MySQL select 10 random rows from 600K rows fast

Question

How can I best write a query that selects 10 rows randomly from a total of 600k

User · Answer

Use the below simple query to get random data from a table   SELECT user firstname   COUNT DISTINCT usr fk id  cnt FROM userdetails  GROUP BY usr fk id  ORDER BY cnt ASC   LIMIT 10

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If you want one random record  no matter if there are gapes between ids    PREPARE stmt FROM  SELECT   FROM  table name  LIMIT 1 OFFSET     SET  count    SELECT         FLOOR RAND     COUNT         FROM  table name     EXECUTE stmt USING  count    Source  https   www warpconduit net 2011 03 23 selecting-a-random-record-using-mysql-benchmark-results  comment-1266

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I Use this query   select floor RAND      SELECT MAX key  FROM table   from table limit 10   query time 0 016s

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I improved the answer  Riedsio had  This is the most efficient query I can find on a large  uniformly distributed table with gaps  tested on getting 1000 random rows from a table that has   2 6B rows      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max     SELECT MAX id  FROM table     1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION  SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1    Let me unpack what s going on     max     SELECT MAX id  FROM table    I m calculating and saving the max  For very large tables  there is a slight overhead for calculating MAX id  each time you need a row  SELECT FLOOR rand      max    1 as rand    Gets a random id  SELECT id FROM table INNER JOIN       on id  gt  rand LIMIT 1   This fills in the gaps  Basically if you randomly select a number in the gaps  it will just pick the next id  Assuming the gaps are uniformly distributed  this shouldn t be a problem     Doing the union helps you fit everything into 1 query so you can avoid doing multiple queries  It also lets you save the overhead of calculating MAX id   Depending on your application  this might matter a lot or very little   Note that this gets only the ids and gets them in random order  If you want to do anything more advanced I recommend you do this   SELECT t id  t name -- etc  etc FROM table t INNER JOIN        SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max     SELECT MAX id  FROM table     1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1  UNION      SELECT id FROM table INNER JOIN  SELECT FLOOR RAND      max    1 as rand  r on id  gt  rand LIMIT 1    x ON x id   t id ORDER BY t id

User · Answer

Here is a game changer that may be helpfully for many   I have a table with 200k rows  with sequential id s  I needed to pick N random rows  so I opt to generate random values based in the biggest ID in the table  I created this script to find out which is the fastest operation   logTime    query  SELECT COUNT id  FROM tbl    logTime    query  SELECT MAX id  FROM tbl    logTime    query  SELECT id FROM tbl ORDER BY id DESC LIMIT 1    logTime      The results are    Count  36 8418693542479 ms Max  0 241041183472 ms Order  0 216960906982 ms   Based in this results  order desc is the fastest operation to get the max id  Here is my answer to the question   SELECT GROUP CONCAT n SEPARATOR      g FROM       SELECT FLOOR RAND               SELECT id FROM tbl ORDER BY id DESC LIMIT 1        n FROM tbl LIMIT 10  a      SELECT   FROM tbl WHERE id IN   result     FYI  To get 10 random rows from a 200k table  it took me 1 78 ms  including all the operations in the php side

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Well if you have no gaps in your keys and they are all numeric you can calculate random numbers and select those lines  but this will probably not be the case   So one solution would be the following   SELECT   FROM table WHERE key  gt   FLOOR RAND   MAX id   LIMIT 1   which will basically ensure that you get a random number in the range of your keys and then you select the next best which is greater  you have to do this 10 times   however this is NOT really random because your keys will most likely not be distributed evenly   It s really a big problem and not easy to solve fulfilling all the requirements  MySQL s rand   is the best you can get if you really want 10 random rows   There is however another solution which is fast but also has a trade off when it comes to randomness  but may suit you better  Read about it here  How can i optimize MySQL  39 s ORDER BY RAND   function   Question is how random do you need it to be    Can you explain a bit more so I can give you a good solution   For example a company I worked with had a solution where they needed absolute randomness extremely fast  They ended up with pre-populating the database with random values that were selected descending and set to different random values afterwards again   If you hardly ever update you could also fill an incrementing id so you have no gaps and just can calculate random keys before selecting    It depends on the use case

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I am getting fast queries  around 0 5 seconds  with a slow cpu  selecting 10 random rows in a 400K registers MySQL database non-cached 2Gb size  See here my code  Fast selection of random rows in MySQL  time  microtime float      sql  SELECT COUNT    FROM pages    rquery  BD Ejecutar  sql   list  num records  mysql fetch row  rquery   mysql free result  rquery     sql  quot SELECT id FROM pages WHERE RAND    num records lt 20    ORDER BY RAND   LIMIT 0 10 quot    rquery  BD Ejecutar  sql   while list  id  mysql fetch row  rquery        if  id in   id in   quot   id quot       else  id in  quot  id quot     mysql free result  rquery     sql  quot SELECT id url FROM pages WHERE id IN  id in  quot    rquery  BD Ejecutar  sql   while list  id  url  mysql fetch row  rquery        logger  quot  id   url quot  1     mysql free result  rquery     time  microtime float  - time   logger  quot num records  num records quot  1   logger  quot  id in quot  1   logger  quot Time elapsed   lt b gt  time segundos lt  b gt  quot  1

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The following should be fast  unbiased and independent of id column  However it does not guarantee that the number of rows returned will match the number of rows requested   SELECT   FROM t WHERE RAND    lt   SELECT 10   COUNT    FROM t    Explanation  assuming you want 10 rows out of 100 then each row has 1 10 probability of getting SELECTed which could be achieved by WHERE RAND    lt  0 1  This approach does not guarantee 10 rows  but if the query is run enough times the average number of rows per execution will be around 10 and each row in the table will be selected evenly

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One way that i find pretty good if there s an autogenerated id is to use the modulo operator      For Example  if you need 10 000 random records out 70 000  you could simplify this by saying you need 1 out of every 7 rows  This can be simplified in this query   SELECT   FROM      table  WHERE      id        FLOOR           SELECT count 1  FROM table             10000         0    If the result of dividing target rows by total available is not an integer  you will have some extra rows than what you asked for  so you should add a LIMIT clause to help you trim the result set like this   SELECT   FROM      table  WHERE      id        FLOOR           SELECT count 1  FROM table             10000         0 LIMIT 10000    This does require a full scan  but it is faster than ORDER BY RAND  and in my opinion simpler to understand than other options mentioned in this thread  Also if the system that writes to the DB creates sets of rows in batches you might not get such a random result as you where expecting

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If you have just one Read-Request  Combine the answer of  redsio with a temp-table  600K is not that much    DROP TEMPORARY TABLE IF EXISTS tmp randorder  CREATE TABLE tmp randorder  id int 11  not null auto increment primary key  data id int 11    INSERT INTO tmp randorder  data id  select id from datatable    And then take a version of  redsios Answer   SELECT dt   FROM         SELECT  RAND                           SELECT MAX id                          FROM tmp randorder   AS id          AS rnd  INNER JOIN tmp randorder rndo on rndo id between rnd id - 10 and rnd id   10  INNER JOIN datatable AS dt on dt id   rndo data id  ORDER BY abs rndo id - rnd id   LIMIT 1    If the table is big  you can sieve on the first part   INSERT INTO tmp randorder  data id  select id from datatable where rand    lt  0 01    If you have many read-requests   Version  You could keep the table tmp randorder persistent  call it datatable idlist  Recreate that table in certain intervals  day  hour   since it also will get holes  If your table gets really big  you could also refill holes   select l data id as whole  from datatable idlist l left join datatable dt on dt id   l data id where dt id is null  Version  Give your Dataset a random sortorder column either directly in datatable or in a persistent extra table datatable sortorder  Index that column  Generate a Random-Value in your Application  I ll call it  rand    select l   from datatable l  order by abs random sortorder -  rand  desc  limit 1     This solution discriminates the  edge rows  with the highest and the lowest random sortorder  so rearrange them in intervals  once a day

User · Answer

Its very simple and single line query   SELECT   FROM Table Name ORDER BY RAND   LIMIT 0 10

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I needed a query to return a large number of random rows from a rather large table  This is what I came up with  First get the maximum record id   SELECT MAX id  FROM table name    Then substitute that value into   SELECT   FROM table name WHERE id  gt  FLOOR RAND     max  LIMIT n    Where max is the maximum record id in the table and n is the number of rows you want in your result set  The assumption is that there are no gaps in the record id s although I doubt it would affect the result if there were  haven t tried it though   I also created this stored procedure to be more generic  pass in the table name and number of rows to be returned  I m running MySQL 5 5 38 on Windows 2008  32GB  dual 3GHz E5450  and on a table with 17 361 264 rows it s fairly consistent at   03 sec    11 sec to return 1 000 000 rows   times are from MySQL Workbench 6 1  you could also use CEIL instead of FLOOR in the 2nd select statement depending on your preference   DELIMITER     USE  schema name      DROP PROCEDURE IF EXISTS  random rows      CREATE PROCEDURE  random rows  IN tab name VARCHAR 64   IN num rows INT  BEGIN  SET  t   CONCAT  SET  max  SELECT MAX id  FROM   tab name       PREPARE stmt FROM  t  EXECUTE stmt  DEALLOCATE PREPARE stmt   SET  t   CONCAT       SELECT   FROM        tab name        WHERE id gt FLOOR RAND    max  LIMIT        num rows    PREPARE stmt FROM  t  EXECUTE stmt  DEALLOCATE PREPARE stmt  END      then  CALL  schema name  random rows  table name   n

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I ve looked through all of the answers  and I don t think anyone mentions this possibility at all  and I m not sure why  If you want utmost simplicity and speed  at a minor cost  then to me it seems to make sense to store a random number against each row in the DB   Just create an extra column  random number  and set it s default to RAND     Create an index on this column  Then when you want to retrieve a row generate a random number in your code  PHP  Perl  whatever  and compare that to the column  SELECT FROM tbl WHERE random number  gt    random LIMIT 1  I guess although it s very neat for a single row  for ten rows like the OP asked you d have to call it ten separate times  or come up with a clever tweak that escapes me immediately

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I think here is a simple and yet faster way  I tested it on the live server in comparison with a few above answer and it was faster    SELECT   FROM  table name  WHERE id  gt    SELECT FLOOR  MAX id    RAND    FROM  table name    ORDER BY id LIMIT 30       Took 0 0014secs against a table of 130 rows  SELECT   FROM  table name  WHERE 1 ORDER BY RAND   LIMIT 30     Took 0 0042secs against a table of 130 rows   SELECT name FROM random AS r1 JOIN     SELECT CEIL RAND                       SELECT MAX id                      FROM random   AS id      AS r2 WHERE r1 id  gt   r2 id ORDER BY r1 id ASC LIMIT 30     Took 0 0040secs against a table of 130 rows

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Simple query that has excellent performance and works with gaps   SELECT   FROM tbl AS t1 JOIN  SELECT id FROM tbl ORDER BY RAND   LIMIT 10  as t2 ON t1 id t2 id   This query on a 200K table takes 0 08s and the normal version  SELECT   FROM tbl ORDER BY RAND   LIMIT 10  takes 0 35s on my machine    This is fast because the sort phase only uses the indexed ID column  You can see this behaviour in the explain   SELECT   FROM tbl ORDER BY RAND   LIMIT 10    SELECT   FROM tbl AS t1 JOIN  SELECT id FROM tbl ORDER BY RAND   LIMIT 10  as t2 ON t1 id t2 id   Weighted Version  https   stackoverflow com a 41577458 893432

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I guess this is the best possible way    SELECT id  id   RAND    AS random no  first name  last name FROM user ORDER BY random no

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A great post handling several cases  from simple  to gaps  to non-uniform with gaps   http   jan kneschke de projects mysql order-by-rand   For most general case  here is how you do it   SELECT name   FROM random AS r1 JOIN         SELECT CEIL RAND                           SELECT MAX id                          FROM random   AS id          AS r2  WHERE r1 id  gt   r2 id  ORDER BY r1 id ASC  LIMIT 1   This supposes that the distribution of ids is equal  and that there can be gaps in the id list  See the article for more advanced examples

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How to select random rows from a table   From here  Select random rows in MySQL  A quick improvement over  table scan  is to use the index to pick up random ids   SELECT   FROM random            SELECT id AS sid         FROM random         ORDER BY RAND            LIMIT 10       tmp WHERE random id   tmp sid

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SELECT column FROM table ORDER BY RAND   LIMIT 10   Not the efficient solution but works

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You can easily use a random offset with a limit PREPARE stm from  select   from table limit 10 offset     SET  total    select count    from table   SET   offset   FLOOR RAND      total   EXECUTE stm using   offset   You can also apply a where clause like so PREPARE stm from  select   from table where available true limit 10 offset     SET  total    select count    from table where available true   SET   offset   FLOOR RAND      total   EXECUTE stm using   offset   Tested on 600 000 rows  700MB  table query execution took  0 016sec HDD drive  EDIT  The offset might take a value close to the end of the table  which will result in the select statement returning less rows  or maybe only 1 row   to avoid this we can check the offset again after declaring it  like so SET  rows count   10  PREPARE stm from  quot select   from table where available true limit   offset   quot   SET  total    select count    from table where available true   SET   offset   FLOOR RAND      total   SET   offset    SELECT IF  total-  offset lt  rows count   offset- rows count   offset    SET   offset    SELECT IF   offset lt 0 0   offset    EXECUTE stm using  rows count   offset

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Another simple solution would be ranking the rows and fetch one of them randomly and with this solution you won t need to have any  Id  based column in the table   SELECT d   FROM   SELECT  t      rownum     rownum   1 AS rank FROM mytable AS t       SELECT  rownum    0  AS r       SELECT  cnt     SELECT RAND      SELECT COUNT    FROM mytable    AS n   d WHERE rank  gt    cnt LIMIT 10    You can change the limit value as per your need to access as many rows as you want but that would mostly be consecutive values   However  if you don t want consecutive random values then you can fetch a bigger sample and select randomly from it  something like      SELECT   FROM   SELECT d   FROM       SELECT  c      rownum     rownum   1 AS rank     FROM buildbrain  commits  AS c           SELECT  rownum    0  AS r           SELECT  cnt     SELECT RAND      SELECT COUNT    FROM buildbrain  commits     AS rnd   d  WHERE rank  gt    cnt LIMIT 10000    t ORDER BY RAND   LIMIT 10

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This is how I do it   select    from table with 600k rows where rand    lt  10 600000 limit 10   I like it because does not require other tables  it is simple to write  and it is very fast to execute

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All the best answers have been already posted  mainly those referencing the link http   jan kneschke de projects mysql order-by-rand     I want to pinpoint another speed-up possibility - caching  Think of why you need to get random rows  Probably you want display some random post or random ad on a website  If you are getting 100 req s  is it really needed that each visitor gets random rows  Usually it is completely fine to cache these X random rows for 1 second  or even 10 seconds   It doesn t matter if 100 unique visitors in the same 1 second get the same random posts  because the next second another 100 visitors will get different set of posts   When using this caching you can use also some of the slower solution for getting the random data as it will be fetched from MySQL only once per second regardless of your req s

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From book   Choose a Random Row Using an Offset Still another technique that avoids problems found in the preceding alternatives is to count the rows in the data set and return a random number between 0 and the count  Then use this number as an offset when querying the data set  rand    quot SELECT ROUND RAND      SELECT COUNT    FROM Bugs   quot    offset    pdo- gt query  rand - gt fetch PDO  FETCH ASSOC    sql    quot SELECT   FROM Bugs LIMIT 1 OFFSET  offset quot    stmt    pdo- gt prepare  sql    stmt- gt execute   offset     rand bug    stmt- gt fetch     Use this solution when you can   t assume contiguous key values and you need to make sure each row has an even chance of being selected

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This is super fast and is 100  random even if you have gaps    Count the number x of rows that you have available SELECT COUNT    as rows FROM TABLE Pick 10 distinct random numbers a 1 a 2     a 10 between 0 and x Query your rows like this  SELECT   FROM TABLE LIMIT 1 offset a i for i 1     10   I found this hack in the book SQL Antipatterns from Bill Karwin

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I used this http   jan kneschke de projects mysql order-by-rand  posted by Riedsio  i used the case of a stored procedure that returns one or more random values       DROP TEMPORARY TABLE IF EXISTS rands     CREATE TEMPORARY TABLE rands   rand id INT         loop me  LOOP         IF cnt  lt  1 THEN           LEAVE loop me          END IF           INSERT INTO rands            SELECT r1 id              FROM random AS r1 JOIN                    SELECT  RAND                                      SELECT MAX id                                     FROM random   AS id                     AS r2             WHERE r1 id  gt   r2 id             ORDER BY r1 id ASC             LIMIT 1           SET cnt   cnt - 1        END LOOP loop me    In the article he solves the problem of gaps in ids causing not so random results by maintaining a table  using triggers  etc   see the article   I m solving the problem by adding another column to the table  populated with contiguous numbers  starting from 1  edit  this  column is added to the temporary table created by the subquery at runtime  doesn t affect your permanent table       DROP TEMPORARY TABLE IF EXISTS rands     CREATE TEMPORARY TABLE rands   rand id INT         loop me  LOOP         IF cnt  lt  1 THEN           LEAVE loop me          END IF           SET  no gaps id    0           INSERT INTO rands            SELECT r1 id              FROM  SELECT id   no gaps id     no gaps id   1 AS no gaps id FROM random  AS r1 JOIN                    SELECT  RAND                                      SELECT COUNT                                       FROM random   AS id                     AS r2             WHERE r1 no gaps id  gt   r2 id             ORDER BY r1 no gaps id ASC             LIMIT 1           SET cnt   cnt - 1        END LOOP loop me    In the article i can see he went to great lengths to optimize the code  i have no ideea if how much my changes impact the performance but works very well for me

[mysql] MySQL select 10 random rows from 600K rows fast

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