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How to create fast database queries

Archive for March, 2011

MySQL: GROUP BY in UNION

with 2 comments

From Stack Overflow:

I have a query where I have a custom developed UDF that is used to calculate whether or not certain points are within a polygon (first query in UNION) or circular (second query in UNION) shape.

SELECT  a.geo_boundary_id, …
FROM     geo_boundary_vertex a, …
…
GROUP BY
        a.geo_boundary_id
UNION
SELECT  b.geo_boundary_id, …
FROM     geo_boundary b, …
…
GROUP BY
        b.geo_boundary_id

When I run an explain for the query I get filesort for both queries within the UNION.

Now, I can split the queries up and use the ORDER BY NULL trick to get rid of the filesort however when I attempt to add that to the end of a UNION it doesn't work.

How do I get rid of the filesort?

In MySQL, GROUP BY also implies ORDER BY on the same set of expressions in the same order. That's why it adds an additional filesort operation to sort the resultset if it does not come out naturally sorted (say, from an index).

This is not always a desired behavior, and MySQL manual suggests adding ORDER BY NULL to the queries where sorting is not required. This can improve performance of the queries.

Let's create a sample table and see:

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Written by Quassnoi

March 30th, 2011 at 11:00 pm

Posted in MySQL

MySQL: splitting aggregate queries

with 2 comments

Answering questions asked on the site.

Victor asks:

I have a table which I will call sale to protect the innocent:

Sale
id product price amount date

I need to retrieve ultimate values of price, amount and date for each product:

SELECT  product,
        MIN(price), MAX(price),
        MIN(amount), MAX(amount),
        MIN(date), MAX(date)
FROM    sale
GROUP BY
        product

The query only returns about 100 records.

I have all these fields indexed (together with product), but this still produces a full table scan over 3,000,000 records.

How do I speed up this query?

To retrieve the ultimate values of the fields, MySQL would just need to make a loose index scan over each index and take the max and min value of the field for each product.

However, the optimizer won't do when multiple indexes are involved. Instead, it will revert to a full scan.

There is a workaround for it. Let's create a sample table and see them:

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Written by Quassnoi

March 28th, 2011 at 11:00 pm

Posted in MySQL