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Is UNION query slow?
So this one takes 0.063. But if I combine it in a UNION (doesn’t matter if it’s UNION ALL OR DISTINCT OR WHATEVER) it just takes about 0.400 seconds.
How do you optimize a UNION query?
UNION ALL is much faster than UNION Combine results, sort, remove duplicates and return the set. Queries with UNION can be accelerated in two ways. Switch to UNION ALL or try to push ORDER BY, LIMIT and WHERE conditions inside each subquery.
What is the use of UNION all in MySQL?
The SQL UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It does not remove duplicate rows between the various SELECT statements (all rows are returned). Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types.
Are UNION queries efficient?
UNION ALL is faster than UNION because plain UNION is expecting that within two joined datasets are duplicates which need to be removed. If you can ensure (by inner WHERE clauses) that there will be no duplicates, it’s far better to use UNION ALL and let database engine optimize the inner selects.
Does Union affect performance?
Use UNION ALL instead of UNION whenever is possible That is why UNION ALL is faster. Because it does not remove duplicated values in the query. If there are few rows (let’s say 1000 rows), there is almost no performance difference between UNION and UNION ALL.
What can I use instead of union all?
There are several alternatives to the union SQL operator:
- Use UNION ALL.
- Execute each SQL separately and merge and sort the result sets within your program!
- Join the tables.
- In versions, 10g and beyond, explore the MODEL clause.
- Use a scalar subquery.
Which is the slowest query for Union all?
The following runs slow. Plan breaks apart t1 and t2 (which were also views) and assembles them as a big series of unions. The time filters are being applied properly on the individual components, but it is still very slow: SELECT FROM v WHERE time >= AND time <
Why is the Union all view so slow?
Your UNION ALL view is slower because, behind the scene, records from both SELECT #1 and #2 are combined in a temporary table first, which is created on the fly, and then your SELECT FROM v WHERE time >=
Why are Union queries so slow in InnoDB?
Sorting (via ORDER BY) may take extra time for whatever (select or union) it is attached to. It is very unlikely to take less time. Simply put, the Optimizer aims to do whatever is fastest, which might happen to be sorted. All these statements apply to InnoDB; MyISAM, which is not supported much, may be missing some recent optimizations.
How to query for ” Union all ” in SQL?
CREATE OR REPLACE FUNCTION CallMyView (t1 date, t2 date) RETURNS TABLE (d date, etc.) AS $$ BEGIN RETURN QUERY SELECT time, etc. FROM t1 WHERE time >= t1 AND time < t2 UNION ALL SELECT time, etc. FROM t2 WHERE time >= t1 AND time < t2; END; $$ LANGUAGE plpgsql; SELECT * FROM CallMyView (….);