UNION ALL and LIMIT in MySQL

user_449931 0
Viewed 36317

Let's say I want to perform this query: 

(SELECT a FROM t1 WHERE a=10 AND B=1) 
UNION ALL 
(SELECT a FROM t2 WHERE a=11 AND B=2) 
UNION ALL 
(SELECT a FROM t3 WHERE a=12 AND B=3) 
ORDER BY a LIMIT 1000;

Is MySQL smart enough to skip "t3" if 550 results are available in "t1" and 450 in "t2"?

I'm looking at MySQL docs (http://dev.mysql.com/doc/refman/5.1/en/union.html) but can't seem to find the answer.

mysql
5 Answers

Currently, MySQL will perform all selects on a union even if there are enough rows in the first few queries, as @Yuki Inoue mentioned in their answer. Using @user1477929's answer, you could re-write your query as:

(SELECT a FROM t1 WHERE a=10 AND B=1 LIMIT 1000) 
UNION ALL 
(SELECT a FROM t2 WHERE a=11 AND B=2 LIMIT 1000) 
UNION ALL 
(SELECT a FROM t3 WHERE a=12 AND B=3 LIMIT 1000) 
ORDER BY a LIMIT 1000;

which will give you at most 1000 rows, and never scan more than 3000.

user_4048610

If you have a very large table, then you can execute following query to see how smart MySQL optimizer is...

(
  select * from very_large_table
  union all
  select * from very_large_table
) limit 1

Sadly, this query takes very long time, compared to following query.

select * from very_large_table limit 1

It seems that whenever result of UNION is needed, MySQL first calculates the entire temporary table of the UNION result.

version

  • MySQL 8.0.11
user_30900680

Yes, MySql is smart enough to skip t3 if the results returned from t1 and t2 are equal to the LIMIT. In fact, it's the default behavior.

user_112314670
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