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From: Tom Lane <[email protected]>
To: Jelte Fennema-Nio <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: psql: Greatly speed up "\d tablename" when not using regexes
Date: Wed, 10 Apr 2024 16:11:51 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAGECzQS7-564414-vV+XrPBj=4gHW6mPT8eObjq5=RrEseRcAw@mail.gmail.com>
References: <CAGECzQRqysy0eJMKR5he3gwtLrT87f9u5CQQua6B_XNwMnUtFA@mail.gmail.com>
	<[email protected]>
	<CAGECzQS7-564414-vV+XrPBj=4gHW6mPT8eObjq5=RrEseRcAw@mail.gmail.com>

Jelte Fennema-Nio <[email protected]> writes:
> On Wed, 10 Apr 2024 at 20:06, Tom Lane <[email protected]> wrote:
>> Really?  ISTM this argument is ignoring an optimization the backend
>> has understood for a long time.

> Interesting. I didn't know about that optimization. I can't check
> right now, but probably the COLLATE breaks that optimization.

Not for me.

# explain select * from pg_class where relname ~ '^(foo)$' collate "en_US";
                                         QUERY PLAN                                          
---------------------------------------------------------------------------------------------
 Index Scan using pg_class_relname_nsp_index on pg_class  (cost=0.27..8.29 rows=1 width=263)
   Index Cond: (relname = 'foo'::text)
   Filter: (relname ~ '^(foo)$'::text COLLATE "en_US")
(3 rows)

Also, using -E:

# \d foo
/******** QUERY *********/
SELECT c.oid,
  n.nspname,
  c.relname
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relname OPERATOR(pg_catalog.~) '^(foo)$' COLLATE pg_catalog.default
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 2, 3;
/************************/

# explain SELECT c.oid,
  n.nspname,
  c.relname
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relname OPERATOR(pg_catalog.~) '^(foo)$' COLLATE pg_catalog.default
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 2, 3;
                                                QUERY PLAN                                                
----------------------------------------------------------------------------------------------------------
 Sort  (cost=9.42..9.42 rows=1 width=132)
   Sort Key: n.nspname, c.relname
   ->  Nested Loop Left Join  (cost=0.27..9.41 rows=1 width=132)
         Join Filter: (n.oid = c.relnamespace)
         ->  Index Scan using pg_class_relname_nsp_index on pg_class c  (cost=0.27..8.32 rows=1 width=72)
               Index Cond: (relname = 'foo'::text)
               Filter: ((relname ~ '^(foo)$'::text) AND pg_table_is_visible(oid))
         ->  Seq Scan on pg_namespace n  (cost=0.00..1.04 rows=4 width=68)
(8 rows)


There may be an argument for psql to do what you suggest,
but so far it seems like duplicative complication.

If there's a case you can demonstrate where "\d foo" doesn't optimize
into an indexscan, we should look into exactly why that's happening,
because I think the cause must be more subtle than this.

			regards, tom lane

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