t1 VALUES (1),(2);
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--move_file $MYSQLTEST_VARDIR/mysqld.1/data/mysql/table_stat.MYD $MYSQLTEST_VARDIR/mysqld.1/data/mysql/table_stat.MYD.save
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FLUSH TABLES;
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SET use_stat_tables=PREFERABLY;
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EXPLAIN SELECT * FROM t1;
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# Cleanup
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--move_file $MYSQLTEST_VARDIR/mysqld.1/data/mysql/table_stat.MYD.save $MYSQLTEST_VARDIR/mysqld.1/data/mysql/table_stat.MYD
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SET use_stat_tables=DEFAULT;
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DROP TABLE t1;
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Actual result:
CREATE TABLE t1 (i INT) ENGINE=MyISAM;
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INSERT INTO t1 VALUES (1),(2);
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FLUSH TABLES;
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SET use_stat_tables=PREFERABLY;
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EXPLAIN SELECT * FROM t1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 0
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Warnings:
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Error 29 File './mysql/table_stat.MYD' not found (Errcode: 2)
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Error 1017 Can't find file: 'table_stat' (errno: 2)
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SET use_stat_tables=DEFAULT;
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DROP TABLE t1;
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bzr version-info
revision-id: elenst@ubuntu11.home-20120903180152-r1n87m6pcvzffh63
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date: 2012-09-03 22:01:52 +0400
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build-date: 2012-09-09 01:58:25 +0300
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revno: 3367
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Workaround: if EXPLAIN produces warnings about corrupted stat tables, set use_stat_tables to NEVER.
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