Select MAX value from two tables: which one is better

Question

I have two ways to select MAX values from two tables but don't know which one is better or faster. i checked from mysql workbench but in case both query i'm getting 0.00 Sec execution time

First query

SELECT MAX(s) from (
  SELECT sheetid  s FROM csheets
  UNION all
  SELECT sheetid s FROM queueitems
) as t

Explain

Second Query

SELECT MAX(s) from (
  SELECT MAX(sheetid)  s FROM csheets
  UNION all
  SELECT MAX(sheetid) s FROM queueitems
) as t

Explain So question is which one better in terms of speed?

Answer 1

The second one would be better, because the aggregation is performed before, you will have one step less on the temporary table than with the first query.

However take a look at the execution plan, with the EXPLAIN .