The **rum** module provides access method to work with `RUM` index. It is based

on the `GIN` access methods code.

`GIN` index allows to perform fast full text search using `tsvector` and

`tsquery` types. But full text search with GIN index has several problems:

- Slow ranking. It is need position information about lexems to ranking. `GIN`

index doesn't store positions of lexems. So after index scan we need additional

heap scan to retreive lexems positions.

- There isn't phrase search with `GIN` index. This problem relates with previous

problem. It is need position information to perform phrase search.

- Slow ordering by timestamp. `GIN` index can't store some related information

in index with lexemes. So it is necessary to perform additional heap scan.

`RUM` solves this problems by storing additional information in posting tree.

For example, positional information of lexemes or timestamps. You can get an

idea of `RUM` by the following picture:


Drawback of `RUM` is that it has slower build and insert time than `GIN`.

It is because we need to store additional information besides keys and because

`RUM` uses generic WAL.

* PostgreSQL version is 9.6+.

**rum** module provides next operators.

| timestamp <=> timestamp | float8 | Returns distance between two timestamps.

| timestamp <=| timestamp | float8 | Returns distance only for left timestamps.

| timestamp |=> timestamp | float8 | Returns distance only for right timestamps.

**rum** provides next operator classes

For type: `tsvector`

This operator class stores `tsvector` lexemes with positional information. Supports

ordering by `<=>` operator and prefix search. There is the example.

FROM test_rum

WHERE a @@ to_tsquery('english', 'beautiful | place')

ORDER BY a <=> to_tsquery('english', 'beautiful | place');

FROM test_rum

WHERE a @@ to_tsquery('english', 'place | situation')

ORDER BY a <=> to_tsquery('english', 'place | situation');

For type: `tsvector`

This operator class stores hash of `tsvector` lexemes with positional information.

Supports ordering by `&lt;=&gt;` operator. But **doesn't** support prefix search.

For type: `timestamp`

Operator class provides fast search and ordering by timestamp fields. Supports

ordering by `&lt;=&gt;`, `&lt;=&#124;` and `&#124;=&gt;` operators. Can be used

with `rum_tsvector_timestamp_ops` operator class.

For type: `timestamptz`

Operator class provides fast search and ordering by timestamptz fields. Supports

ordering by `&lt;=&gt;`, `&lt;=&#124;` and `&#124;=&gt;` operators. Can be used

with `rum_tsvector_timestamptz_ops` operator class.

For type: `tsvector`

This operator class stores `tsvector` lexems with timestamp field. There is the example:

Let us assume we have the table:

CREATE TABLE tsts (id int, t tsvector, d timestamp);

\copy tsts from 'rum/data/tsts.data'

CREATE INDEX tsts_idx ON tsts USING rum (t rum_tsvector_timestamp_ops, d)

WITH (attach = 'd', to = 't');

Now we can execute the following queries:

EXPLAIN (costs off)

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;

QUERY PLAN

Limit

-> Index Scan using tsts_idx on tsts

Index Cond: (t @@ '''wr'' & ''qh'''::tsquery)

Order By: (d <=> 'Mon May 16 14:21:25 2016'::timestamp without time zone)

(4 rows)

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: `tsvector`

See comments for `rum_tsvector_timestamp_ops` operator class.

For type: `tsvector`

This operator class stores hash of `tsvector` lexems with timestamp field.

**Doesn't** support prefix search.

For type: `tsvector`

This operator class stores hash of `tsvector` lexems with timestamptz field.

**Doesn't** support prefix search.

For type: `tsquery`

Stores branches of query tree in additional information. For example we have the table:

CREATE TABLE query (q tsquery, tag text);

INSERT INTO query VALUES ('supernova & star', 'sn'),

('black', 'color'),

('big & bang & black & hole', 'bang'),

('spiral & galaxy', 'shape'),

('black & hole', 'color');

CREATE INDEX query_idx ON query USING rum(q);

Now we can execute the following fast query:

SELECT * FROM query

WHERE to_tsvector('black holes never exists before we think about them') @@ q;

q | tag

'black' | color

'black' & 'hole' | color

(2 rows)