quick selection of a random row from a large table in mysql

Question

What is a fast way to select a random row from a large mysql table?

I\'m working in php, but I\'m interested in any solution even if it\'s in another language.

Answer 1

Quick and dirty method:

SET @COUNTER=SELECT COUNT(*) FROM your_table;

SELECT PrimaryKey
FROM your_table
LIMIT 1 OFFSET (RAND() * @COUNTER);

The complexity of the first query is O(1) for MyISAM tables.

The second query accompanies a table full scan. Complexity = O(n)

Dirty and quick method:

Keep a separate table for this purpose only. You should also insert the same rows to this table whenever inserting to the original table. Assumption: No DELETEs.

CREATE TABLE Aux(
  MyPK INT AUTO_INCREMENT,
  PrimaryKey INT
);

SET @MaxPK = (SELECT MAX(MyPK) FROM Aux);
SET @RandPK = CAST(RANDOM() * @MaxPK, INT)
SET @PrimaryKey = (SELECT PrimaryKey FROM Aux WHERE MyPK = @RandPK);

If DELETEs are allowed,

SET @delta = CAST(@RandPK/10, INT);

SET @PrimaryKey = (SELECT PrimaryKey
                   FROM Aux
                   WHERE MyPK BETWEEN @RandPK - @delta AND @RandPK + @delta
                   LIMIT 1);

The overall complexity is O(1).