Bug #3504 HIGH_PRIORITY doesn't take any affect with UNION
Submitted: 19 Apr 2004 8:18 Modified: 3 May 2004 15:24
Reporter: Victoria Reznichenko Email Updates:
Status: Not a Bug Impact on me:
None 
Category:MySQL Server Severity:S3 (Non-critical)
Version: OS:Any
Assigned to: CPU Architecture:Any

[19 Apr 2004 8:18] Victoria Reznichenko 
Description:
HIGH_PRIORITY in the SELECT statement doesn't take any affect if query contains UNION. Even if LOW_PRIORITY keyword is used in the UPDATE statement. UPDATE statement is always executed first.

How to repeat:
Just execute query with SELECT HIGH_PRIORITY .. UNION SELECT .. and UPDATE statement.
[3 May 2004 15:24] MySQL Verification Team 
This is acutally a documentation issue.

I will forward this to our documentation department.

But behaviour is like this.

Setting HIGH_PRIORITY is forbidden in UNION's.

If you  set it in any other SELECT except for the first one, you will get a syntax error. 

If you set it in the first one, it will be silently re-set to normal priority.

This is due to the simple fact that during parsing there is no way of knowing whether this is simple SELECT or no.
[3 May 2004 22:20] Paul DuBois 
I've added a note about this in the manual now.