Ratheesh K J wrote:
Hello all,
I have a doubt on Index Merge.
We are currently using MySQL server 4.1.11. As a part of DB management I am
currently doing an analysis on all the Tables.
While looking into the table structures, their columns and Indexes I found that
most of the tables have an Index on fields that have a very low cardinality.
For an estimate let me say that there were indexes on fields with cardinality
17 for a table with 13 lac rows. So i decided to remove the Index on such
fields.
I made this decision because I assume that the probability of MySQL optimizer
choosing such indexes is very low. MySQL would always choose a better index
than this.
Now i doubt my assumption when I move to MySQL server 5.0.X. In MySQL 5 there
is a concept of Index Merge. So was it right for me to remove these indexes if
we were to use MySQL 5?
How much of a difference in terms of performance would removal of Index make in
MySQL 4.1.11?
How much of a difference in terms of performance would retaining of Index make
in MySQL 5?
You can test those scenarios yourself.
You can use "ignore index" like this:
select blah from table IGNORE INDEX (index_name) WHERE ....;
so that will tell mysql specifically not to use that index without you
having to drop it and recreate it.
Comes in handy ;)
And it *always* depends on the situation (queries you run a lot vs
queries that run once a month) and data you have. Asking us not to take
that into account is silly.
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