GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
RedHat 6.2(Zoot) |
|
 |
create_index(l) |
 |
CREATE INDEX − Constructs a secondary index
CREATE [ UNIQUE
] INDEX index_name ON table
[ USING acc_name ] ( column [ ops_name]
[, ...] )
CREATE [ UNIQUE ] INDEX index_name ON table
[ USING acc_name ] ( func_name(
r">colle> [, ... ]) ops_name )
INPUTS
|
UNIQUE |
Causes the system to check for duplicate values in the table when the index is created (if data already exist) and each time data is added. Attempts to insert or update non-duplicate data will generate an error. |
index_name
The name of the index to be created.
|
table |
The name of the table to be indexed. |
acc_name
the name of the access method which is to be used for the index. The default access method is BTREE. Postgres provides three access methods for secondary indexes:
|
BTREE |
an implementation of the Lehman-Yao high-concurrency btrees. | ||
|
RTREE |
implements standard rtrees using Guttman’s quadratic split algorithm. | ||
|
HASH |
an implementation of Litwin’s linear hashing. | ||
|
column |
The name of a column of the table.
ops_name
An associated operator class. See below for details.
func_name
A user-defined function, which returns a value that can be indexed.
OUTPUTS
|
CREATE |
The message returned if the index is successfully created. |
ERROR: Cannot create index: ’index_name’ already exists.
This error occurs if it is impossible to create the index.
CREATE INDEX constructs an index index_name on the specified table.
Tip: Indexes are primarily used to enhance database performance. But inappropriate use will result in slower performance.
In the first syntax shown above, the key fields for the index are specified as column names; a column may also have an associated operator class. An operator class is used to specify the operators to be used for a particular index. For example, a btree index on four-byte integers would use the int4_ops class; this operator class includes comparison functions for four-byte integers. The default operator class is the appropriate operator class for that field type.
In the second syntax shown above, an index is defined on the result of a user-defined function func_name applied to one or more attributes of a single class. These functional indices can be used to obtain fast access to data based on operators that would normally require some transformation to apply them to the base data.
Postgres provides btree, rtree and hash access methods for secondary indices. The btree access method is an implementation of the Lehman-Yao high-concurrency btrees. The rtree access method implements standard rtrees using Guttman’s quadratic split algorithm. The hash access method is an implementation of Litwin’s linear hashing. We mention the algorithms used solely to indicate that all of these access methods are fully dynamic and do not have to be optimized periodically (as is the case with, for example, static hash access methods).
NOTES
The Postgres query optimizer will consider using btree
indices in a scan whenever an indexed attribute is involved
in a comparison using one of: <, <=, =, >=,
>
Both box classes support indices on the box data type in Postgres. The difference between them is that bigbox_ops scales box coordinates down, to avoid floating point exceptions from doing multiplication, addition, and subtraction on very large floating-point coordinates. If the field on which your rectangles lie is about 20,000 units square or larger, you should use bigbox_ops. The poly_ops operator class supports rtree indices on polygon data.
The Postgres query optimizer will consider using an rtree index whenever an indexed attribute is involved in a comparison using one of: <<, &<, &>, >>, @, ~=, &&
The Postgres query optimizer will consider using a hash index whenever an indexed attribute is involved in a comparison using the = operator.
Currently, only the BTREE access method supports multi-column indexes. Up to 7 keys may be specified.
Use drop_index(l) to remove an index.
The int24_ops operator class is useful for constructing indices on int2 data, and doing comparisons against int4 data in query qualifications. Similarly, int42_ops support indices on int4 data that is to be compared against int2 data in queries.
The following select list returns all ops_names:
SELECT
am.amname AS acc_name,
opc.opcname AS ops_name,
opr.oprname AS ops_comp
FROM pg_am am, pg_amop amop,
pg_opclass opc, pg_operator opr
WHERE amop.amopid = am.oid AND
amop.amopclaid = opc.oid AND
amop.amopopr = opr.oid
ORDER BY acc_name, ops_name, ops_comp
To create a btree index on the field title in the table films:
CREATE UNIQUE
INDEX title_idx
ON films (title);
SQL92
CREATE INDEX is a Postgres language extension.
There is no CREATE INDEX command in SQL92.
|
create_index(l) | |