warehouse_db=# create table item (item_id integer not null,item_name text,item_price numeric,item_data text);
CREATE TABLE
warehouse_db=# create index item_idx on item(item_id);
CREATE INDEX
warehouse_db=# di item_idx
List of relations
Schema | Name | Type | Owner | Table
--------+----------+-------+----------+-------
public | item_idx | index | postgres | item
(1 row)
warehouse_db=# h create index
Command: CREATE INDEX
Description: define a new index
Syntax:
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ name ] ON table_name [ USING method ]
( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ WITH ( storage_parameter = value [, ... ] ) ]
[ TABLESPACE tablespace_name ]
[ WHERE predicate ]
warehouse_db=# di
List of relations
Schema | Name | Type | Owner | Table
--------+--------------------------------+-------+----------+---------------
public | PRIM_KEY | index | postgres | warehouse_tb1
public | PRM_KEY | index | postgres | history
public | cards_card_id_owner_number_key | index | postgres | cards
public | item_idx | index | postgres | item
public | item_item_id_idx | index | postgres | item
public | movies_title_copies_excl | index | postgres | movies
public | tools_pkey | index | postgres | tools
(7 rows)
warehouse_db=# di item_item_id_idx
List of relations
Schema | Name | Type | Owner | Table
--------+------------------+-------+----------+-------
public | item_item_id_idx | index | postgres | item
(1 row)
warehouse_db=# drop index item_item_id_idx ;
DROP INDEX
http://www.postgresql.org/docs/9.4/static/sql-createindex.html.
types of index
single index
create index index_name on table_name(column);
warehouse_db=# create index item_single_index on item (item_id);
CREATE INDEX
warehouse_db=# create index item_multi_index on item (item_id,item_price);
CREATE INDEX
partial index:creating an index on the subset of the table
CREATE INDEX index_name ON table_name (column) WHERE (condition);
warehouse_db=# CREATE INDEX item_partial_index ON item (item_id) WHERE
(item_id < 106);
warehouse_db=# d item;
Table "item"
Column | Type | Modifiers
------------+--------------------+-----------
item_id | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_index" btree (item_id)
"item_multi_index" btree (item_id, item_price)
"item_partial_index" btree (item_id) WHERE item_id < 106
the unique index
a unique index can be created on any column;it not only creates an index ,but also
enforces uniqueness of the column.
warehouse_db=# CREATE UNIQUE INDEX item_unique_idx ON item (item_id);
CREATE INDEX
Time: 485.644 ms
warehouse_db=# d item_unique_idx;
List of relations
Schema | Name | Type | Owner | Table
--------+-----------------+-------+----------+-------
public | item_unique_idx | index | postgres | item
(1 row)
we can create a unique index explicitly using the CREATE UNIQUE
INDEX command and that it can be created implicitly by declaring a primary key on a table.
warehouse_db=# create table item
warehouse_db-# (
warehouse_db(# item_unique integer primary key,
warehouse_db(# item_name text,
warehouse_db(# item_price numeric,
warehouse_db(# item_data text);
CREATE TABLE
warehouse_db=# d item
Table "public.item"
Column | Type | Modifiers
-------------+---------+-----------
item_unique | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (item_unique), tablespace "tbs_yl"
Tablespace: "tbs_yl"
Here is an example of an implicit creation of a unique index by defining unique
constraints:
warehouse_db=# alter table item add constraint primary_key unique(item_unique);
ALTER TABLE
warehouse_db=# d item;
Table "public.item"
Column | Type | Modifiers
-------------+---------+-----------
item_unique | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (item_unique), tablespace "tbs_yl"
"primary_key" UNIQUE CONSTRAINT, btree (item_unique), tablespace "tbs_yl"
Tablespace: "tbs_yl"
The ALTER command adds a unique constraint to the item_id column and can be used as
the primary key.
explicitlycreate a unique index explicitly using the already discussed CREATE INDEX
command as follows:
warehouse_db=# create table item(
warehouse_db(# item_id integer primary key,
warehouse_db(# item_name text,
warehouse_db(# item_price numeric,
warehouse_db(# item_data text);
CREATE TABLE
warehouse_db=#
warehouse_db=# d item
Table "public.item"
Column | Type | Modifiers
------------+---------+-----------
item_id | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (item_id), tablespace "tbs_yl"
Tablespace: "tbs_yl"
warehouse_db=# create unique index idx_unique_id on item(item_id);
CREATE INDEX
warehouse_db=# d item;
Table "public.item"
Column | Type | Modifiers
------------+---------+-----------
item_id | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (item_id), tablespace "tbs_yl"
"idx_unique_id" UNIQUE, btree (item_id), tablespace "tbs_yl"
Tablespace: "tbs_yl"
warehouse_db=# insert into item values (1,'boxing',200,'glaves');
INSERT 0 1
warehouse_db=# insert into item values (1,'hockey',300,'shoes');
ERROR: duplicate key value violates unique constraint "item_pkey"
DETAIL: Key (item_id)=(1) already exists.
warehouse_db=# insert into item values (2,'hockey',300,'shoes');
INSERT 0 1
the expression index
For example, if we want to search for a case-insensitive item name,
then the normal way of doing this is as follows:
warehouse_db=# SELECT * FROM item WHERE UPPER(item_name) LIKE 'COFFEE';
The preceding query will scan each row or table and convert item_name to uppercase and
compare it with COFFEE; this is really expensive. The following is the command to create
an expression index on the item_name column:
warehouse_db=# create index item_expression_index on item(upper(item_name));
CREATE INDEX
warehouse_db=# d item;
Table "public.item"
Column | Type | Modifiers
------------+---------+-----------
item_id | integer | not null
item_name | text |
item_price | numeric |
item_data | text |
Indexes:
"item_pkey" PRIMARY KEY, btree (item_id), tablespace "tbs_yl"
"idx_unique_id" UNIQUE, btree (item_id), tablespace "tbs_yl"
"item_expression_index" btree (upper(item_name)), tablespace "tbs_yl"
Tablespace: "tbs_yl"
the implicit index
An index that is created automatically by the database is called an implicit index. The
primary key or unique constraint implicitly creates an index on that column.
index
creation on a table is a very expensive operation, and on a sizeably huge table, it can take
hours to build an index. This can cause difficulty in regards to performing any write
operations. To solve this issue, PostgreSQL has the concurrent index, which is useful
when you need to add indexes in a live database.
The syntax of a concurrent index is as follows:
CREATE INDEX CONCURRENTLY index_name ON table_name using btree(column);
The concurrent index is slower than the normal index because it completes index building
in two parts. This can be explained with the help of the following example:
warehouse_db=# CREATE INDEX idx_id ON item (item_id);
Time: 8265.473 ms
Time taken in creating a concurrent index idx_id using CREATE INDEX CONCURRENTLY:
warehouse_db=# CREATE INDEX CONCURRENTLY idx_id ON item (item_id);
Time: 51887.942 ms
index types
PostgreSQL supports the B-tree, hash, GiST, and GIN index methods. The index method
or type can be selected via the USING method. Different types of indexes have different
purposes, for example, the B-tree index is effectively used when a query involves the
range and equality operators and the hash index is effectively used when the equality
operator is used in a query.
Here is a simple example of how to use the index types:
warehouse_db=# CREATE INDEX index_name ON table_name USING btree(column);
the B-tree index
The B-tree index is effectively used when a query involves the equality operator (=) and
range operators (<, <=, >, >=, BETWEEN, and IN).
the hash index
Hash indexes are utilized when a query involves simple equivalent operators only. Here,
we create a hash index on the item table. You can see in the following example that the
planner chooses the hash index in the case of an equivalent operator and does not utilize
the hash index in the case of the range operator:
the hash index is the best for queries that have equivalent operators in the
WHERE clause. This can be explained with the help of the following example:
warehouse_db=# EXPLAIN SELECT COUNT(*) FROM item WHERE item_id = 100;
QUERY PLAN
------------------------------------------------------------------
Aggregate (cost=8.02..8.03 rows=1 width=0)
-> Index Scan using item_hash_index on item (cost=0.00..8.02 rows=1
width=0)
Index Cond: (item_id = 100)
(3 rows)
The hash index method is not suitable for range operators, so the planner will not select a
hash index for range queries:
warehouse_db=# EXPLAIN SELECT COUNT(*) FROM item WHERE item_id > 100;
QUERY PLAN
------------------------------------------------------------------
Aggregate (cost=25258.75..25258.76 rows=1 width=0)
-> Seq Scan on item (cost=0.00..22759.00 rows=999900 width=0)
Filter: (item_id > 100)
(3 rows)
To get the size of a table and an index, we can use the following:
SELECT pg_relation_size('table_name')
AS table_size,pg_relation_size('index_name') index_size
FROM pg_tables WHERE table_name like 'table_name';
the GiST index
The Generalized Search Tree (GiST) index provides the possibility to create custom
data types with indexed access methods. It additionally provides an extensive set of
queries.
It can be utilized for operations beyond equivalent and range comparisons. The GiST
index is lossy, which means that it can create incorrect matches.
The syntax of the GiST index is as follows:
warehouse_db=# CREATE INDEX index_name ON table_name USING
gist(column_name);
the GIN index
“GIN stands for Generalized Inverted Index. GIN is designed for handling cases
where the items to be indexed are composite values, and the queries to be handled by
the index need to search for element values that appear within the composite items.
For example, the items could be documents, and the queries could be searches for
documents containing specific words”
Here is the syntax for the creation of a GIN index:
warehouse_db=# CREATE INDEX index_name ON table_name USING
gin(column_name);
The GIN index requires three times more space than GiST, but is three times faster than
GiST.
warehouse_db=# create extension pg_trgm ;
CREATE EXTENSION
Time: 117.645 ms
warehouse_db=# create table words(lineno int,simple_words text,special_words text);
CREATE TABLE
Time: 32.913 ms
warehouse_db=# insert into words values (generate_series(1,2000000),md5(random()::text),md5(random()::text));
INSERT 0 2000000
Time: 18268.619 ms
warehouse_db=# select count(*) from words where simple_words like '%a31%' and special_words like '%a31%';
count
-------
115
(1 row)
Time: 669.342 ms
warehouse_db=# create index words_idx on words (simple_words,special_words);
CREATE INDEX
Time: 22136.229 ms
warehouse_db=# select count(*) from words where simple_words like '%a31%' and special_words like '%a31%';
count
-------
115
(1 row)
Time: 658.988 ms
warehouse_db=# create index words_idx on words using gin(simple_words gin_trgm_ops,special_words gin_trgm_ops);
ERROR: relation "words_idx" already exists
Time: 0.952 ms
warehouse_db=# drop index words_idx ;
DROP INDEX
Time: 75.698 ms
warehouse_db=# create index words_idx on words using gin(simple_words gin_trgm_ops,special_words gin_trgm_ops);
CREATE INDEX
Time: 271499.350 ms
warehouse_db=# select count(*) from words where simple_words like '%a31%' and special_words like '%a31%';
count
-------
115
(1 row)
Time: 10.260 ms
http://www.sai.msu.su/~megera/wiki/Gin
http://www.postgresql.org/docs/9.4/static/pgtrgm.html
index bloating
As the architecture of PostgreSQL is based on MVCC, tables have the difficulty of dead
rows. Rows that are not visible to any transaction are considered dead rows. In a
continuous table, some rows are deleted or updated. These operations cause dead space in
a table. Dead space can potentially be reused when new data is inserted. Due to a lot of
dead rows, bloating occurs. There are various reasons for index bloating, and it needs to
be fixed to achieve more performance, because it hurts the performance of the database.
AUTO VACUUM is the best obviation from bloating, but it is a configurable parameter and can
be incapacitated or erroneously configured. There are multiple ways to fix index bloating;
To know more about MVCC, check out
http://www.postgresql.org/docs/current/static/mvcc-intro.html
dump and restore
In the case of bloating, the simplest way of prevention is to back up the table utilizing
pg_dump, drop the table, and reload the data into the initial table. This is an expensive
operation and sometimes seems too restrictive.
VCUUM
Vacuuming the table using the VACUUM command is another solution that can be used to fix
the bloat. The VACUUM command reshuffles the rows to ensure that the page is as full as
possible, but database file shrinking only happens when there are 100 percent empty pages
at the end of the file. This is the only case where VACUUM is useful to reduce the bloat. Its
syntax is as follows:
VACUUM table_name
The following example shows the usage of VACUUM on the item table:
warehouse_db=# VACUUM item;
The other way of using VACUUM is as follows:
warehouse_db=# VACUUM FULL item;
CLUSTER
As we discussed previously, rewriting and reordering of rows can fix the issue that can be
indirectly achieved using dump/restore, but this is an expensive operation. The other way
to do this is the CLUSTER command, which is used to physically reorder rows based on the
index. The CLUSTER command is used to create a whole initial copy of the table and the old
copy of the data is dropped. The CLUSTER command requires enough space, virtually twice
the disk space, to hold the initial organized copy of the data. Its syntax is as follows:
CLUSTER table_name USING index_name
As we discussed previously, rewriting and reordering of rows can fix the issue that can be
indirectly achieved using dump/restore, but this is an expensive operation. The other way
to do this is the CLUSTER command, which is used to physically reorder rows based on the
index. The CLUSTER command is used to create a whole initial copy of the table and the old
copy of the data is dropped. The CLUSTER command requires enough space, virtually twice
the disk space, to hold the initial organized copy of the data. Its syntax is as follows:
CLUSTER table_name USING index_name
Reindexing
If an index becomes inefficient due to bloating or data becomes randomly scattered, then
reindexing is required to get the maximum performance from the index. Its syntax is as
follows:
warehouse_db=# REINDEX TABLE item;
points to ponder
When using an index, you need to keep in mind the following things:
It will make sense to index a table column when you have a handsome number of
rows in a table.
When retrieving data, you need to make sure that good candidates for an index are
foreign keys and keys where min() and max () can be used when retrieving data.
This means column selectivity is very important to index effectively.
Don’t forget to remove unused indexes for better performance. Also, perform
REINDEX on all indexes once a month to clean up the dead tuples.
Use table partitioning along with an index if you have large amounts of data.
When you are indexing columns with null values, consider using a conditional index
with WHERE column_name IS NOT NULL.