Re: BUG #19385: Normal SELECT generates an ineffecifient query plan compare to the prepared SELECT.

On 21/1/26 12:57, Andrei Lepikhov wrote:
> On 21/1/26 07:53, PG Bug reporting form wrote:
>> The following bug has been logged on the website:
>>
>> Bug reference:      19385
>> Logged by:          Chi Zhang
>> Email address:      798604270(at)qq(dot)com
>> PostgreSQL version: 18.1
>> Operating system:   ubuntu 24.04 with docker
>> Description:
>>
>> Hi,
>>
>> In the following test case, there are two equivalent simple SELECTs with
>> DISTINCT, however, the normal SELECT is slower than the prepared SELECT.
>> Given that prepared SELECT statements typically generate suboptimal query
>> plans due to the presence of unknown literals, one would expect prepared
>> SELECT to be slower than normal SELECT. However, in this example, the
>> prepared SELECT executes faster, suggesting that there may still be
>> room for
>> optimization in the query plan generation for normal SELECT.
> There is no equivalence.
>
> In the 'simple query' case with such a big LIMIT, you force the planner
> to choose a full-scan-optimal path. In the generic case, the planner
> uses 'magic constants' to provide some glue during planning, and LIMIT
> 10 is one of them, as we see from your example.
>
> I once proposed an alternative solution: use a 'reference' value to
> create a generic plan, as SQL Server does. If you're really interested
> in such optimisation, support it!
>

That is [1] the reference to the patch that allows you to choose
specific constants when building a generic plan.

[1]
https://www.postgresql.org/message-id/flat/19919494-92a8-4905-a250-6cf17b89f7c3(at)gmail(dot)com

--
regards, Andrei Lepikhov,
pgEdge