Quentin Bennett wrote:
I think that you can use the left most columns of the index, without including
the remainder.
That's wasn't my understanding of how things work, but I've just checked
the documentation, and it looks like you're right:
<docs>
MySQL cannot use a partial index if the columns do not form a leftmost
prefix of the index. Suppose that you have the |SELECT| statements shown
here:
SELECT * FROM /|tbl_name|/ WHERE col1=/|val1|/;
SELECT * FROM /|tbl_name|/ WHERE col1=/|val1|/ AND col2=/|val2|/;
SELECT * FROM /|tbl_name|/ WHERE col2=/|val2|/;
SELECT * FROM /|tbl_name|/ WHERE col2=/|val2|/ AND col3=/|val3|/;
If an index exists on |(col1, col2, col3)|, only the first two queries
use the index. The third and fourth queries do involve indexed columns,
but |(col2)| and |(col2, col3)| are not leftmost prefixes of |(col1,
col2, col3)|.
</docs>
That's pretty strange. Anyway, what output do you get if you put
'explain ' in front of your queries? Are the indexes being used?
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