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

on the GIN access methods code.

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.

Before build and install**rum** you should ensure following:

* PostgreSQL version is 9.6.

* PostgreSQL version is 9.6+.

Typical installation procedure may look like this:

The**rum** module provides the access method**rum** and the operator class

**rum_tsvector_ops**.

The module providesnew operators.

**rum** module providesnext operators.

| Operator | Returns | Description

| -------------------- | ------- | ----------------------------------------------

| tsvector<=> tsquery | float4 | Returns distance between tsvector and tsquery.

| 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.

Let us assume we have the table:

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','beautiful | place')

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

t | rank

The situation is most beautiful |0.0303964

(3 rows)

FROM test_rum

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

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

FROM test_rum

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

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

t | rank

The situation is most beautiful |0.0303964

It lookslike a beautiful place |0.0303964

(2 rows)

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:

CREATETABLEtsts (idint, t tsvector, dtimestamp);

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

CREATEINDEXtsts_idxON 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 tstsWHERE t @@'wr&qh'ORDER BY d<=>'2016-05-16 14:21:25'LIMIT5;

QUERY PLAN

-> Index Scan using tsts_idxon 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 tstsWHERE t @@'wr&qh'ORDER BY d<=>'2016-05-16 14:21:25'LIMIT5;

id | d | ?column?

355 | Mon May1614:21:22.3267242016 |2.673276

354 | Mon May1613:21:22.3267242016 |3602.673276

371 | Tue May1706:21:22.3267242016 |57597.326724

406 | Wed May1817:21:22.3267242016 |183597.326724

415 | Thu May1902:21:22.3267242016 |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:

CREATETABLEquery (q tsquery, tagtext);

INSERT INTO queryVALUES ('supernova & star','sn'),

('black','color'),

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

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

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

CREATEINDEXquery_idxON 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)

Alexander Korotkov <a.korotkov@postgrespro.ru> Postgres Professional Ltd., Russia