Greg Stark <[EMAIL PROTECTED]> writes: > Tom Lane <[EMAIL PROTECTED]> writes: >> However, it looks to me like the subquery-scan-outside plan probably >> is the faster one, on both my machine and yours. I get
> Woah, that's pretty whacky. It seems like it ought to be way faster to do a > single sequential scan and return two records for each tuple read rather than > do an entire unnecessary sequential scan, even if most or even all of the > second one is cached. The problem is the CPU expense of executing "SELECT 1 UNION SELECT 2" over and over. Doing that for every row of the outer table adds up. We were both testing on relatively small tables --- I suspect the results would be different if the outer table were too large to fit in disk cache. I am not sure why the planner did not choose to stick a Materialize node atop the Subquery Scan, though. It looks to me like it should have considered that option --- possibly the undercharging for Subquery Scan is the reason it wasn't chosen. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]