How to create fast database queries

Archive for May 9th, 2009

Creating indexes

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Answering the questions asked on the site.

Dima asks:

I have a single rather large table with 18 columns.

Data in a table are continuous time stamped records.

On one hand, the application is appending new records to this table; on the other hand, users can do various queries.

My question is: whether I should use composite indexes or few single column indexes, or a mix of the two?

There could be all sorts of combinations in WHERE clause, I don't want to limit users in their choices.

Is there any common sense strategy to decide on indexes in my case?

Unfortunately you forgot to mention what RDBMS you are using, so I'll assume MySQL. The principles are all the same.

Generally speaking, an index on (column1, column2, column3) can be used for any kind of predicate that can be represented in the following form:

ROW(@value1_start, @value2_start, @value3_start) < ROW(column1, column2, column3) < ROW(@value1_end, @value2_end, @value3_end)

In other words, if all the records are sorted by column1, column2, column3, the condition should select a contiguous block of such a sorted set of records.

All @value's here can match and can be +Infinity or -Infinity.

Let's look to some real world predicates and decide whether they are sargable (i. e. an index search can be applied against them). We assume that all columns are INTEGER:
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Written by Quassnoi

May 9th, 2009 at 11:00 pm

Posted in MySQL