2008/12/14 Dmitry Koterov <dmitry@koterov.ru>:
> Hello.
>
> Suppose I have the following index:
>
> CREATE INDEX idx ON tbl USING btree (abc, def, id)
>
> and perform the query with index scan:
>
> SELECT md5(id)
> FROM tbl
> WHERE abc=1 AND def=2
> LIMIT 200
>
> 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.
>
> (Some time ago I have read something about this behaviour somewhere, but now
> cannot find it in the PostgreSQL documentation. Possibly it were words about
> another database, not Postges?)
>
>
Maybe here? http://www.postgresql.org/docs/8.3/static/mvcc.html
David
