When I used explain I found that the query planner use a seq scan to execute a query on a table containing about 2 millions rows.However I'm creating an index.Why does the planner uses seq scan in place of index scan?Does the execution of index scan is slower with table containing a huge amount of data?
Please avoid posting to multiple lists at once.
An index doesn't contain visibility information so every record located in the index must also be checked on the storage table to determine if it is visible to the current transaction and thus is valid to be returned. If it does need to be returned the table itself is also needed to get the rest of the information. Thus the index scan itself involves additional non-value-added (NVA) effort so far as the query is concerned. If a large fraction (I've seen estimates of 10%) of the table is estimated to be returned the additional cost involved with scanning the entire table will less than the NVA cost of walking through the index and then fetching records from the table anyway.
Also: "Fetching rows separately is much more expensive than reading them sequentially."
