Pavan Deolasee wrote: > On Thu, Apr 16, 2015 at 2:47 PM, Greg Stark <st...@mit.edu> wrote:
> > From a holistic point of view the question is how many times is a given > > hit chain going to need to be followed before it's pruned. Or to put it > > another way, how expensive is creating a hot chain. Does it cause a single > > prune? a fixed number of chain readers followed by a prune? Does the amount > > of work depend on the workload or is it consistent? > > IMO the size or traversal of the HOT chain is not that expensive compared > to the cost of either pruning too frequently, which generates WAL as well > as makes buffers dirty. OTOH cost of less frequent pruning could also be > very high. It can cause severe table bloat which may just stay for a very > long time. Even if dead space is recovered within a page, truncating a > bloated heap is not always possible. I think you're failing to consider that in the patch there is a distinction between read-only page accesses and page updates. During a page update, HOT cleanup is always done even with the patch, so there won't be any additional bloat that would not be there without the patch. It's only the read-only accesses to the patch that skip the HOT pruning. Of course, as Greg says there will be some additional scans of the HOT chain by read-only processes. -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
