RUM: improved inverted index for full-text search based on GIN index

RUM is an extension which adds a RUM index to Postgresql.

RUM index is based on GIN that stores additional per-entry information in a posting tree. For example, positional information of lexemes or timestamps. In comparison to GIN it can use this information to make faster index-only scans for:

• Phrase search
• Text search with ranking by text distance operator
• Text SELECTs with ordering by some non-indexed additional column e.g. by timestamp.

RUM works best in scenarios when the possible keys are highly repeatable. I.e. all texts are composed of a limited amount of words, so per-lexeme indexing gives significant speed-up in searching texts containing word combinations or phrases.

Main operators for ordering are:

tsvector <=> tsquery | float4 | Distance between tsvector and tsquery. value <=> value | float8 | Distance between two values.

Where value is timestamp, timestamptz, int2, int4, int8, float4, float8, money and oid

Usage#

Enable the extension#

You can get started with rum by enabling the extension in your Supabase dashboard.

Syntax#

For type: tsvector

To understand the following you may need first to see Official PostgreSQL documentation on text search

rum_tsvector_ops

CREATE TABLE test_rum(t text, a tsvector);

CREATE TRIGGER tsvectorupdate
BEFORE UPDATE OR INSERT ON test_rum
FOR EACH ROW EXECUTE PROCEDURE tsvector_update_trigger('a', 'pg_catalog.english', 't');

INSERT INTO test_rum(t) VALUES ('The situation is most beautiful');
INSERT INTO test_rum(t) VALUES ('It is a beautiful');
INSERT INTO test_rum(t) VALUES ('It looks like a beautiful place');

CREATE INDEX rumidx ON test_rum USING rum (a rum_tsvector_ops);

And we can execute tsvector selects with ordering by text distance operator:

SELECT t, a `<=>` to_tsquery('english', 'beautiful | place') AS rank
    FROM test_rum
    WHERE a @@ to_tsquery('english', 'beautiful | place')
    ORDER BY a `<=>` to_tsquery('english', 'beautiful | place');
                t                |  rank
---------------------------------+---------
 It looks like a beautiful place | 8.22467
 The situation is most beautiful | 16.4493
 It is a beautiful               | 16.4493
(3 rows)

rum_tsvector_addon_ops

CREATE TABLE tsts (id int, t tsvector, d timestamp);
CREATE INDEX tsts_idx ON tsts USING rum (t rum_tsvector_addon_ops, d)
    WITH (attach = 'd', to = 't');

Now we can execute the selects with ordering distance operator on attached column:

SELECT id, d, d `<=>` '2016-05-16 14:21:25' FROM tsts WHERE t @@ 'wr&qh' ORDER BY d `<=>` '2016-05-16 14:21:25' LIMIT 5;
 id  |                d                |   ?column?
-----+---------------------------------+---------------
 355 | Mon May 16 14:21:22.326724 2016 |      2.673276
 354 | Mon May 16 13:21:22.326724 2016 |   3602.673276
 371 | Tue May 17 06:21:22.326724 2016 |  57597.326724
 406 | Wed May 18 17:21:22.326724 2016 | 183597.326724
 415 | Thu May 19 02:21:22.326724 2016 | 215997.326724
(5 rows)

For type: anyarray

rum_anyarray_ops

This operator class stores anyarray elements with length of the array. It supports operators &&, @>, <@, =, % operators. It also supports ordering by <=> operator.

CREATE TABLE test_array (i int2[]);
INSERT INTO test_array VALUES ('{}'), ('{0}'), ('{1,2,3,4}'), ('{1,2,3}'), ('{1,2}'), ('{1}');
CREATE INDEX idx_array ON test_array USING rum (i rum_anyarray_ops);

Now we can execute the query using index scan:

SELECT * FROM test_array WHERE i && '{1}' ORDER BY i `<=>` '{1}' ASC;
     i
-----------
 {1}
 {1,2}
 {1,2,3}
 {1,2,3,4}
(4 rows)

rum_anyarray_addon_ops

The does the same with anyarray index as rum_tsvector_addon_ops i.e. allows to order select results using distance operator by attached column.

Limitations#

RUM has slower build and insert times than GIN due to:

1. It is bigger due to the additional attributes stored in the index.
2. It uses generic WAL records.

Resources#

• Official RUM documentation