On 4 August 2016 at 00:56, Bruce Momjian <bruce@momjian.us> wrote:
> On Wed, Aug 3, 2016 at 07:28:52PM -0400, Bruce Momjian wrote:
>> With LITE, you can avoid the creation of duplicate-value index entries
>> for indexes without changed column values by using a bitmap in place of
>> the tid item number (16 bits). It can't remove dead tids.
>
> How would you handle the case where there are two LITE index entries
> pointing to two different update chains on the same page?
> When you
> search the page for the first heap chain, could the second index entry
> find the same chain. How would you know which index entry is which
> chain?
It's easiest to understand this is by imagining each LITE pointer
pointing to a whole page. The chains aren't followed during the scan,
individual heap tuple versions are treated as they would be by a seq
scan.
That is more expensive than we might like, so the bitmap/linepointer
thing is just an extra tweak to avoid scanning the whole block. The
bitmap refers to ranges of linepointers, not chains. i.e. 0-15, 16-31,
32-47 etc
> Would you only add a LITE index entry when there isn't an
> existing index entry for the same values and heap page? That seems
> quite complicated.
The insertion algorithm is described. Doesn't seem complicated to me.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services