"Ron Johnson" <ron.l.johnson@xxxxxxx> writes: >> The main disadvantage is that it's not clear when it would actually be faster. >> Generally index scans are slower than reading the whole table and sorting. >> Probably it would have to run an SPI query to use the planner to find the best >> way to get the rows it wants. > > I believe you, but it's totally counter to prima-fascia logic. > > Scanning the whole table means that you have to read in a whole > bunch of columns that you don't really give a rat's arse about, and > thus is a waste, whereas directly reading an existing index means > that you've got perfect locality of data, since you're only reading > what you care about. Well you still have to read the table either way. The main difference is you'll end up reading it in a random access order which is slower and also means potentially reading parts of it many times over before you're done. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com
