Re: HOT updates in index-less tables

On Sat, 2010-11-13 at 10:51 -0500, Tom Lane wrote:
> If a table has no indexes, we will always decide that any same-page
> update operation is a HOT update, since obviously it isn't modifying
> any indexed columns.  But is there any benefit to doing so? 

If we do the in-page "mini vacuum" even without HOT, then there should
be no benefit from index-less HOT updates. 

If we don't try the mini vacuum in this case, then some current
behaviuors could become much worse, say a table with one row and load of
updates.

> I don't
> see one offhand, and it has a downside: we're very likely to
> encounter broken HOT chains if an index is created later.  That leads
> to the sort of unexpected behavior exhibited here:
> http://archives.postgresql.org/pgsql-performance/2010-11/msg00216.php
> 
> I'm thinking maybe HeapSatisfiesHOTUpdate should be changed so that it
> always returns false if the relation has no indexes, which could be
> checked cheaply via relation->rd_rel->relhasindex.
> 
>             regards, tom lane
> 

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Hannu Krosing
PostgreSQL Infinite Scalability and Performance Consultant
PG Admin Book: http://www.2ndQuadrant.com/books/