On 1 May 2004 at 13:18, Jochem van Dieten wrote:
> Yes, really :-)
>
> When a tuple is inserted the visible-to-all flag is set to false.
> The effect of this is that every index scan that finds this tuple
> has to visit the heap to verify visibility. If it turns out the
> tuple is not only visible to the current transaction, but to all
> current transactions, the visible-to-all flag can be set to true.
> This is non-critical, because if it is set to false scans will
> not miss the tuple, they will just visit the heap to verify
> visibility.
>
> The moment the heap tuple is updated/deleted the visible-to-all
> flag needs to be set to false again in all indexes. This is
> critical, and the I/O and (dead)lock costs of unsetting the
> visible-to-all flag are unknown and might be big enough to ofset
> any advantage on the selects.
>
> But I believe that for applications with a "load, select, drop"
> usage pattern (warehouses, archives etc.) having this
> visible-to-all flag would be a clear winner.
>
> Jochem
>
If needs be this index maintenance could be set as a configuration
option. It is likely that database usage patterns are reasonably well
known for a particular installation. This option could be set on or off
dependant on typical transactions.
In my case with frequent large/complex selects and few very short
insert/updates I think it could be a big performance boost. If it works :-)
Regards,
Gary.
