How to create fast database queries

Archive for August 11th, 2009

Efficient date range query: SQL Server

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From Stack Overflow:

I have a table A that has a startDate and an endDate as 2 DATETIME columns besides some more other columns.

I have another table B that has one DATETIME column (call it dates column).

This is in SQL Server 2005.

How to best set up the indexes etc to get the following:

FROM a, b
WHERE a.startDate >= b.dates
AND a.endDate < b.dates [/sourcecode] Both tables have several thousand records.

Queries of this kind involve searching a table for a range that contains a given constant and plain B-Tree indexes are not good for this query.

A B-Tree index (or any other index that relies on sorting) is efficient for searching a column in a range between a minimal and maximal value (an unknown value between two constants).

However, this query is just the opposite: we search for a constaint between two unknown values.

If we create the indexes on startDate and endDate and issue a query with a condition like this:

startDate <= value AND endDate > value

, this will just give two range condition on two distinct indexes:

-INF <= startDate <= value
value < endDate <= +INF

Each of these ranges is too large to be efficient for searching, though their intersection can be quite small.

There is a special class of indexes: R-Tree indexes, which handle the conditions as described above just perfectly. They are mostly used to query 2-dimensional GIS data, that's why in MySQL they are called SPATIAL indexes.

In both these systems they can only be defined over geometry data which implies 2-dimensional objects (Points, LineStrings and such).

There is a way to use these indexes to speed up the query described above (see the article behind the link on how to do it in MySQL):

However, SQL Server 2005 does not support SPATIAL indexes.

In theory, an R-Tree index could be built over 1-dimensional data just as well, but no database of the major four supports it (as for now).

However, quite a close structure can be implemented using plain B-Tree indexes, and in this article I'll show how to do it.

Let's create a sample table:
Read the rest of this entry »

Written by Quassnoi

August 11th, 2009 at 11:00 pm

Posted in SQL Server