On Fri, Sep 07, 2007 at 10:30:30AM -0400, Mark Mielke wrote: > Kenneth Marshall wrote: >> I understand that a hash value is a many-to-one mapping. That is the >> point of the flag in the index. The flag means that there is only one >> item in the heap corresponding to that hash value. In this case we >> know that the value in the heap is the correct one and a possibly >> very expensive string comparison can be skipped. Given that the hash >> function is doing its job, almost every string comparison can be skipped. >> How long would it take to compare 1-32K of data? How much CPU usage? >> With this field in place, you only need to check tuple visibility > The value comparison cannot be skipped. I do not think you understand the > many-to-one mapping in its entirety. > > Example: > > Table has: a(1), b(2), c(3) > Index has: 1 => 1 (unique), 2 => 2 (unique), 3 => 3 (unique) > > Query: > > select * from table where key = 'z'; > > If 'z' hashes to '3' (completely possible), then the index record 3 points > to tuple 3, and it "exists". Only, it doesn't because 'a' <> 'z'. You MUST > check the value. > > Cheers, > mark > Yes, you are completely correct. Thank you for the reminder.
Regards, Ken ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
