UNION vs UNION ALL Performance

When I was comparing performance of UNION vs MySQL 5.0 index merge algorithm Sinisa pointed out I should be using UNION ALL instead of simple UNION in my benchmarks, and he was right. Numbers would be different but it should not change general point of having optimization of moving LIMIT inside of union clause being cool thing.

But So is UNION ALL indeed faster than UNION DISTINCT (the UNION is shortcut for UNION DISTINCT) ?

Indeed it is. I did not have the same data as I used for the other test but I created similar test case – table with separate indexes on “a” and “b” columns with cardinality of 100, having about 40.000.000 of rows

This original query was taking about 22 seconds.

As I modified it:

The query time dropped to about 6 seconds which is 3.5 times faster – quite considerable improvement.

As you can notice I added “i!=5” clause – this is what allows us to ensure we do not have duplicate rows in result set matching both conditions and so result will be same as query with “i=5 or j=5” where clause.

I also tried this original query (which uses index merge method in MySQL 5.0):

Such query takes 4 seconds so if you do not need to trick with order by and limit using index merge is faster than UNION as it indeed should be.

So why UNION ALL is faster than UNION DISTINCT ?

The first informed guess would be – because UNION ALL does not need to use temporary table to store result set, however this is not correct – both UNION ALL and UNION distinct use temporary table for result generation. Perhaps one more thing for Optimizer Team to look into.

Interesting enough the fact UNION and UNION ALL require temporary table can only be seen in SHOW STATUS – EXPLAIN does not want to tell you this shameful fact:

In fact EXPLAIN output is the same for UNION and UNION ALL (which is too bad as execution for them is obviously different).

The difference in execution speed comes from the fact UNION requires internal temporary table with index (to skip duplicate rows) while UNION ALL will create table without such index.

This also explains why difference becomes larger when on disk table is required (as in this case) – Hash indexes used by MEMORY table are very efficient and do not give so much overhead.

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Comments (16)

  • Kiran

    Is UNION on same table going to make some lock or create more connection to database?

    October 5, 2007 at 12:00 am
  • Xaprb

    I read the source for UNION once (Mark Leith pointed me to it). It’s not very much code. The only difference between the two is the temporary table has the “distinct” property set, as I recall. So of course that means a unique index on all columns, which is bound to be slow.

    We should mention this in our book… it’s such an instinct for me to use UNION ALL that I forgot about it. I’ll go look and see if we have mentioned it anywhere.

    October 5, 2007 at 6:07 pm
  • peter


    I think the most important thing here is the fact even UNION ALL uses temporary table, while it could simply be sending result sets one after another in many cases, possibly with little conversion to adjust data types.

    The unique index is in fact not index on all columns but some form of hash index – with MyISAM key limit index on all columns would not work for tables with long rows not to mention BLOBs

    October 6, 2007 at 2:01 am
  • Scott Marlowe

    Union All is generally faster than union because it doesn’t have to do a sort / unique step.

    With union all, you smoosh two data sets together not caring if there are dups or not. union (distinct) has to then go that extra step to remove dups.


    November 8, 2007 at 10:48 am
  • peter

    Hey Scott,

    This is what I mentioned too. The point was not to see if it is faster but to get the measure of the thing in numbers.

    November 8, 2007 at 2:06 pm
  • ramasubramanian.G


    March 21, 2008 at 2:45 am
  • Sebastian Gomez Morales

    great article! (as usual)
    thanks a lot.
    i was looking for this and this text was very helpful
    keep the good work, boys

    March 25, 2008 at 3:45 pm
  • Charu


    Can someone answer my related query posted in the mysql performance forum.Query can be found here:



    May 18, 2008 at 11:55 pm
  • art

    Hey, thanks for the article, helped me also.

    September 9, 2009 at 3:16 am
  • hic


    I have a query like
    1)select distinct(col) from(select distinct a col from tab_a
    union all
    select distinct b col from tab_b
    union all
    select distinct c col from tab_c
    union all
    select distinct d col from tab_d

    2)select distinct(col) from(select distinct a col from tab_a
    union all
    select b col from tab_b
    union all
    select c col from tab_c
    union all
    select d col from tab_d

    3)select distinct(col) from(select distinct a col from tab_a
    select distinct b col from tab_b
    select distinct c col from tab_c
    select distinct d col from tab_d

    I thought like option 2 will be better to achieve this but the explain plan shows less cost for option 1. Can anybody tell me which option will be better.

    May 2, 2012 at 6:29 am
  • mohamat167

    Thanks Peter, it is useful for me.

    June 16, 2012 at 9:17 pm
  • Rahul

    suppose register_num = 1005 , register_num=2000 are present in the table.
    when i query using union i get the two rows as result wherein at result 0 i will have register_num = 1005 details and at result 1 i will have register_num=2000 details.
    Here I want to know if it is possible to get the details of register_num=2000 at result 1 when register_num=1005 is not present in DB. and get the details of register_num=1005 at result 0 when register_num=2000 is not present in DB

    September 10, 2012 at 9:33 pm
  • Kebba Foon

    Peter this article is a life saver, i never ever bother to check the union and union all statements always stick with union untill recently while i was working on a project, have to query sets everything works fine and when set to union different results. for the life of me could never understood why, i even concidered re-writting part my application code to do two seperate query – that will have cause me a lot of time and effort. Thanks P for saving me.

    November 26, 2013 at 5:53 am

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