On 25th of January, Tom Lane committed patch:
Implement ALTER TABLE ADD UNIQUE/PRIMARY KEY USING INDEX. This feature allows a UNIQUE OR pkey CONSTRAINT TO be created USING an already-existing UNIQUE INDEX. While the CONSTRAINT isn't very functionally different from the bare index, it's nice TO be able TO do that FOR documentation purposes. The main advantage OVER just issuing a plain ALTER TABLE ADD UNIQUE/PRIMARY KEY IS that the INDEX can be created WITH CREATE INDEX CONCURRENTLY, so that there IS NOT a long INTERVAL WHERE the TABLE IS locked against updates. ON the way, refactor SOME OF the code IN DefineIndex() AND index_create() so that we don't have to pass through those functions in order to create the index constraint's catalog entries. Also, IN parse_utilcmd.c, pass around the ParseState pointer IN struct CreateStmtContext TO save ON notation, AND ADD error location pointers TO SOME error reports that didn't have one before. Gurjeet Singh, reviewed by Steve Singer and Tom Lane
I, personally, am extremely happy about it.
First, let's see the syntax:
$ CREATE TABLE test ( i int4 NOT NULL, z text ); CREATE TABLE $ CREATE UNIQUE INDEX test_idx ON test (i); CREATE INDEX
Above creates simple table, with unique index, but which is not primary key or unique constraint. But now I can:
$ ALTER TABLE test ADD PRIMARY KEY USING INDEX test_idx; ALTER TABLE $ \d test TABLE "public.test" COLUMN | TYPE | Modifiers --------+---------+----------- i | INTEGER | NOT NULL z | text | Indexes: "test_idx" PRIMARY KEY, btree (i)
$ ALTER TABLE test ADD UNIQUE USING INDEX test_idx; ALTER TABLE $ \d test TABLE "public.test" COLUMN | TYPE | Modifiers --------+---------+----------- i | INTEGER | NOT NULL z | text | Indexes: "test_idx" UNIQUE CONSTRAINT, btree (i)
Why is that so cool?
Indexes do accumulate bloat. It happens. And, while you can do “REINDEX" – it's locking operation.
But – you can create new index concurrently, with the same specification as bloated indexes, but under different name and then drop old index. This creation can be done CONCURRENTLY which makes it non-locking.
Which is great, but you can't really do it with PRIMARY KEYS or UNIQUE CONSTRAINTS, as you can't drop index (old, bloated) while having constraint. The best that we could do so far, when we had bloated primary key index, was to create new unique index, concurrently, and drop primary key.
This approach leads to problems with tools which use PRIMARY KEYs.
Luckily now – you can add primary key back to table, using recreated, bloat-free index. Or even – add primary key to table “on the fly", with very short lock only.
Sweet. Doesn't solve all problems – for example foreign keys pointing to primary key, but it's a good start.