MySQL queries slow after a DELETE

I have a two part PHP script.

1. The first that deletes some rows in the database
2. The second that triggers SELECT queries

The second script alone runs in about 开发者_如何学C0.2 sec. When both parts are processed, the second part takes 5 sec. The next time the second script runs alone, it's back to 0.2 sec.

Any clue?

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Its likely that the query has cached so it runs faster the second time, more info here. If you add SQL_NO_CACHE to the query then you should be able to determine if the cache is a factor.

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Your Query-Cache for a table is 'invalidated' each time the table is updated.

The result is that any UPDATE/DELETE/INSERT to your table, will clear the query-cache of that table, and force a fresh disk read for the next SELECT. The result will be a slower query.

Here's a link to the MySQL DOCs.

If your table is incredibly large, you may want to investigate MyISAM tables and a separate Key-Cache for yet better read performance. MyISAM = fast reads, fast writes, horrible concurrency for writes/udpates. InnoDB = mediocre speed for reads, good concurrency for writes/updates.

-- J Jorgenson --