Horton

Re: On disable_cost

Laurenz Albe <laurenz.albe@cybertec.at>

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From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Jim Finnerty <jfinnert@amazon.com>, pgsql-hackers@postgresql.org
Date: 2019-12-11T06:23:51Z
Lists: pgsql-hackers 
On Tue, 2019-12-10 at 15:50 -0700, Jim Finnerty wrote:
> As a proof of concept, I hacked around a bit today to re-purpose one of the
> bits of the Cost structure to mean "is_disabled" so that we can distinguish
> 'disabled' from 'non-disabled' paths without making the Cost structure any
> bigger.  In fact, it's still a valid double.  The obvious choice would have
> been to re-purpose the sign bit, but I've had occasion to exploit negative
> costs before so for this POC I used the high-order bit of the fractional
> bits of the double.  (see Wikipedia for double precision floating point for
> the layout).
> 
> The idea is to set a special bit when disable_cost is added to a cost. 
> Dedicating multiple bits instead of just 1 would be easily done, but as it
> is we can accumulate many disable_costs without overflowing, so just
> comparing the cost suffices.

Doesn't that rely on a specific implementation of double precision (IEEE)?
I thought that we don't want to limit ourselves to platforms with IEEE floats.

Yours,
Laurenz Albe

Commits

  1. Doc: add detail about EXPLAIN's "Disabled" property

  2. Adjust EXPLAIN's output for disabled nodes

  3. Fix order of parameters in a cost_sort call

  4. Show number of disabled nodes in EXPLAIN ANALYZE output.

  5. Treat number of disabled nodes in a path as a separate cost metric.

  6. Remove grotty use of disable_cost for TID scan plans.