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.
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) DESC LIMIT 1;
Replace the PostgreSQL major version number here by the one you're running
(pick either 9.1 or 9.2):
apt-get install postgresql-9.2-prefix
psql ...
=# create extension prefix;
Replace the PostgreSQL major version number here by the one you're running
(pick either 91 or 92):
yum install prefix92
psql ...
=# create extension prefix;
Check $PATH, then
make
make install
psql <connection string> -c 'CREATE EXTENSION prefix;'
The make install step might have to be done as root, and the psql one
has to be done as a PostgreSQL superuser.
If you're running a very old PostgreSQL version such as 8.3, 8.4 of
9.0 you can still install this extension manually:
psql ...
=# \i /usr/share/postgresql/X.Y/extension/prefix--1.2.0.sql
You still have to edit this example to replace X.Y with your local
PostgreSQL version number, such as 8.4.
It's as easy as:
DROP TYPE prefix_range CASCADE;
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);
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.
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
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).
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.
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)
This TESTS.md page is more developper oriented material, but still of interest.