Re: How to enable index fast in MyISAM table

[Gejun Shen]

Rick James Wrote:
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> > explain showes it use correct process_sid index.
>
> > KEY `process_sid` (`process_sid`,`action`),
> > select xxx from servicetable where
> process_sid="xxx" and action in (xxx, ...) order
> by session_seq
>
> That index is optimal for that query.

Yes. I think so.

> > explain partitions select count(*) from
> servicetable where timestamp>"2014-03-21 00:00:00"
> and timestamp<"2014-03-22 00:00:00" and
> date="2014-03-21";
>
> That failed to say "Using index". File a bug at
> bugs.mysql.com

I think it not a bug. In this case, date is used for partition pruning, and timestamp will be used as the search index. But when date=xxx" is evaluated, the date information need be read from data file and it can't use index only.

Compared to the statement with no partition pruning(no "date=xxx"), the query needn't go to data file for information. However, it need go to every partition for result. The fortunate thing is most partitions will return nothing except one partition and the merging will still be efficient.

Please point me out if I am wrong.


> I do not know if it matters that `date` and
> `timestamp` are NULLable, but it might.

I will try to test it and share the result.

PS: It's said sharding will added to MySQL. Is it true? Do you have some recommendation on sharding libraries for MySQL?