David Siegal <dsiegal@brave.cs.uml.edu> writes:
> If I were to do:
> SELECT nationality, ((COUNT(*) * 100)/(select count(*) from member)) as
> percentage FROM member GROUP BY nationality ORDER BY nationality;
> would this repeatedly execute the inner query over and over?
No, it'd do it just once, because the sub-select has no dependency on
the outer select. (If the sub-select used any variables from the outer
level, then it'd have to be done over at each outer row.)
You can tell the difference between these cases by looking at EXPLAIN
output for the query: do-once sub-selects are labeled InitPlans,
do-every-time sub-selects are labeled SubPlans.
regards, tom lane