Re: Is index enough to do simple fetch, or table is always used too?

2008/12/14 Dmitry Koterov <dmitry@koterov.ru>:

> The question: if the table "tbl" scanned to fetch "id" and calculate
> md5(id), or the value of "id" is brought directly from "idx" index with no
> table data access at all? The second behaviour is logical: why should we
> access the table if all the needed data is already in the index entry...

In fact not all the required information is in the index. Postgresql
uses Multi-version-concurrency-control, which means there may be multi
versions of the same row. Postgresql must hit the heap (table) no get
the visibility information.