public inbox for [email protected]  

help / color / mirror / Atom feed

From: Tom Lane <[email protected]>
To: David G. Johnston <[email protected]>
Cc: Adrian Garcia Badaracco <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Wrapping a where clause to preserve rows with nulls
Date: Wed, 18 Dec 2024 23:38:08 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKFQuwa2jcBnsZxHmNn7_1MjcmOZx_fU93epBx9+orw3yMNgKw@mail.gmail.com>
References: <CAE8z92GN0-5J=4q6RSsPGk_OP-Y_EHVNgQimTWwcq+BE+HGqRQ@mail.gmail.com>
	<CAKFQuwa2jcBnsZxHmNn7_1MjcmOZx_fU93epBx9+orw3yMNgKw@mail.gmail.com>

"David G. Johnston" <[email protected]> writes:
> On Wednesday, December 18, 2024, Adrian Garcia Badaracco <
> [email protected]> wrote:
>> Is there any way to include the rows where the predicate evaluates to null
>> while still using an index?

> ... A btree index, which handles =, can’t be told to behave
> differently and so cannot fulfill your desire to produce rows where the
> stored value is null; it can only produce those equal to 5000.

Not in a single scan, no.  But multiple scans are possible:

regression=# create table t (id int unique);
CREATE TABLE
regression=# explain select * from t where id = 5000 or id is null;
                                  QUERY PLAN                                  
------------------------------------------------------------------------------
 Bitmap Heap Scan on t  (cost=8.42..18.98 rows=14 width=4)
   Recheck Cond: ((id IS NULL) OR (id = 5000))
   ->  BitmapOr  (cost=8.42..8.42 rows=14 width=0)
         ->  Bitmap Index Scan on t_id_key  (cost=0.00..4.25 rows=13 width=0)
               Index Cond: (id IS NULL)
         ->  Bitmap Index Scan on t_id_key  (cost=0.00..4.16 rows=1 width=0)
               Index Cond: (id = 5000)
(7 rows)

The OP was quite unclear about what semantics he wants for
multiple-variable WHERE clauses, but maybe something like this
would work:

WHERE (original-clause) OR x IS NULL OR y IS NULL OR ...

where each variable mentioned in original-clause is allowed
to also be NULL.  Or perhaps what is wanted is

WHERE (original-clause) OR (x IS NULL AND y IS NULL AND ...)

??

			regards, tom lane

view thread (4+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: Re: Wrapping a where clause to preserve rows with nulls
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox