cc young cc young - 1 year ago 173
SQL Question

How to create an index for elements of an array in PostgreSQL?

With this schema:

create table object (
obj_id serial primary key,
name varchar(80) not null unique,
description text,
tag_arr int[]

create table tag (
tag_id serial primary key,
label varchar(20) not null unique

An object may have any number of tags attached. Instead of an
object X tag
table, I wish to keep
s in an array so they can be easily fetched with the object record.

How do I create an index on
so that each element of
is an index?

That said, are there better ways to solve this problem?


This could be achieved with:

create table obj_x_tag(
obj_id references object,
tag_id references tag,
constraint obj_x_tag_pk primary key( obj_id, tag_id )

select obj_id, name, description, array_agg( tag_id )
from object o
join obj_x_tag x using( obj_id )
group by 1, 2;

But to me it makes more sense to simply keep the array of
s in a column and dispense with the cross table and

It was suggested to use PostgresQL SQL: Converting results to array. The problem, as noted, is that "this doesn't actually index individual array values, but instead indexes the entire array"

It was also suggested to use pg's
) index. The problem - to me - seems that the index is for the standard pg set-based array operators, not necessarily optimized for finding one element of an array, but rather where one array contains another, intersects with another - for me it's counter-intuitive that, size-wise and speed-wise, such a wide solution is correct for such a narrow problem. Also, the
extension seems limited to
, not covering
, limiting its usefulness.

Answer Source

You can create GIN indexes on any 1-dimensional array with standard Postgres.
Details ion the manual (last chapter).

While operating with integer arrays the additional supplied module intarray provide a lot more operators. Install it (once per database) with


You can create GIN or GIST indexes on integer arrays. There is an example in the manual.
CREATE EXTENSION requires PostgreSQL 9.1 or later. For older versions you need to run the supplied script.

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