Arjen Lentz wrote:
Hi Mark
On 01/08/2009, at 3:15 AM, MARK CALLAGHAN wrote:
Why does UNION ALL use a temp table? Many users are surprised by this. It would be faster to return rows from each branch of a UNION ALL query directly to the network.
Actually, it doesn't! I just tested it with a random MySQL 5.0 If it's UNION ALL and no ORDER BY is used, no tmp table is used. So, it acts as you wish already.
I had this in my brain already, but verified by running a SELECT * FROM tbl UNION ALL SELECT * FROM tbl and running SHOW SESSION STATUS LIKE 'created_tmp_tables' before and after. No change.
If you run EXPLAIN on it, it'll create a tmp table, but that's an artefact of EXPLAIN rather than the query since the query itself is not actually executed then.
Cheers, Arjen.
Arjen, Set a breakpoint in select_union::send_data and you'll see how a temporary table is filled in. Regards, Igor.