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Poor Performance with tables over 1Million entries
Posted by: József Rekedt-Nagy
Date: April 08, 2009 05:47PM


I'm wondering if it is expected to have poor performance when using tables with more than 1M entries.

Have the following scenario:
Table I stores items(itemId, otherIrrelevantData)
Table D stores data(dataId,val)
Table M stores the mapping between the 2 above, let's call it DataMap(itemId,dataId). 2Indexes, 1 on (itemId,dataId) the other is on (dataId,itemId)

The relation is M-N (many to many), so any itemId can have many related data entries (estimation for avarage is 100) and a single dataId can belong to many itemIds (avarage here is higher, 10k or more).

The test database has just 1M mapping recorded in the DataMap table (M).

Q: What's the fastest/proper way to get items if the filter is matching of 3dataIds? (dataId of 1,2,3 for example)
Tried these queries so far:
1. Joining the map 3x is painfully slow: ~0.4sec
SELECT * FROM DataMap m, DataMap m2, DataMap m3
WHERE m.dataId=1 AND m2.itemId=m.itemId AND m2.dataId=2 AND m3.itemId=m2.itemId AND m3.dataId=3

2. Used OR and Group By with Having: ~0.3sec
WHERE dataId=1 OR dataId=2 OR dataId=3

According to EXPLAIN the query DOES use indexes in both cases, no temporary or file sorting is done. (USING INDEX and/or USING WHERE, USING INDEX only)

Speed is UNacceptable, must stay under 0.01s with any combination of ids.
Storage Engine is INNODB for all tables btw.

am I missing something obvious here? 1M rows can't be too much for MySql, right?

Thanks for any feedback or ideas in advance, greatly appreciated.

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