This module is written by Dimitri Fontaine with a great amount of help from RhodiumToad (formely known as AndrewSN), who was the one advising for a GiST opclass to solve the prefix matching problem.

Presentation

Prefix matching is both very common and important in telephony applications, where call routing and costs depend on matching caller/callee phone number to an operator prefix.

Let's say the prefixes table is called prefixes, a typical query will try to match a phone number to the longest prefix in the table:

  SELECT *
    FROM prefixes
   WHERE prefix @> '0123456789'
ORDER BY length(prefix)
   LIMIT 1;

Installation

Check $PATH, then

make
make install
psql <connection string> -f prefix.sql <database>

The make install step might have to be done as root, and the psql one has to be done as a PostgreSQL superuser.

Uninstall

It's as easy as:

DROP TYPE prefix_range CASCADE;

Usage

Table and index creation

create table prefixes (
       prefix    prefix_range primary key,
       name      text not null,
       shortname text,
       status    char default 'S',
       check( status in ('S', 'R') )
);
comment on column prefixes.status is 'S:   - R: reserved';
\copy prefixes from 'prefixes.fr.csv' with delimiter ; csv quote '"'
create index idx_prefix on prefixes using gist(prefix gist_prefix_range_ops);

Simple tests:

dim=# select '123'::prefix_range @> '123456';
 ?column?
----------
 t
(1 row)

Please note earlier versions of prefix didn't use any restriction nor join selectivity estimator functions for the @> operator, so you had to set enable_seqscan to off to use the index. That's no more the case, the @> operator uses contsel and contjoinsel and the planner is able to figure out by itself when to use the index or not.

If you don't understand previous mention, ignore it and use a more recent version of prefix than 0.2.

Forcing seqcan

dim=# select * from ranges where prefix @> '0146640123';
 prefix |      name      | shortname | state
--------+----------------+-----------+-------
 0146[] | FRANCE TELECOM | FRTE      | S
(1 row)
Time: 4,071 ms
dim=# select * from ranges where prefix @> '0100091234';
  prefix  |    name    | shortname | state
----------+------------+-----------+-------
 010009[] | LONG PHONE | LGPH      | S
(1 row)
Time: 4,110 ms

Using the Index

dim=# select * from ranges where prefix @> '0146640123';
 prefix |      name      | shortname | state
--------+----------------+-----------+-------
 0146[] | FRANCE TELECOM | FRTE      | S
(1 row)
Time: 1,036 ms
dim=# select * from ranges where prefix @> '0100091234';
  prefix  |    name    | shortname | state
----------+------------+-----------+-------
 010009[] | LONG PHONE | LGPH      | S
(1 row)
Time: 0,771 ms

As of version 1.0, prefix_range GiST index supports also queries using the <@, && and = operators (see below).

creating prefix_range, cast to and from text

There's a constructor function:

prefix=# select prefix_range('123');
 prefix_range
--------------
 123
(1 row)
prefix=# select prefix_range('123', '4', '5');
 prefix_range
--------------
 123[4-5]
(1 row)

Casting from unknown literal or text is as easy as usual:

prefix=# select '123'::prefix_range;
 prefix_range
--------------
 123
(1 row)
prefix=# select x, x::prefix_range from (values('123'), ('123[4-5]'), ('[2-3]')) as t(x);
    x     |    x
----------+----------
 123      | 123
 123[4-5] | 123[4-5]
 [2-3]    | [2-3]
(3 rows)

And two casts are provided:

CREATE CAST (text as prefix_range) WITH FUNCTION prefix_range(text) AS IMPLICIT;
CREATE CAST (prefix_range as text) WITH FUNCTION text(prefix_range);

Which means you can use text expression in a context where a prefix_range is expected and it'll get implicit casting, but prefix_range to text has to be asked explicitely, so that you don't get strange behavior.

Provided operators

The prefix module is all about indexing prefix lookups, but in order to be able to do this with some efficiency, it has to know a lot about prefix ranges, such as basic comparing, containment, union, intersection and overlapping.

The operators <=, <, =, <>, >= and > are read as usual, @> is read contains, <@ is read is contained by, && is read overlaps, and | is "union" and "&" is "intersect".

prefix=# select a, b,
  a <= b as "<=", a < b as "<", a = b as "=", a <> b as "<>", a >= b as ">=", a > b as ">",
  a @> b as "@>", a <@ b as "<@", a && b as "&&"
from  (select a::prefix_range, b::prefix_range
         from (values('123', '123'),
                     ('123', '124'),
                     ('123', '123[4-5]'),
                     ('123[4-5]', '123[2-7]'),
                     ('123', '[2-3]')) as t(a, b)
      ) as x;
    a     |    b     | <= | < | = | <> | >= | > | @> | <@ | &&
----------+----------+----+---+---+----+----+---+----+----+----
 123      | 123      | t  | f | t | f  | t  | f | t  | t  | t
 123      | 124      | t  | t | f | t  | f  | f | f  | f  | f
 123      | 123[4-5] | t  | t | f | t  | f  | f | t  | f  | t
 123[4-5] | 123[2-7] | f  | f | f | t  | t  | t | f  | t  | t
 123      | [2-3]    | t  | t | f | t  | f  | f | f  | f  | f
(5 rows)
prefix=# select a, b, a | b as union, a & b as intersect
  from  (select a::prefix_range, b::prefix_range
           from (values('123', '123'),
                       ('123', '124'),
                       ('123', '123[4-5]'),
                       ('123[4-5]', '123[2-7]'),
                       ('123', '[2-3]')) as t(a, b)
        ) as x;
    a     |    b     |  union   | intersect
----------+----------+----------+-----------
 123      | 123      | 123      | 123
 123      | 124      | 12[3-4]  |
 123      | 123[4-5] | 123      | 123
 123[4-5] | 123[2-7] | 123[2-7] | 123[4-5]
 123      | [2-3]    | [1-3]    |
(5 rows)

See also

This Tests page is more developper oriented material, but still of interest.