From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: pgsql: doc: add examples of creative use of unique expression indexes
Date: Thu, 21 May 2020 19:49:41 -0400
Message-ID: <20200521234941.GB30409@momjian.us> (raw)
In-Reply-To: <20200421003129.GA21241@momjian.us>
References: <E1ikvbp-0005jW-E9@gemulon.postgresql.org>
<2304.1586532634@sss.pgh.pa.us>
<20200410214821.GC24988@momjian.us>
<13031.1586560889@sss.pgh.pa.us>
<20200411001709.GD24988@momjian.us>
<20200411122532.GE24988@momjian.us>
<8755.1587414092@sss.pgh.pa.us>
<20200421003129.GA21241@momjian.us>
On Mon, Apr 20, 2020 at 08:31:29PM -0400, Bruce Momjian wrote:
> On Mon, Apr 20, 2020 at 04:21:32PM -0400, Tom Lane wrote:
> > [ sorry, I'd lost track of this thread ]
> >
> > Bruce Momjian <bruce@momjian.us> writes:
> > > I now remember that I wrote the first IS NULL in:
> > > CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL;
> > > in hope that if someone is looking for the null value in the column, the
> > > IS NULL would allow the index to be used to find it, while 1 or true
> > > would not.
> >
> > Well, that's not the case:
> >
> > regression=# create index tenk1_null_index on tenk1((1)) where ten is null;
> > CREATE INDEX
> > regression=# explain select * from tenk1 where ten is null;
> > QUERY PLAN
> > --------------------------------------------------------------------------------
> > Index Scan using tenk1_null_index on tenk1 (cost=0.12..8.14 rows=1 width=244)
> > (1 row)
> >
> > (Maybe it was true at some time in the past, but not any more.)
> >
> > Also, it complicates the example, and since you didn't explain the
> > reason for the complication, I think it's pretty confusing.
> >
> > But really I still don't see the need for these additional examples
> > at all. It's especially weird that what you want to do is have
> > some examples on that page have <example> markup and others not.
>
> OK, seems like only you and I care about this issue, which I take to
> mean that we should minimize what we are adding here. What the attached
> patch does is to remove the previous commit, and just add a sentence to
> the last example to mention the ability restrict a column to a single
> NULL.
Patch applied.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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: pgsql-committers@postgresql.org
Cc: bruce@momjian.us, tgl@sss.pgh.pa.us, pgsql-docs@postgresql.org
Subject: Re: pgsql: doc: add examples of creative use of unique expression indexes
In-Reply-To: <20200521234941.GB30409@momjian.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox