Simon Riggs <si...@2ndquadrant.com> wrote: > This topic has been discussed many times, yet I have never seen an > assessment that explains WHY we would want to do index-only scans. In databases with this feature, it's not too unusual for a query which uses just an index to run one or more orders of magnitude faster than a query which has to randomly access the heap for each index entry. That seems like enough evidence of its possible value in PostgreSQL to proceed to the point where benchmarks become possible. I'm assuming that, like all other features added as performance optimizations, it won't be committed until there are benchmarks showing the net benefit. As a thought experiment, picture the relative costs of scanning a portion of an index in index sequence, and being done, versus scanning a portion of an index in index sequence and jumping to a random heap access for each index entry as you go. -Kevin
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