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From: Tom Lane <[email protected]>
To: Peter J. Holzer <[email protected]>
Cc: [email protected]
Subject: Re: Request for Insights on ID Column Migration Approach
Date: Fri, 27 Sep 2024 18:37:35 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <CAE_cSysRBuzmGoRGknPX6Y5sws7qgh0iitYLfQj3g3iiSygzDw@mail.gmail.com>
	<[email protected]>

"Peter J. Holzer" <[email protected]> writes:
> As you can see, adding the primary key takes just as much time as
> creating the unique index. So it doesn't look like PostgreSQL is able to
> take advantage of the existing index (which makes sense since it still
> has to create a new index).

No, but you can attach an existing unique index as a primary key:

regression=# create table t1 (f1 int not null);
CREATE TABLE
regression=# create unique index t1_f1 on t1(f1);
CREATE INDEX
regression=# alter table t1 add primary key using index t1_f1;
ALTER TABLE

If you build the unique index concurrently, this needn't involve
much downtime.

			regards, tom lane

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