I would like to add a unique constraint to a DB field in one of my production databases. Unfortunately, the following statement is blocking:

ALTER TABLE package ADD CONSTRAINT unique_package_id UNIQUE (package_id);

Reading the relevant part from the PostgreSQL manual:

Note: Adding a constraint using an existing index can be helpful in situations where a new constraint needs to be added without blocking table updates for a long time. To do that, create the index using CREATE INDEX CONCURRENTLY, and then install it as an official constraint using this syntax. See the example below.

I tried:

CREATE UNIQUE INDEX CONCURRENTLY package_tmp_id_idx ON answer (package_id);

Followed by:

          unique_package_id UNIQUE (package) USING INDEX package_tmp_id_idx;

Which failed with:

ERROR:  syntax error at or near "package_tmp_id_idx"
LINE 1: ...T unique_package_id UNIQUE (answer_id) USING INDEX package_tmp_id_idx...

I've tried playing with the syntax, but could not get it to work.

What's the right syntax to create a UNIQUE constraint using an existing UNIQUE index?

  • I tend not to add a unique constraint at all. This way, it is somewhat easier to solve issues with bloated indexes on heavily used tables. – dezso Nov 1 '14 at 12:37

You don't specify the column for the unique constraint. That's not necessary because the column list is defined through the index:

     unique_package_id UNIQUE USING INDEX package_tmp_id_idx;

There is an example for that in the manual:

You need to scroll down to the end of the page

  • Thanks. This query works syntactically, but it does not return for a long time (more than 5 minutes on a table with 3,000 rows). Could it be waiting for other transactions to finish? – Adam Matan Nov 1 '14 at 6:46
  • 1
    @AdamMatan: yes the ALTER TABLE needs an exclusive lock (although for only a short time). If you have uncommitted transactions, then those will block this. Check for sessions that have the state "idle in transaction" in pg_stat_activity – a_horse_with_no_name Nov 1 '14 at 7:48
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    What is the purpose of adding the CONSTRAINT if the UNIQUE INDEX already exists? I tried testing around a bit and it seems that creating the UNIQUE INDEX was sufficient to ensure uniqueness - trying to add duplicate data resulted in duplicate key value violates unique constraint 'package_tmp_id_idx'. I assume I'm missing something though, otherwise it wouldn't be in the manual, so any information is appreciated! – dwanderson Mar 19 '18 at 21:18

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