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Hi all, A silly idea: a composite index on objectId, subChunkId (beware the order) may avoid to create a temporary table for each subChunkId? Here's an interesting documentation about the weird behaviour of MySQL query optimizer: http://dasini.net/blog/2009/02/18/optimisation-de-requetes-comprendre-loptimiseur-de-mysql/ And the official doc: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html FYI, Daniel and Jacek have ran some tests about it. Cheers, Fabrice ######################################################################## Use REPLY-ALL to reply to list To unsubscribe from the QSERV-L list, click the following link: https://listserv.slac.stanford.edu/cgi-bin/wa?SUBED1=QSERV-L&A=1