CREATE INDEX

Function

CREATE INDEX creates an index in a specified table.

Indexes are primarily used to enhance database performance (though inappropriate use can result in database performance deterioration). You are advised to create indexes on:

• Columns that are often queried
• Join conditions. For a query on joined columns, you are advised to create a composite index on the columns, for example, select * from t1 join t2 on t1.a=t2.a and t1.b=t2.b. You can create a composite index on columns a and b in table t1.
• Columns having filter criteria (especially scope criteria) of a where clause
• Columns that appear after order by, group by, and distinct

Partitioned tables do not support concurrent index creation, partial index creation, and NULL FIRST.

Precautions

• Indexes consume storage and computing resources. Creating too many indexes has negative impact on database performance (especially the performance of data import. Therefore, you are advised to import the data before creating indexes). Therefore, create indexes only when they are necessary.
• All functions and operators used in an index definition must be immutable, that is, their results must depend only on their parameters and never on any outside influence (such as the contents of another table or the current time). This restriction ensures that the behavior of the index is well-defined. To use a customized function in an index expression or WHERE clause, remember to mark the function immutable when you create it.
• Partitioned table indexes are classified into LOCAL indexes and GLOBAL indexes. A LOCAL index binds to a specific partition, and a GLOBAL index corresponds to the entire partitioned table.
• Column-store tables support B-tree and PSORT indexes. If the two indexes are used, you cannot create expression, partial, and unique indexes.
• Column-store tables support GIN indexes, rather than partial indexes and unique indexes. If GIN indexes are used, you can create expression indexes. However, an expression in this situation cannot contain empty splitters, empty columns, or multiple columns.

Syntax

• Create an index on a table.

  CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [schema_name.]index_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 ];

• Create an index on a partitioned table.

  CREATE [ UNIQUE ] INDEX [ [schema_name.]index_name ] ON table_name [ USING method ]
      ( {{ column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS LAST ] }[, ...] )
      [ LOCAL [ ( { PARTITION index_partition_name [ TABLESPACE index_partition_tablespace ] } [, ...] ) ] | GLOBAL ]
      [ WITH ( { storage_parameter = value } [, ...] ) ]
      [ TABLESPACE tablespace_name ];

Parameter Description

• UNIQUE

  Creates a unique index. In this way, the system checks whether new values are unique in the index column. Attempts to insert or update data which would result in duplicate entries will generate an error.

  Currently, only B-tree supports UNIQUE indexes.

• CONCURRENTLY

  Create an index (with ShareUpdateExclusiveLock) in non-blocking DML mode. A normal CREATE INDEX acquires exclusive lock on the table on which the index depends, blocking other accesses until the index drop can be completed. If this keyword is specified, DML is not blocked during the creation.

  • This option can only specify a name of one index.

  • The CREATE INDEX statement can be run within a transaction, but CREATE INDEX CONCURRENTLY cannot.

  • Column-store tables，temporary tables and partitioned tables do not support CREATE INDEX CONCURRENTLY.

    NOTE:

    • This keyword is specified when an index is created. The entire table needs to be scanned twice and built. When the table is scanned for the first time, an index is created and the read and write operations are not blocked. During the second scan, changes that have occurred since the first scan are merged and updated.
    • The table needs to be scanned and built twice, and all existing transactions that may modify the table must be completed. This means that the creation of the index takes a longer time than normal. In addition, the CPU and I/O consumption also affects other services.
    • If an index build fails, it leaves an "unusable" index. This index is ignored by the query, but it still consumes the update overhead. In this case, you are advised to delete the index and try CREATE INDEX CONCURRENTLY again.
    • After the second scan, index creation must wait for any transaction that holds a snapshot earlier than the snapshot taken by the second scan to terminate. In addition, the ShareUpdateExclusiveLock (level 4) added during index creation conflicts with a lock whose level is greater than or equal to 4. Therefore, when such an index is created, the system is prone to hang or deadlock. For example:
    • If two sessions create an index concurrently for the same table, a deadlock occurs.
    • If a session creates an index concurrently for a table and another session drops a table, a deadlock occurs.
    • There are three sessions. Session 1 locks table a and does not commit it. Session 2 creates an index concurrently for table b. Session 3 writes data to table a. Before the transaction of session 1 is committed, session 2 is blocked.
    • The transaction isolation level is set to repeatable read (read committed by default). Two sessions are started. Session 1 writes data to table a and does not commit it. Session 2 creates an index concurrently for table b. Before the transaction of session 1 is committed, session 2 is blocked.

• schema_name

  Specifies the schema name.

  Value range: an existing schema name

• index_name

  Specifies the name of the index to be created. The schema of the index is the same as that of the table.

  Value range: a string. It must comply with the naming convention.

• table_name

  Specifies the name of the table to be indexed (optionally schema-qualified).

  Value range: an existing table name

• USING method

  Specifies the name of the index method to be used.

  Value range:

  • btree: B-tree indexes store key values of data in a B+ tree structure. This structure helps users to quickly search for indexes. B-tree supports comparison queries with a scope specified.

  • gin: GIN indexes are reverse indexes and can process values that contain multiple keys (for example, arrays).

  • gist: GiST indexes are suitable for the set data type and multidimensional data types, such as geometric and geographic data types.The currently supported data types are box, point, poly, circle, tsvector, tsquery, range.

  • Psort: psort index. It is used to perform partial sort on column-store tables.

    Row-store tables support the following index types: btree (default), gin, and gist. Column-store tables support the following index types: Psort (default), btree, and gin.

    NOTE: Column-store tables support GIN indexes only for the tsvector type. That is, the input parameter for creating a column-store GIN index must be the return value of the to_tsvector function. This method is commonly used for GIN indexes.

• column_name

  Specifies the name of the column on which an index is to be created.

  Multiple columns can be specified if the index method supports multi-column indexes. A maximum of 32 columns can be specified.

• expression

  Specifies an expression based on one or more columns of the table. The expression usually must be written with surrounding parentheses, as shown in the syntax. However, the parentheses can be omitted if the expression has the form of a function call.

  Expression can be used to obtain fast access to data based on some transformation of the basic data. For example, an index computed on upper(col) would allow the clause WHERE upper(col) = 'JIM' to use an index.

  If an expression contains IS NULL, the index for this expression is invalid. In this case, you are advised to create a partial index.

• COLLATE collation

  Assigns a collation to the column (which must be of a collatable data type). If no collation is specified, the default collation is used.

• opclass

  Specifies the name of an operator class. An operator class can be specified for each column of an index. The operator class identifies the operators to be used by the index for that column. For example, a B-tree index on the type int4 would use the int4_ops class; this operator class includes comparison functions for values of type int4. In practice, the default operator class for the column's data type is sufficient. The operator class applies to data with multiple sorts. For example, users might want to sort a complex-number data type either by absolute value or by real part. They could do this by defining two operator classes for the data type and then selecting the proper class when making an index.

• ASC

  Specifies an ascending (default) sort order.

• DESC

  Specifies a descending sort order.

• NULLS FIRST

  Specifies that null values appear before non-null values in the sort ordering. This is the default when DESC is specified.

• NULLS LAST

  Specifies that null values appear after non-null values in the sort ordering. This is the default when DESC is not specified.

• LOCAL

  Specifies that the partitioned index to be created is a LOCAL index.

• GLOBAL

  Specifies the partitioned index to be created as a GLOBAL index. If no keyword is specified, a GLOBAL index is created by default.

• WITH ( {storage_parameter = value} [, … ] )

  Specifies the storage parameter used for an index.

  Value range:

  Only index GIN supports parameters FASTUPDATE and GIN_PENDING_LIST_LIMIT. Indexes other than GIN and psort support the FILLFACTOR parameter.

  • FILLFACTOR

    The fill factor of an index is a percentage from 10 to 100.

    Value range: 10-100

  • FASTUPDATE

    Specifies whether fast update is enabled for the GIN index.

    Value range: : ON and OFF

    Default value: ON

  • GIN_PENDING_LIST_LIMIT

    Specifies the maximum capacity of the pending list of the GIN index when fast update is enabled for the GIN index.

    Value range: 64-INT_MAX. The unit is KB.

    Default value: The default value of gin_pending_list_limit depends on gin_pending_list_limit specified in GUC parameters. By default, the value is 4 MB.

• TABLESPACE tablespace_name

  Specifies the tablespace for an index. If no tablespace is specified, the default tablespace is used.

  Value range: an existing table name

• WHERE predicate

  Creates a partial index. A partial index is an index that contains entries for only a portion of a table, usually a portion that is more useful for indexing than the rest of the table. For example, if you have a table that contains both billed and unbilled orders where the unbilled orders take up a small fraction of the total table and yet that is an often used portion, you can improve performance by creating an index on just that portion. In addition, WHERE with UNIQUE can be used to enforce uniqueness over a subset for a table.

  Value range: The predicate expression can only refer to columns of the underlying table, but it can use all columns, not just the ones being indexed. Currently, subqueries and aggregate expressions are forbidden in WHERE.

• PARTITION index_partition_name

  Specifies the name of an index partition.

  Value range: a string. It must comply with the naming convention.

• TABLESPACE index_partition_tablespace

  Specifies the tablespace of an index partition.

  Value range: If this parameter is not specified, the value of index_tablespace is used.

Examples

-- Create the tpcds.ship_mode_t1 table.
mogdb=# create schema tpcds;
mogdb=# CREATE TABLE tpcds.ship_mode_t1
(
    SM_SHIP_MODE_SK           INTEGER               NOT NULL,
    SM_SHIP_MODE_ID           CHAR(16)              NOT NULL,
    SM_TYPE                   CHAR(30)                      ,
    SM_CODE                   CHAR(10)                      ,
    SM_CARRIER                CHAR(20)                      ,
    SM_CONTRACT               CHAR(20)
)
;

-- Create a common index on the SM_SHIP_MODE_SK column in the tpcds.ship_mode_t1 table.
mogdb=# CREATE UNIQUE INDEX ds_ship_mode_t1_index1 ON tpcds.ship_mode_t1(SM_SHIP_MODE_SK);

-- Create a B-tree index on the SM_SHIP_MODE_SK column in the tpcds.ship_mode_t1 table.
mogdb=# CREATE INDEX ds_ship_mode_t1_index4 ON tpcds.ship_mode_t1 USING btree(SM_SHIP_MODE_SK);

-- Create an expression index on the SM_CODE column in the tpcds.ship_mode_t1 table:
mogdb=# CREATE INDEX ds_ship_mode_t1_index2 ON tpcds.ship_mode_t1(SUBSTR(SM_CODE,1 ,4));

-- Create a partial index on the SM_SHIP_MODE_SK column where SM_SHIP_MODE_SK is greater than 10 in the tpcds.ship_mode_t1 table.
mogdb=# CREATE UNIQUE INDEX ds_ship_mode_t1_index3 ON tpcds.ship_mode_t1(SM_SHIP_MODE_SK) WHERE SM_SHIP_MODE_SK>10;

-- Rename an existing index.
mogdb=# ALTER INDEX tpcds.ds_ship_mode_t1_index1 RENAME TO ds_ship_mode_t1_index5;

-- Set the index as unusable.
mogdb=# ALTER INDEX tpcds.ds_ship_mode_t1_index2 UNUSABLE;

-- Rebuild an index.
mogdb=# ALTER INDEX tpcds.ds_ship_mode_t1_index2 REBUILD;

-- Delete an existing index.
mogdb=# DROP INDEX tpcds.ds_ship_mode_t1_index2;

-- Delete the table.
mogdb=# DROP TABLE tpcds.ship_mode_t1;

-- Create a tablespace.
mogdb=# CREATE TABLESPACE example1 RELATIVE LOCATION 'tablespace1/tablespace_1';
mogdb=# CREATE TABLESPACE example2 RELATIVE LOCATION 'tablespace2/tablespace_2';
mogdb=# CREATE TABLESPACE example3 RELATIVE LOCATION 'tablespace3/tablespace_3';
mogdb=# CREATE TABLESPACE example4 RELATIVE LOCATION 'tablespace4/tablespace_4';
-- Create the tpcds.customer_address_p1 table.
mogdb=# CREATE TABLE tpcds.customer_address_p1
(
    CA_ADDRESS_SK             INTEGER               NOT NULL,
    CA_ADDRESS_ID             CHAR(16)              NOT NULL,
    CA_STREET_NUMBER          CHAR(10)                      ,
    CA_STREET_NAME            VARCHAR(60)                   ,
    CA_STREET_TYPE            CHAR(15)                      ,
    CA_SUITE_NUMBER           CHAR(10)                      ,
    CA_CITY                   VARCHAR(60)                   ,
    CA_COUNTY                 VARCHAR(30)                   ,
    CA_STATE                  CHAR(2)                       ,
    CA_ZIP                    CHAR(10)                      ,
    CA_COUNTRY                VARCHAR(20)                   ,
    CA_GMT_OFFSET             DECIMAL(5,2)                  ,
    CA_LOCATION_TYPE          CHAR(20)
)
TABLESPACE example1
PARTITION BY RANGE(CA_ADDRESS_SK)
(
   PARTITION p1 VALUES LESS THAN (3000),
   PARTITION p2 VALUES LESS THAN (5000) TABLESPACE example1,
   PARTITION p3 VALUES LESS THAN (MAXVALUE) TABLESPACE example2
)
ENABLE ROW MOVEMENT;
-- Create the partitioned table index ds_customer_address_p1_index1 without specifying the index partition name.
mogdb=# CREATE INDEX ds_customer_address_p1_index1 ON tpcds.customer_address_p1(CA_ADDRESS_SK) LOCAL;
-- Create the partitioned table index ds_customer_address_p1_index2 with the name of the index partition specified.
mogdb=# CREATE INDEX ds_customer_address_p1_index2 ON tpcds.customer_address_p1(CA_ADDRESS_SK) LOCAL
(
    PARTITION CA_ADDRESS_SK_index1,
    PARTITION CA_ADDRESS_SK_index2 TABLESPACE example3,
    PARTITION CA_ADDRESS_SK_index3 TABLESPACE example4
)
TABLESPACE example2;

-- Create a GLOBAL partitioned index.
mogdb=CREATE INDEX ds_customer_address_p1_index3 ON tpcds.customer_address_p1(CA_ADDRESS_ID) GLOBAL;

-- If no keyword is specified, a GLOBAL partitioned index is created by default.
mogdb=CREATE INDEX ds_customer_address_p1_index4 ON tpcds.customer_address_p1(CA_ADDRESS_ID);

-- Change the tablespace of the partitioned table index CA_ADDRESS_SK_index2 to example1.
mogdb=# ALTER INDEX tpcds.ds_customer_address_p1_index2 MOVE PARTITION CA_ADDRESS_SK_index2 TABLESPACE example1;

-- Change the tablespace of the partitioned table index CA_ADDRESS_SK_index3 to example2.
mogdb=# ALTER INDEX tpcds.ds_customer_address_p1_index2 MOVE PARTITION CA_ADDRESS_SK_index3 TABLESPACE example2;

-- Rename a partitioned table index.
mogdb=# ALTER INDEX tpcds.ds_customer_address_p1_index2 RENAME PARTITION CA_ADDRESS_SK_index1 TO CA_ADDRESS_SK_index4;

-- Delete the created indexes and the partitioned table.
mogdb=# DROP INDEX tpcds.ds_customer_address_p1_index1;
mogdb=# DROP INDEX tpcds.ds_customer_address_p1_index2;
mogdb=# DROP TABLE tpcds.customer_address_p1;
-- Delete the tablespace.
mogdb=# DROP TABLESPACE example1;
mogdb=# DROP TABLESPACE example2;
mogdb=# DROP TABLESPACE example3;
mogdb=# DROP TABLESPACE example4;

-- Create a column-store table and its GIN index:
mogdb=# create table cgin_create_test(a int, b text) with (orientation = column);
CREATE TABLE
mogdb=# create index cgin_test on cgin_create_test using gin(to_tsvector('ngram', b));
CREATE INDEX

Suggestions

• create index

  You are advised to create indexes on:

  • Columns that are often queried
  • Join conditions. For a query on joined columns, you are advised to create a composite index on the columns, for example, select * from t1 join t2 on t1.a=t2.a and t1.b=t2.b. You can create a composite index on columns a and b in table t1.
  • Columns having filter criteria (especially scope criteria) of a where clause
  • Columns that appear after order by, group by, and distinct

  Constraints:

  • Partitioned tables do not support partial indexes or the NULL FIRST feature.
  • A unique LOCAL index to be created must include a partitioned column and all the partition keys. This constraint does not apply to GLOBAL indexes.
  • When a GLOBAL index is created on a partitioned table, the following constraints apply:
    • Expression indexes and partial indexes are not supported.
    • Row-store tables are not supported.
    • Only B-tree indexes are supported.
  • In the same attribute column, the LOCAL index and GLOBAL index of a partition cannot coexist.
  • GLOBAL index supports a maximum of 31 columns.
  • If the alter statement does not have UPDATE GLOBAL INDEX, then the original GLOBAL index will be invalid, and other indexes will be used for query; if the alter statement has UPDATE GLOBAL INDEX, then the original GLOBAL index is still valid and the index function is correct.