Christopher Kings-Lynne <[EMAIL PROTECTED]> wrote: > Are you sure that postgres evaluates that subselect more than once? > It looks to me like it returns a constant result for every row and > hence it will be evaluated once per statement, not once per row. I'm > no expert tho. Can someone answer this? > > And if the subselect changes for each row (ie. it's a correlated > subquery) then you cannot use the variable anyway. > > It seems to me that if postgres doesn't consider count(*) as a > constant then perhaps it should be taught to? Should be safe > shouldn't it? I guess if a function in your select statemnt is > inserting a row then there's trouble. But if there is, then the > sum/count(*) is nonsensical anyway. > > Chris >
It looks like it (7.2.x): # time psql genline -c "select id from xxxx" > /dev/null real 0m0.694s user 0m0.147s sys 0m0.025s # time psql genline -c "select id,id||'/'||(select count(*) from xxxx) as x from xxxx" > /dev/null real 0m2.202s user 0m0.263s sys 0m0.040s # time psql genline -c "select id,(select count(*) from bildsekvens) as x from xxxx" > /dev/null real 0m1.479s user 0m0.254s sys 0m0.047s They were taken from a busy system, but i ran the several times showing about the same result. Magnus ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
