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From: Laurenz Albe <[email protected]>
To: Yuri Kutsko <[email protected]>
To: [email protected]
Subject: Re: EXPLAIN(GENERIC_PLAN) failing for some queries
Date: Thu, 08 Jan 2026 09:27:16 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CABvjvhruTCeDfx6XMoG5xEY74tJHip1zxHny6TFXEruL4=sKsQ@mail.gmail.com>
References: <CABvjvhruTCeDfx6XMoG5xEY74tJHip1zxHny6TFXEruL4=sKsQ@mail.gmail.com>

On Wed, 2026-01-07 at 14:57 -0800, Yuri Kutsko wrote:
> I want to identify every query in my company’s database that potentially uses full table scans.
> To do this, I wrote a simple PL/pgSQL function that generates an execution plan for each query
> from pg_stat_statements and then searches the plan for the 'Seq Scan' pattern.
> Most queries contain parameters, so I use EXPLAIN (GENERIC_PLAN). This works for approximately
> 80% of queries, but it fails in the following scenarios:
> 
> 1. Planner cannot infer parameter types without explicit casts
> The following fails: EXPLAIN (GENERIC_PLAN) SELECT * FROM tbl WHERE id = $1 + $2;
> The following works: EXPLAIN (GENERIC_PLAN) SELECT * FROM tbl WHERE id = $1::integer + $2::integer;
> 
> 2. Parameter is used in EXTRACT
> The following fails: EXPLAIN (GENERIC_PLAN) SELECT * FROM tbl WHERE EXTRACT($1 FROM date_col) = 1;
> The following works: EXPLAIN (GENERIC_PLAN) SELECT * FROM tbl WHERE date_part($1, date_col) = 1;
> 
> 3. Parameter is used in type 'string' notation
> The following fails: EXPLAIN (GENERIC_PLAN) SELECT int $1;
> The following works: EXPLAIN (GENERIC_PLAN) SELECT $1::int;
> 
> I modified my procedure to account for these scenarios, but I am not sure whether there are other
> issues with EXPLAIN (GENERIC_PLAN) that I am not aware of.
> Problem #1 is mentioned in the EXPLAIN documentation
> (https://www.postgresql.org/docs/18/sql-explain.html) at the bottom of the page, but the other cases are not.
> My questions are:
> Are all of the issues described above expected behavior for EXPLAIN (GENERIC_PLAN)?

Yes.

Note that your cases #2 and #3 are illegal SQL, since you cannot use a parameter in these places,
only string literals.  I'd say that your problem is that you are using strings from pg_stat_statements,
which ignores the value of constants.  Replacing literals with placeholders can result in incorrect
SQL statements.

> Are there other known scenarios where EXPLAIN (GENERIC_PLAN) will fail?

It is expected to fail for all statements with syntax errors...

> Should the EXPLAIN documentation be updated to list scenarios in which EXPLAIN (GENERIC_PLAN) can fail?

I don't think that #2 and #3 deserve documentation, and I'd say it doesn't need to be documented
that EXPLAIN will fail for syntactically incorrect SQL.

Yours,
Laurenz Albe

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