On Thu, 29 Apr 2004 19:09:09 -0400, Joseph Shraibman <[EMAIL PROTECTED]> wrote: >How does the analyzer/planner deal with rows clustered together?
There's a correlation value per column. Just try SELECT attname, correlation FROM pg_stats WHERE tablename = '...'; if you are interested. It indicates how well the hypothetical order of tuples if sorted by that column corresponds to the physical order. +1.0 is perfect correlation, 0.0 is totally chaotic, -1.0 means reverse order. The optimizer is more willing to choose an index scan if correlation for the first index column is near +/-1. > What if the data in the table happens to be close >together because it was inserted together originally? Having equal values close to each other is not enough, the values should be increasing, too. Compare 5 5 5 4 4 4 7 7 7 2 2 2 6 6 6 3 3 3 8 8 8 low correlation and 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 correlation = 1.0 ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings