About clustered index in postgres

Question

I\'m using psql to access a postgres database. When viewing the metadata of a table, is there any way to see whether an index of a table is a clustered index?

I hear

Answer 1

is there any way to see whether an index of a table is a clustered index

PostgreSQL does not have a clustered index, so you won't be able to see them.

I heard that the PRIMARY KEY of a table is automatically associated with a clustered index, is it true?

No, that's not true (see above)

You can manually cluster a table along an index, but this is nothing that will be maintained automatically (as e.g. with SQL Server's clustered indexes).

For more details, see the description of the CLUSTER command in the manual.

Answer 2

PostgreSQL does not have direct implementation of CLUSTER index like Microsoft SQL Server.

Reference Taken from this Blog:

In PostgreSQL, we have one CLUSTER command which is similar to Cluster Index.

Once you create your table primary key or any other Index, you can execute the CLUSTER command by specifying that Index name to achieve the physical order of the Table Data.

When a table is clustered, it is physically reordered based on the index information. Clustering is a one-time operation: when the table is subsequently updated, the changes are not clustered. That is, no attempt is made to store new or updated rows according to their index order.

Syntax of Cluster:

First time you must execute CLUSTER using the Index Name.

CLUSTER table_name USING index_name;

Cluster the table:

Once you have executed CLUSTER with Index, next time you should execute only CLUSTER TABLE because It knows that which index already defined as CLUSTER.

CLUSTER table_name;

Answer 3

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Cluster Indexing

A cluster index means telling the database to store the close values actually close to one another on the disk. They can uniquely identify the rows in the SQL table. Every table can have exactly one one clustered index. A cluster index can covers more than one column. By default, a column with a primary key already has a clustered index.

dictionary

A dictionary itself a table with clustered index. Because all the data physically stored in alphabetical order.

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Non Cluster Indexing

Non clustered indexing are like simple indexing of a book. They are just used for fast retrieval of data. Not sure to have unique data. A non clustered index contains the non clustered index keys and each keys contains the data location pointer. Just a book content index contains the key of a topic or chapter and the page location of that.

book content index

A book content table just remember the content name and its page location. It is not sure that the data is unique. Because same paragraph or text line or word can be placed many times.

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PostgreSQL Indexing

PostgreSQL automatically create indexes for PRIMARY KEY and every UNIQUE constraints of a table. Login to a database in PostgreSQL terminal and type \d table_name. All stored indexes will be visualized. If there exist the a clustered index that will also be identified.

Creating a table

CREATE TABLE IF NOT EXISTS profile(
    uid serial NOT NULL UNIQUE PRIMARY KEY,
    username varchar(30) NOT NULL UNIQUE,
    phone varchar(11) NOT NULL UNIQUE,
    age smallint CHECK(age>12),
    address text NULL
);

3 index will be created automatically. All these indexes are non clustered

"profile_pkey" PRIMARY KEY, btree (uid)
"profile_phone_key" UNIQUE CONSTRAINT, btree (phone)
"profile_username_key" UNIQUE CONSTRAINT, btree (username)

Create our own index with uid and username

CREATE INDEX profile_index ON profile(uid, username);

This is actually a non cluster index. Now make it to the cluster index

Making a non cluster index to a cluster one

ALTER TABLE profile CLUSTER ON profile_index;

check the table \d profile. it will show like this

                                     Table "public.profile"
  Column  |         Type          | Collation | Nullable |               Default
----------+-----------------------+-----------+----------+--------------------------------------
 uid      | integer               |           | not null | nextval('profile_uid_seq'::regclass)
 username | character varying(30) |           | not null |
 phone    | character varying(11) |           | not null |
 age      | smallint              |           |          |
 address  | text                  |           |          |
Indexes:
    "profile_pkey" PRIMARY KEY, btree (uid)
    "profile_phone_key" UNIQUE CONSTRAINT, btree (phone)
    "profile_username_key" UNIQUE CONSTRAINT, btree (username)
    "profile_index" btree (uid, username) CLUSTER
Check constraints:
    "profile_age_check" CHECK (age > 12)

See, profile_index now CLUSTER

Now, re-cluster the table so that the table can follow the cluster index role

CLUSTER profile;

Answer 4

If you want to know if a given table is CLUSTERed using SQL, you can use the following query to show the index being used (tested in Postgres versions 9.5 and 9.6):

SELECT
  i.relname AS index_for_cluster
FROM
  pg_index AS idx
JOIN
  pg_class AS i
ON
  i.oid = idx.indexrelid
WHERE
  idx.indisclustered
  AND idx.indrelid::regclass = 'your_table_name'::regclass;

Answer 5

Note that PostgreSQL uses the term "clustered index" to use something vaguely similar and yet very different to SQL Server.

If a particular index has been nominated as the clustering index for a table, then psql's \d command will indicate the clustered index, e.g.,

Indexes:
    "timezone_description_pkey" PRIMARY KEY, btree (timezone) CLUSTER

PostgreSQL does not nominate indices as clustering indices by default. Nor does it automatically arrange table data to correlate with the clustered index even when so nominated: the CLUSTER command has to be used to reorganise the table data.

Answer 6

In PostgreSQL the clustered attribute is held in the metadata of the corresponding index, rather than the relation itself. It is the indisclustered attribute in pg_index catalogue. Note, however, that clustering relations within postgres is a one-time action: even if the attribute is true, updates to the table do not maintain the sorted nature of the data. To date, automatic maintenance of data clustering remains a popular TODO item.

There is often confusion between clustered and integrated indexes, particularly since the popular textbooks use conflicting names, and the terminology is different again in the manuals of postgres and SQL server (to name just two). When I talk about an integrated index (also called a main index or primary index) I mean one in which the relation data is contained in the leaves of the index, as opposed an external or secondary index in which the leaves contain index entries that point to the table records. The former type is necessarily always clustered. Unfortunately postgres only supports the latter type. Anyhow, the fact that an integrated (primary) index is always clustered may have given rise to the belief that "a PRIMARY KEY of a table is automatically associated with a clustered index". The two statements sound similar, but are different.