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a2e4449bfc
The problem is that select queries executed concurrently with a concurrent insert on a MyISAM table could be cached if the select started after the query cache invalidation but before the unlock of tables performed by the concurrent insert. This race could happen because the concurrent insert was failing to prevent cache of select queries happening at the same time. The solution is to add a 'uncacheable' status flag to signal that a concurrent insert is being performed on the table and that queries executing at the same time shouldn't cache the results.
73 lines
2.3 KiB
Text
73 lines
2.3 KiB
Text
flush status;
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set query_cache_type=DEMAND;
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set global query_cache_size= 1024*768;
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drop table if exists t1;
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create table t1 (a varchar(100));
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insert into t1 values ('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'),('bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb');
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Activate debug hook and attempt to retrieve the statement from the cache.
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set session debug='+d,wait_in_query_cache_insert';
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select SQL_CACHE * from t1;;
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On a second connection; clear the query cache.
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show status like 'Qcache_queries_in_cache';
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Variable_name Value
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Qcache_queries_in_cache 1
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set global query_cache_size= 0;
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Signal the debug hook to release the lock.
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select id from information_schema.processlist where state='wait_in_query_cache_insert' into @thread_id;
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kill query @thread_id;
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Show query cache status.
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show status like 'Qcache_queries_in_cache';
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Variable_name Value
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Qcache_queries_in_cache 0
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set global query_cache_size= 0;
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use test;
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drop table t1;
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SET @old_concurrent_insert= @@GLOBAL.concurrent_insert;
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SET @old_query_cache_size= @@GLOBAL.query_cache_size;
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DROP TABLE IF EXISTS t1, t2;
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CREATE TABLE t1 (a INT);
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CREATE TABLE t2 (a INT);
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INSERT INTO t1 VALUES (1),(2),(3);
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SET GLOBAL concurrent_insert= 1;
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SET GLOBAL query_cache_size= 1024*512;
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SET GLOBAL query_cache_type= ON;
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# Switch to connection con1
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SET SESSION debug='+d,wait_after_query_cache_invalidate';
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# Send concurrent insert, will wait in the query cache table invalidate
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INSERT INTO t1 VALUES (4);
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# Switch to connection default
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# Wait for concurrent insert to reach the debug point
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# Switch to connection con2
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# Send SELECT that shouldn't be cached
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SELECT * FROM t1;
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a
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1
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2
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3
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# Switch to connection default
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# Notify the concurrent insert to proceed
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SELECT ID FROM INFORMATION_SCHEMA.PROCESSLIST
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WHERE STATE = 'wait_after_query_cache_invalidate' INTO @thread_id;
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KILL QUERY @thread_id;
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# Switch to connection con1
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# Gather insert result
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SHOW STATUS LIKE "Qcache_queries_in_cache";
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Variable_name Value
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Qcache_queries_in_cache 0
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# Test that it's cacheable
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SELECT * FROM t1;
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a
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1
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2
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3
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4
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SHOW STATUS LIKE "Qcache_queries_in_cache";
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Variable_name Value
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Qcache_queries_in_cache 1
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# Disconnect
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# Restore defaults
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RESET QUERY CACHE;
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DROP TABLE t1,t2;
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SET GLOBAL concurrent_insert= DEFAULT;
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SET GLOBAL query_cache_size= DEFAULT;
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SET GLOBAL query_cache_type= DEFAULT;
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