[nycphp-talk] Re: MySQL doubt
zaunere at gmail.com
Sat Jan 15 02:41:47 EST 2005
> > Both queries have the same where clause.
> That's what I missed. I too would have expected the parser to execute
> the WHERE clause before the SELECT clause.
> Is this another case where MySQL is nonstandard?
It is standard. MyISAM has a special optimization that allows a COUNT(*) to
read the table definition directly, whereas most cases, this cannot hold
true. For example, a row with a NULL in a COUNT(somecolumn)'d isn't
counted. A WHERE or GROUP BY generally requires a real calculation to be
performed because the contents of each row cannot be assumed, and for InnoDB
tables, this can never be assumed because of multi-versioning.
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