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How to create fast database queries

Archive for October 1st, 2009

Bitwise operations and indexes

with 2 comments

From Stack Overflow:

Are the following queries efficient in MySQL:

SELECT  *
FROM    table
WHERE   field & number = number
-- to find values with superset of number's bits
SELECT  *
FROM    table
WHERE   field | number = number
-- to find values with subset of number's bits

, if an index for the field has been created?

If not, is there a way to make it run faster?

An index can be used for the following things:

  1. To limit the number of records scanned
  2. To lower the number of row lookups

When doing a full table scan, every record should be fetched and examined. If the table contains say, 1,000,000 records, and each record is 100 bytes long, then 100 Mb worth of data should be processed. These data are usually scanned sequentially.

An index row, on the other hand, contains the key value, the row pointer and some additional information (pointers to the parents and children). On a MyISAM table, each row of the index on an INT column occupies 10 bytes (4 (sizeof(INT)) + 6 (default MyISAM pointer size)) plus some overhead (block headers, leaf pointers etc).

Even if we cannot build a range on the index and need to look over all index values, 10 Mb is far less than 100 Mb.

However, scanning the index has two drawbacks:

  1. Traversing the B-Tree is more slow than a full table scan, since the former is not sequential
  2. When the condition we search for matches the index key, we should follow the pointer to fetch the unindexed values from the table row. This also takes some time.

Both these things can be optimized:

  • Oracle has INDEX FAST FULL SCAN access method. It scans the index rows in their physical order, not logical. The key order is not maintained in such a scan, but we not always need it anyway.

    This makes the index scanning sequential (and taking the constant time)

  • PostgreSQL has bitmap access method. PostgreSQL cannot disregard the index order as Oracle does so it has to traverse the tree anyway. But instead of following the pointer to fetch the values it fills a special bitmap with a bit set for each matching row.

    The order of bits corresponds to the physical order of rows.

    When it's over with setting bits it just reads the bitmap and fetches the rows with the bits set. Since the bits are ordered, the rows fetched are ordered too, so this scan is also sequential. Scanning the bitmap takes constant time (which is usually negligible since bitmaps are very small), pure time for fetching the rows depends on the number of bits set but is no longer than a full scan (since the scan is sequential).

MySQL, however, does neither of these optimizations.

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Written by Quassnoi

October 1st, 2009 at 11:00 pm

Posted in MySQL