On Tue, May 10, 2011 at 9:34 PM, Bruce Momjian <br...@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

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