Mysql Union time V.S. separate query one by one [closed]

Answer 1

Since UNION ALL with n subqueries could be executed as n selects, a smart DBMS should be able do at most the same work as n separate selects.

That leaves round trip time, which is n * rtt for n queries and rtt for the UNION ALL.

In theory, a smart DBMS should always be answer a single UNION ALL faster than n separate queries.
In practice, as usual, all bets are off without testing.

Answer 2

The UNION operator requires duplicate tuples (rows) be removed the result set before any rows are returned. That's effectively a SORT UNIQUE operation. That's relatively inexpensive for small result sets, but for massive sets, it can be resource intensive on the server time (i.e. take a long time.)

In theory, combining the queries with a UNION ALL operator rather than a UNION operator would be fastest, since it would eliminate (n-1) roundtrips to the database, vs running queries separately. But for large values of n, you are going to run into practical limits on the size of the SQL text (max packet size).

Given the choice between UNION operator and separate queries, for a large result set, the separate queries are going to be less resource intensive on the server side.

In short, it's really a tradeoff between the heavy lifting for each query, vs. the heavy lifting of a SORT UNIQUE operation.