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 UNION Clause

The UNION clause lets you combine the result sets of multiple queries. By default, the result sets are combined as if the DISTINCT operator was applied.

query_1 UNION [DISTINCT | ALL] query_2

The UNION keyword by itself is the same as UNION DISTINCT. Because eliminating duplicates can be a memory-intensive process for a large result set, prefer UNION ALL where practical. (That is, when you know the different queries in the union will not produce any duplicates, or where the duplicate values are acceptable.)

When an ORDER BY clause applies to a UNION ALL or UNION query, in Impala 1.4 and higher, the LIMIT clause is no longer required. To make the ORDER BY and LIMIT clauses apply to the entire result set, turn the UNION query into a subquery, SELECT from the subquery, and put the ORDER BY clause at the end, outside the subquery.

First, set up some sample data, including duplicate 1 values:

[localhost:21000] > create table few_ints (x int); [localhost:21000] > insert into few_ints values (1), (1), (2), (3); [localhost:21000] > set default_order_by_limit=1000;

This example shows how UNION ALL returns all rows from both queries, without any additional filtering to eliminate duplicates. For the large result sets common with Impala queries, this is the most memory-efficient technique.

[localhost:21000] > select x from few_ints order by x; +---+ | x | +---+ | 1 | | 1 | | 2 | | 3 | +---+ Returned 4 row(s) in 0.41s [localhost:21000] > select x from few_ints union all select x from few_ints; +---+ | x | +---+ | 1 | | 1 | | 2 | | 3 | | 1 | | 1 | | 2 | | 3 | +---+ Returned 8 row(s) in 0.42s [localhost:21000] > select * from (select x from few_ints union all select x from few_ints) as t1 order by x; +---+ | x | +---+ | 1 | | 1 | | 1 | | 1 | | 2 | | 2 | | 3 | | 3 | +---+ Returned 8 row(s) in 0.53s [localhost:21000] > select x from few_ints union all select 10; +----+ | x | +----+ | 10 | | 1 | | 1 | | 2 | | 3 | +----+ Returned 5 row(s) in 0.38s

This example shows how the UNION clause without the ALL keyword condenses the result set to eliminate all duplicate values, making the query take more time and potentially more memory. The extra processing typically makes this technique not recommended for queries that return result sets with millions or billions of values.

[localhost:21000] > select x from few_ints union select x+1 from few_ints; +---+ | x | +---+ | 3 | | 4 | | 1 | | 2 | +---+ Returned 4 row(s) in 0.51s [localhost:21000] > select x from few_ints union select 10; +----+ | x | +----+ | 2 | | 10 | | 1 | | 3 | +----+ Returned 4 row(s) in 0.49s [localhost:21000] > select * from (select x from few_ints union select x from few_ints) as t1 order by x; +---+ | x | +---+ | 1 | | 2 | | 3 | +---+ Returned 3 row(s) in 0.53s