On Wed, Jan 13, 2016 at 10:47 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Vladimir Sitnikov <sitnikov.vladimir@gmail.com> writes:
>> Note: I state that mixing "kinds" of bind values is a bad application
>> design anyway. In other words, application developer should understand
>> if a query is DWH-like (requires replans) or OLTP-like (does not
>> require replans). Agreed?
>
> No, not agreed. As was already pointed out upthread, such information
> is not available in many use-cases for the plancache.
>
> The real problem here IMO is inaccurate plan cost estimates, and that's
> not something that there is any easy fix for.
Not really. Even if the cost estimates for all of the plans tried are
perfectly accurate, you'll have only seen 5 values when you decide to
switch to a generic plan. If the 6th, 60th, 600th, or 6000th
execution uses a parameter where a custom plan would have been a big
win, you will blindly use the generic plan anyway and lose bigtime.
On the other hand, if first five plans are all equivalent to each
other and to the generic plan, then you've spent the cost of uselessly
replanning six times instead of just caching the first plan and being
done with it. I'm aware of an actual case where that extra
re-planning causes a serious performance problem, aggregated across
many queries and many backends.
This isn't the first complaint about this mechanism that we've gotten,
and it won't be the last. Way too many of our users are way more
aware than they should be that the threshold here is five rather than
any other number, which to me is a clear-cut sign that this needs to
be improved. How to improve it is a harder question. We lack the
ability to do any kind of sensitivity analysis on a plan, so we can't
know whether there are other parameter values that would have resulted
in a different plan, nor can we test whether a particular set of
parameter values would have changed the outcome.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company