Fetching Random Rows from a Table

Table of Contents

The Problem
Metrics
Case: Consecutive AUTO_INCREMENT without gaps, 1 row returned
Case: Consecutive AUTO_INCREMENT without gaps, 10 rows
Case: AUTO_INCREMENT with gaps, 1 row returned
Case: Extra FLOAT column for randomizing
Case: UUID or MD5 column
Postlog
Brought to you by Rick James

The Problem


(This is Version 2 of my RANDOM blog; the previous one had too many slow algorithms.)

One would like to do "SELECT ... ORDER BY RAND() LIMIT 10" to get 10 rows at random. But this is slow. The optimizer does
    ⚈  Fetch all the rows -- this is costly
    ⚈  Append RAND() to the rows
    ⚈  Sort the rows -- also costly
    ⚈  Pick the first 10.

All the algorithms given below are "fast", but most introduce flaws:
    ⚈  Bias -- some rows are more like to be fetched than others.
    ⚈  Repetitions -- If two random sets contain the same row, they are likely to contain other dups.
    ⚈  Sometimes failing to fetch the desired number of rows.

"Fast" means avoiding reading all the rows. There are many techniques that require a full table scan, or at least an index scan. They are not acceptable for this list. There is even a technique that averages half a scan; it is relegated to a footnote.

Metrics


Here's a way to measure performance without having a big table.
    FLUSH STATUS;
    SELECT ...;
    SHOW SESSION STATUS LIKE 'Handler%';
If some of the "Handler" numbers look like the number of rows in the table, then there was a table scan.

None of the queries presented here need a full table (or index) scan. Each has a time proportional to the number of rows returned.

Virtually all published algorithms involve a table scan. The previously published version of this blog had, embarassingly, several algorithms that had table scans.

Sometimes the scan can be avoided via a subquery. For example, the first of these will do a table scan; the second will not.
    SELECT *  FROM RandTest AS a
        WHERE id = FLOOR(@min + (@max - @min + 1) * RAND());  -- BAD: table scan
    SELECT *
        FROM RandTest AS a
        JOIN (
            SELECT FLOOR(@min + (@max - @min + 1) * RAND()) AS id -- Good; single eval.
             ) b  USING (id);

Case: Consecutive AUTO_INCREMENT without gaps, 1 row returned


    ⚈  Requirement: AUTO_INCREMENT id
    ⚈  Requirement: No gaps in id
    SELECT r.*
        FROM (
            SELECT FLOOR(mm.min_id + (mm.max_id - mm.min_id + 1) * RAND()) AS id
                FROM (
                    SELECT MIN(id) AS min_id,
                           MAX(id) AS max_id
                        FROM RandTest
                     ) AS mm
             ) AS init
        JOIN  RandTest AS r  ON r.id = init.id;
(Of course, you might be able to simplify this. For example, min_id is likely to be 1. Or precalculate limits into @min and @max.)

Case: Consecutive AUTO_INCREMENT without gaps, 10 rows


    ⚈  Requirement: AUTO_INCREMENT id
    ⚈  Requirement: No gaps in id
    ⚈  Flaw: Sometimes delivers fewer than 10 rows
    -- First select is one-time:
    SELECT @min := MIN(id),
           @max := MAX(id)
        FROM RandTest;
    SELECT DISTINCT *
        FROM RandTest AS a
        JOIN (
            SELECT FLOOR(@min + (@max - @min + 1) * RAND()) AS id
                FROM RandTest
                LIMIT 11    -- more than 10 (to compensate for dups)
             ) b  USING (id)
        LIMIT 10;           -- the desired number of rows
The FLOOR expression could lead to duplicates, hence the inflated inner LIMIT. There could (rarely) be so many duplicates that the inflated LIMIT leads to fewer than the desired 10 different rows. One approach to that Flaw is to rerun the query if it delivers too few rows.

A variant:
    SELECT r.*
        FROM (
            SELECT FLOOR(mm.min_id + (mm.max_id - mm.min_id + 1) * RAND()) AS id
                FROM (
                    SELECT MIN(id) AS min_id,
                           MAX(id) AS max_id
                        FROM RandTest
                     ) AS mm
                JOIN ( SELECT id dummy FROM RandTest LIMIT 11 ) z
             ) AS init
        JOIN  RandTest AS r  ON r.id = init.id
        LIMIT 10;
Again, ugly but fast, regardless of table size.

Case: AUTO_INCREMENT with gaps, 1 row returned


    ⚈  Requirement: AUTO_INCREMENT, possibly with gaps due to DELETEs, etc
    ⚈  Flaw: Only semi-random (rows do not have an equal chance of being picked), but it does partially compensate for the gaps
    ⚈  Flaw: The first few rows of the table are less likely to be delivered.

This gets 50 "consecutive" ids (possibly with gaps), then delivers a random 10 of them.
    -- First select is one-time:
    SELECT @min := MIN(id),
           @max := MAX(id)
        FROM RandTest;
    SELECT a.*
        FROM RandTest a
        JOIN ( SELECT id FROM
                ( SELECT id
                    FROM ( SELECT @min + (@max - @min + 1 - 5) * RAND() AS start FROM DUAL ) AS init
                    JOIN RandTest y
                    WHERE    y.id > init.start
                    ORDER BY y.id
                    LIMIT 50           -- Inflated to deal with gaps
                ) z ORDER BY RAND()
               LIMIT 10                -- number of rows desired
             ) r ON a.id = r.id;
Yes, it is complex, but yes, it is fast, regardles of the table size.

Case: Extra FLOAT column for randomizing


(Unfinished: need to check these.)

Assuming `rnd` is a FLOAT (or DOUBLE) populated with RAND() and INDEXed:

    ⚈  Requirement: extra, indexed, FLOAT column
    ⚈  Flaw: Fetches 10 adjacent rows (according to `rnd`), hence not good randomness
    ⚈  Flaw: Near 'end' of table, can't find 10 rows.
    SELECT r.*
        FROM ( SELECT RAND() AS start FROM DUAL ) init
        JOIN RandTest r
        WHERE r.rnd >= init.start
        ORDER BY r.rnd
        LIMIT 10;

    ⚈  These two variants attempt to resolve the end-of-table flaw:
    SELECT r.*
        FROM ( SELECT RAND() * ( SELECT rnd
                          FROM RandTest
                          ORDER BY rnd DESC
                          LIMIT 10,1 ) AS start
             ) AS init
        JOIN RandTest r
        WHERE r.rnd > init.start
        ORDER BY r.rnd
        LIMIT 10;

SELECT @start := RAND(), @cutoff := CAST(1.1 * 10 + 5 AS DECIMAL(20,8)) / TABLE_ROWS FROM information_schema.TABLES WHERE TABLE_SCHEMA = 'dbname' AND TABLE_NAME = 'RandTest'; -- 0.0030 SELECT d.* FROM ( SELECT a.id FROM RandTest a WHERE rnd BETWEEN @start AND @start + @cutoff ) sample JOIN RandTest d USING (id) ORDER BY rand() LIMIT 10;

Case: UUID or MD5 column


    ⚈  Requirement: UUID/GUID/MD5/SHA1 column exists and is indexed.
    ⚈  Similar code/benefits/flaws to AUTO_INCREMENT with gaps.
    ⚈  Needs

7 random HEX digits: RIGHT( HEX( (1<<24) * (1+RAND()) ), 6)

can be used as a `start` for adapting a gapped AUTO_INCREMENT case. If the field is BINARY instead of hex, then UNHEX(RIGHT( HEX( (1<<24) * (1+RAND()) ), 6))

Postlog


Original writing -- July, 2012; not adequate

Not adequately fast:
Another Approach
And another
Excellent for randomness

Rewrite -- July, 2015


Contact me by posting a question at MySQL Forums :: Performance
-- Rick James

MySQL Documents by Rick James

HowTo Techniques for Optimizing Tough Tasks:

Partition Maintenance (DROP+REORG) for time series (includes list of PARTITION uses)
Big DELETEs - how to optimize -- and other chunking advice, plus a use for PARTITIONing
Data Warehouse techniques: Overview   Summary Tables   High speed ingestion  
Entity-Attribute-Value -- a common, poorly performing, design pattern (EAV); plus an alternative
Find the nearest 10 pizza parlors -- efficient searching on Latitude + Longitude (another PARITION use)
Pagination, not with OFFSET, LIMIT
Techniques on efficiently finding a random row (On beyond ORDER BY RAND())
GUID/UUID Performance (type 1 only)
IP Range Table Performance -- or other disjoint ranges
Rollup Unique User Counts
Alter of a Huge table -- Mostly obviated by 5.6
Latest 10 news articles -- how to optimize the schema and code for such
Build and execute a "Pivot" SELECT (showing rows as columns)
Find largest row for each group

Other Tips, Tuning, Debugging, Optimizations, etc...

Rick's RoTs (Rules of Thumb -- lots of tips)
Memory Allocation (caching, etc)
Character Set and Collation problem solver
Converting from MyISAM to InnoDB -- includes differences between them
Compound INDEXes plus other insights into the mysteries of INDEXing
Cookbook for Creating Indexes
MySQL Limits -- built-in hard limits
Galera, tips on converting to (Percona XtraDB Cluster, MariaDB 10, or manually installed)
5.7's Query Rewrite -- perhaps 5.7's best perf gain, at least for this forum's users
Best of MySQL Forum -- index of lots of tips, discussions, etc

View Rick James's profile on LinkedIn