On Tue, May 10, 2011 at 9:34 PM, Bruce Momjian <bruce@momjian.us> wrote:
> Robert Haas wrote:
>> So, what do we need in order to find our way to index-only scans?
>>
>> 1. The visibility map needs to be crash-safe. The basic idea of
>> index-only scans is that, instead of checking the heap to find out
>> whether each tuple is visible, we first check the visibility map. If
>> the visibility map bit is set, then we know all tuples on the page are
>> visible to all transactions, and therefore the tuple of interest is
>> visible to our transaction. Assuming that a significant number of
>> visibility map bits are set, this should enable us to avoid a fair
>> amount of I/O, especially on large tables, because the visibility map
>> is roughly 8000 times smaller than the heap, and therefore far more
>> practical to keep in cache. However, before we can rely on the
>
> FYI, because the visibility map is only one _bit_ per page, it is 8000 *
> 8 or 64k times smaller than the heap, e.g. one 8k page covers 64MB of
> heap pages. This is important because we rely on this compactness in
> hope that the WAL logging of this information will not be burdensome.
I accuse you of bad math.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
