Re: stats_block_level

Tom Lane wrote:
> I wrote:
> > "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> >> Anybody got any objection to setting it on by default?
>
> > Yes. It's pure overhead with no redeeming social value except to those
> > who actually want to look at that sort of stat, and those who do can
> > certainly turn it on for themselves.
>
> On second thought ... the cost of incrementing n_blocks_read etc is
> certainly negligible. The overhead comes from sending messages to the
> collector, having the collector maintain table entries, writing those
> entries out to a file, etc. And AFAICS all that overhead is expended
> per table: if you touch a relation during a transaction, the ensuing
> costs are identical no matter whether you have stats_block_level or
> stats_row_level or both turned on.
>
> Furthermore, it seems pretty likely that a transaction that creates any
> row-level counts for a table will also create block-level counts, and
> vice versa.
>
> So maybe the *real* question to ask is why we have separate GUCs for
> stats_row_level and stats_block_level. Shouldn't we fold them into a
> single switch? It's hard to see what having just one of them turned on
> will save.

Agreed. Jan had a tendency to add more GUCs than needed "just in case",
but usually "case" never happened.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +