Re: Query about index usage

Hello,
> PostgreSQL can't currently avoid reading the table, because that's
> where the tuple visibility information is stored. We've been making
> progress toward having some way to avoid reading the table for all
> except very recently written tuples, but we're not there yet (in any
> production version or in the 9.0 version to be released this
> summer).
More doubts on how indexes are used by PostgreSQL. It is mentioned that
table data blocks have data about tuple visibility and hence table scans
are always necessary. So how does PostgreSQL reduce the number of blocks
to be read by using indexes? Does this mean that indexes will have
references to all the 'possible' blocks which may contain the data one is
searching for, and then scans all those blocks and eliminates records
which should not be 'visible' to the query being executed? Do index data
get updated as and when data is committed and made 'visible' or is it
that index data get updated as soon as data is changed, before commit is
issued and rollback of transaction results in a rollback of the index data
changes too?
Regards,
Jayadevan

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