Subject Re: [firebird-support] optimizing query
Author Svein Erling Tysvaer
At 18:08 17.07.2003 +0700, you wrote:
>Next question: how do we know that an index is selective enough to be used in
>a query plan?

I don't know (well, except that you don't even index a field having only a
couple of possible values)! But I do know that if an index uniquely
identify the row within a table, then using other indexes as well will
inevitably slow things down. Hence, INDEX (RDB$PRIMARY29,MT_SMSC_IDX)
indicated that MT_SMSC_IDX ought to be eliminated.

Sometimes it makes sense to use more than one index, particularly if
neither of the indexes are very selective, but making a general rule? Well,
that's for the real experts to answer, I haven't got a clue what such a
rule would look like... Trial and error is the main way I've learnt about
these things, and then I've watched the firebird-support (ib-support) group
for as long as it has existed and the ibobjects group for a short while
before that.


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