ALTER TABLE
Synopsis
ALTER TABLE [ IF EXISTS ] [ ONLY ]name [ * ]action [, ... ]ALTER TABLE [ IF EXISTS ] [ ONLY ]name [ * ] RENAME [ COLUMN ]column_name TOnew_column_nameALTER TABLE [ IF EXISTS ] [ ONLY ]name [ * ] RENAME CONSTRAINTconstraint_name TOnew_constraint_nameALTER TABLE [ IF EXISTS ]name RENAME TOnew_nameALTER TABLE [ IF EXISTS ]name SET SCHEMAnew_schemaALTER TABLE ALL IN TABLESPACEname [ OWNED BYrole_name [, ... ] ] SET TABLESPACEnew_tablespace [ NOWAIT ]whereaction is one of: ADD [ COLUMN ]column_namedata_type [ COLLATEcollation ] [column_constraint [ ... ] ] DROP [ COLUMN ] [ IF EXISTS ]column_name [ RESTRICT | CASCADE ] ALTER [ COLUMN ]column_name [ SET DATA ] TYPEdata_type [ COLLATEcollation ] [ USINGexpression ] ALTER [ COLUMN ]column_name SET DEFAULTexpression ALTER [ COLUMN ]column_name DROP DEFAULT ALTER [ COLUMN ]column_name { SET | DROP } NOT NULL ALTER [ COLUMN ]column_name SET STATISTICSinteger ALTER [ COLUMN ]column_name SET (attribute_option =value [, ... ] ) ALTER [ COLUMN ]column_name RESET (attribute_option [, ... ] ) ALTER [ COLUMN ]column_name SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN } ADDtable_constraint [ NOT VALID ] ADDtable_constraint_using_index ALTER CONSTRAINTconstraint_name [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] VALIDATE CONSTRAINTconstraint_name DROP CONSTRAINT [ IF EXISTS ]constraint_name [ RESTRICT | CASCADE ] DISABLE TRIGGER [trigger_name | ALL | USER ] ENABLE TRIGGER [trigger_name | ALL | USER ] ENABLE REPLICA TRIGGERtrigger_name ENABLE ALWAYS TRIGGERtrigger_name DISABLE RULErewrite_rule_name ENABLE RULErewrite_rule_name ENABLE REPLICA RULErewrite_rule_name ENABLE ALWAYS RULErewrite_rule_name CLUSTER ONindex_name SET WITHOUT CLUSTER SET WITH OIDS SET WITHOUT OIDS SET (storage_parameter =value [, ... ] ) RESET (storage_parameter [, ... ] ) INHERITparent_table NO INHERITparent_table OFtype_name NOT OF OWNER TOnew_owner SET TABLESPACEnew_tablespace REPLICA IDENTITY {DEFAULT | USING INDEXindex_name | FULL | NOTHING}andtable_constraint_using_index is: [ CONSTRAINTconstraint_name ] { UNIQUE | PRIMARY KEY } USING INDEXindex_name [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
Description
ALTER TABLE changes the definition of an existing table. There are several subforms described below. Note that the lock level required may differ for each subform. AnACCESS EXCLUSIVE lock is held unless explicitly noted. When multiple subcommands are listed, the lock held will be the strictest one required from any subcommand.
- ADD COLUMN
This form adds a new column to the table, using the same syntax asCREATE TABLE.
- DROP COLUMN [ IF EXISTS ]
This form drops a column from a table. Indexes and table constraints involving the column will be automatically dropped as well. You will need to sayCASCADE if anything outside the table depends on the column, for example, foreign key references or views. IfIF EXISTS is specified and the column does not exist, no error is thrown. In this case a notice is issued instead.
- IF EXISTS
Do not throw an error if the table does not exist. A notice is issued in this case.
- SET DATA TYPE
This form changes the type of a column of a table. Indexes and simple table constraints involving the column will be automatically converted to use the new column type by reparsing the originally supplied expression. The optionalCOLLATE clause specifies a collation for the new column; if omitted, the collation is the default for the new column type. The optionalUSING clause specifies how to compute the new column value from the old; if omitted, the default conversion is the same as an assignment cast from old data type to new. AUSING clause must be provided if there is no implicit or assignment cast from old to new type.
- SET/DROP DEFAULT
These forms set or remove the default value for a column. Default values only apply in subsequentINSERT orUPDATE commands; they do not cause rows already in the table to change.
- SET/DROP NOT NULL
These forms change whether a column is marked to allow null values or to reject null values. You can only useSET NOT NULL when the column contains no null values.
- SET STATISTICS
This form sets the per-column statistics-gathering target for subsequentANALYZE operations. The target can be set in the range 0 to 10000; alternatively, set it to -1 to revert to using the system default statistics target (default_statistics_target). For more information on the use of statistics by thePostgreSQL query planner, refer toSection 14.2.
SET STATISTICS acquires aSHARE UPDATE EXCLUSIVE lock.
- SET (attribute_option =value [, ... ] )
RESET (attribute_option [, ... ] ) This form sets or resets per-attribute options. Currently, the only defined per-attribute options aren_distinct andn_distinct_inherited, which override the number-of-distinct-values estimates made by subsequentANALYZE operations.n_distinct affects the statistics for the table itself, whilen_distinct_inherited affects the statistics gathered for the table plus its inheritance children. When set to a positive value,ANALYZE will assume that the column contains exactly the specified number of distinct nonnull values. When set to a negative value, which must be greater than or equal to -1,ANALYZE will assume that the number of distinct nonnull values in the column is linear in the size of the table; the exact count is to be computed by multiplying the estimated table size by the absolute value of the given number. For example, a value of -1 implies that all values in the column are distinct, while a value of -0.5 implies that each value appears twice on the average. This can be useful when the size of the table changes over time, since the multiplication by the number of rows in the table is not performed until query planning time. Specify a value of 0 to revert to estimating the number of distinct values normally. For more information on the use of statistics by thePostgreSQL query planner, refer toSection 14.2.
Changing per-attribute options acquires aSHARE UPDATE EXCLUSIVE lock.
- SET STORAGE
This form sets the storage mode for a column. This controls whether this column is held inline or in a secondaryTOAST table, and whether the data should be compressed or not.PLAIN must be used for fixed-length values such asinteger and is inline, uncompressed.MAIN is for inline, compressible data.EXTERNAL is for external, uncompressed data, andEXTENDED is for external, compressed data.EXTENDED is the default for most data types that support non-PLAIN storage. Use ofEXTERNAL will make substring operations on very largetext andbytea values run faster, at the penalty of increased storage space. Note thatSET STORAGE doesn't itself change anything in the table, it just sets the strategy to be pursued during future table updates. SeeSection 59.2 for more information.
- ADDtable_constraint [ NOT VALID ]
This form adds a new constraint to a table using the same syntax asCREATE TABLE, plus the optionNOT VALID, which is currently only allowed for foreign key and CHECK constraints. If the constraint is markedNOT VALID, the potentially-lengthy initial check to verify that all rows in the table satisfy the constraint is skipped. The constraint will still be enforced against subsequent inserts or updates (that is, they'll fail unless there is a matching row in the referenced table, in the case of foreign keys; and they'll fail unless the new row matches the specified check constraints). But the database will not assume that the constraint holds for all rows in the table, until it is validated by using theVALIDATE CONSTRAINT option.
- ADDtable_constraint_using_index
This form adds a newPRIMARY KEY orUNIQUE constraint to a table based on an existing unique index. All the columns of the index will be included in the constraint.
The index cannot have expression columns nor be a partial index. Also, it must be a b-tree index with default sort ordering. These restrictions ensure that the index is equivalent to one that would be built by a regularADD PRIMARY KEY orADD UNIQUE command.
IfPRIMARY KEY is specified, and the index's columns are not already markedNOT NULL, then this command will attempt to doALTER COLUMN SET NOT NULL against each such column. That requires a full table scan to verify the column(s) contain no nulls. In all other cases, this is a fast operation.
If a constraint name is provided then the index will be renamed to match the constraint name. Otherwise the constraint will be named the same as the index.
After this command is executed, the index is"owned" by the constraint, in the same way as if the index had been built by a regularADD PRIMARY KEY orADD UNIQUE command. In particular, dropping the constraint will make the index disappear too.
Note: Adding a constraint using an existing index can be helpful in situations where a new constraint needs to be added without blocking table updates for a long time. To do that, create the index usingCREATE INDEX CONCURRENTLY, and then install it as an official constraint using this syntax. See the example below.
- ALTER CONSTRAINT
This form alters the attributes of a constraint that was previously created. Currently only foreign key constraints may be altered.
- VALIDATE CONSTRAINT
This form validates a foreign key or check constraint that was previously created asNOT VALID, by scanning the table to ensure there are no rows for which the constraint is not satisfied. Nothing happens if the constraint is already marked valid.
Validation can be a long process on larger tables. The value of separating validation from initial creation is that you can defer validation to less busy times, or can be used to give additional time to correct pre-existing errors while preventing new errors. Note also that validation on its own does not prevent normal write commands against the table while it runs.
Validation acquires only aSHARE UPDATE EXCLUSIVE lock on the table being altered. If the constraint is a foreign key then aROW SHARE lock is also required on the table referenced by the constraint.
- DROP CONSTRAINT [ IF EXISTS ]
This form drops the specified constraint on a table. IfIF EXISTS is specified and the constraint does not exist, no error is thrown. In this case a notice is issued instead.
- DISABLE/ENABLE [ REPLICA | ALWAYS ] TRIGGER
These forms configure the firing of trigger(s) belonging to the table. A disabled trigger is still known to the system, but is not executed when its triggering event occurs. For a deferred trigger, the enable status is checked when the event occurs, not when the trigger function is actually executed. One can disable or enable a single trigger specified by name, or all triggers on the table, or only user triggers (this option excludes internally generated constraint triggers such as those that are used to implement foreign key constraints or deferrable uniqueness and exclusion constraints). Disabling or enabling internally generated constraint triggers requires superuser privileges; it should be done with caution since of course the integrity of the constraint cannot be guaranteed if the triggers are not executed. The trigger firing mechanism is also affected by the configuration variablesession_replication_role. Simply enabled triggers will fire when the replication role is"origin" (the default) or"local". Triggers configured asENABLE REPLICA will only fire if the session is in"replica" mode, and triggers configured asENABLE ALWAYS will fire regardless of the current replication mode.
- DISABLE/ENABLE [ REPLICA | ALWAYS ] RULE
These forms configure the firing of rewrite rules belonging to the table. A disabled rule is still known to the system, but is not applied during query rewriting. The semantics are as for disabled/enabled triggers. This configuration is ignored forON SELECT rules, which are always applied in order to keep views working even if the current session is in a non-default replication role.
- CLUSTER ON
This form selects the default index for futureCLUSTER operations. It does not actually re-cluster the table.
Changing cluster options acquires aSHARE UPDATE EXCLUSIVE lock.
- SET WITHOUT CLUSTER
This form removes the most recently usedCLUSTER index specification from the table. This affects future cluster operations that don't specify an index.
Changing cluster options acquires aSHARE UPDATE EXCLUSIVE lock.
- SET WITH OIDS
This form adds anoid system column to the table (seeSection 5.4). It does nothing if the table already has OIDs.
Note that this is not equivalent toADD COLUMN oid oid; that would add a normal column that happened to be namedoid, not a system column.
- SET WITHOUT OIDS
This form removes theoid system column from the table. This is exactly equivalent toDROP COLUMN oid RESTRICT, except that it will not complain if there is already nooid column.
- SET (storage_parameter =value [, ... ] )
This form changes one or more storage parameters for the table. SeeStorage Parameters for details on the available parameters. Note that the table contents will not be modified immediately by this command; depending on the parameter you might need to rewrite the table to get the desired effects. That can be done withVACUUM FULL,CLUSTER or one of the forms ofALTER TABLE that forces a table rewrite.
Note: WhileCREATE TABLE allowsOIDS to be specified in theWITH (storage_parameter) syntax,ALTER TABLE does not treatOIDS as a storage parameter. Instead use theSET WITH OIDS andSET WITHOUT OIDS forms to change OID status.
- RESET (storage_parameter [, ... ] )
This form resets one or more storage parameters to their defaults. As withSET, a table rewrite might be needed to update the table entirely.
- INHERITparent_table
This form adds the target table as a new child of the specified parent table. Subsequently, queries against the parent will include records of the target table. To be added as a child, the target table must already contain all the same columns as the parent (it could have additional columns, too). The columns must have matching data types, and if they haveNOT NULL constraints in the parent then they must also haveNOT NULL constraints in the child.
There must also be matching child-table constraints for allCHECK constraints of the parent, except those marked non-inheritable (that is, created withALTER TABLE ... ADD CONSTRAINT ... NO INHERIT) in the parent, which are ignored; all child-table constraints matched must not be marked non-inheritable. CurrentlyUNIQUE,PRIMARY KEY, andFOREIGN KEY constraints are not considered, but this might change in the future.
- NO INHERITparent_table
This form removes the target table from the list of children of the specified parent table. Queries against the parent table will no longer include records drawn from the target table.
- OFtype_name
This form links the table to a composite type as thoughCREATE TABLE OF had formed it. The table's list of column names and types must precisely match that of the composite type; the presence of anoid system column is permitted to differ. The table must not inherit from any other table. These restrictions ensure thatCREATE TABLE OF would permit an equivalent table definition.
- NOT OF
This form dissociates a typed table from its type.
- OWNER
This form changes the owner of the table, sequence, view, materialized view, or foreign table to the specified user.
- SET TABLESPACE
This form changes the table's tablespace to the specified tablespace and moves the data file(s) associated with the table to the new tablespace. Indexes on the table, if any, are not moved; but they can be moved separately with additionalSET TABLESPACE commands. All tables in the current database in a tablespace can be moved by using theALL IN TABLESPACE form, which will lock all tables to be moved first and then move each one. This form also supportsOWNED BY, which will only move tables owned by the roles specified. If theNOWAIT option is specified then the command will fail if it is unable to acquire all of the locks required immediately. Note that system catalogs are not moved by this command, useALTER DATABASE or explicitALTER TABLE invocations instead if desired. Theinformation_schema relations are not considered part of the system catalogs and will be moved. See alsoCREATE TABLESPACE.
- REPLICA IDENTITY
This form changes the information which is written to the write-ahead log to identify rows which are updated or deleted. This option has no effect except when logical replication is in use.DEFAULT (the default for non-system tables) records the old values of the columns of the primary key, if any.USING INDEX records the old values of the columns covered by the named index, which must be unique, not partial, not deferrable, and include only columns markedNOT NULL.FULL records the old values of all columns in the row.NOTHING records no information about the old row. (This is the default for system tables.) In all cases, no old values are logged unless at least one of the columns that would be logged differs between the old and new versions of the row.
- RENAME
TheRENAME forms change the name of a table (or an index, sequence, view, materialized view, or foreign table), the name of an individual column in a table, or the name of a constraint of the table. There is no effect on the stored data.
- SET SCHEMA
This form moves the table into another schema. Associated indexes, constraints, and sequences owned by table columns are moved as well.
All the actions exceptRENAME,SET TABLESPACE andSET SCHEMA can be combined into a list of multiple alterations to apply in parallel. For example, it is possible to add several columns and/or alter the type of several columns in a single command. This is particularly useful with large tables, since only one pass over the table need be made.
You must own the table to useALTER TABLE. To change the schema or tablespace of a table, you must also haveCREATE privilege on the new schema or tablespace. To add the table as a new child of a parent table, you must own the parent table as well. To alter the owner, you must also be a direct or indirect member of the new owning role, and that role must haveCREATE privilege on the table's schema. (These restrictions enforce that altering the owner doesn't do anything you couldn't do by dropping and recreating the table. However, a superuser can alter ownership of any table anyway.) To add a column or alter a column type or use theOF clause, you must also haveUSAGE privilege on the data type.
Parameters
- name
The name (optionally schema-qualified) of an existing table to alter. IfONLY is specified before the table name, only that table is altered. IfONLY is not specified, the table and all its descendant tables (if any) are altered. Optionally,* can be specified after the table name to explicitly indicate that descendant tables are included.
- column_name
Name of a new or existing column.
- new_column_name
New name for an existing column.
- new_name
New name for the table.
- data_type
Data type of the new column, or new data type for an existing column.
- table_constraint
New table constraint for the table.
- constraint_name
Name of a new or existing constraint.
- CASCADE
Automatically drop objects that depend on the dropped column or constraint (for example, views referencing the column).
- RESTRICT
Refuse to drop the column or constraint if there are any dependent objects. This is the default behavior.
- trigger_name
Name of a single trigger to disable or enable.
- ALL
Disable or enable all triggers belonging to the table. (This requires superuser privilege if any of the triggers are internally generated constraint triggers such as those that are used to implement foreign key constraints or deferrable uniqueness and exclusion constraints.)
- USER
Disable or enable all triggers belonging to the table except for internally generated constraint triggers such as those that are used to implement foreign key constraints or deferrable uniqueness and exclusion constraints.
- index_name
The name of an existing index.
- storage_parameter
The name of a table storage parameter.
- value
The new value for a table storage parameter. This might be a number or a word depending on the parameter.
- parent_table
A parent table to associate or de-associate with this table.
- new_owner
The user name of the new owner of the table.
- new_tablespace
The name of the tablespace to which the table will be moved.
- new_schema
The name of the schema to which the table will be moved.
Notes
The key wordCOLUMN is noise and can be omitted.
When a column is added withADD COLUMN, all existing rows in the table are initialized with the column's default value (NULL if noDEFAULT clause is specified). If there is noDEFAULT clause, this is merely a metadata change and does not require any immediate update of the table's data; the added NULL values are supplied on readout, instead.
Adding a column with aDEFAULT clause or changing the type of an existing column will require the entire table and its indexes to be rewritten. As an exception when changing the type of an existing column, if theUSING clause does not change the column contents and the old type is either binary coercible to the new type or an unconstrained domain over the new type, a table rewrite is not needed; but any indexes on the affected columns must still be rebuilt. Adding or removing a systemoid column also requires rewriting the entire table. Table and/or index rebuilds may take a significant amount of time for a large table; and will temporarily require as much as double the disk space.
Adding aCHECK orNOT NULL constraint requires scanning the table to verify that existing rows meet the constraint.
The main reason for providing the option to specify multiple changes in a singleALTER TABLE is that multiple table scans or rewrites can thereby be combined into a single pass over the table.
TheDROP COLUMN form does not physically remove the column, but simply makes it invisible to SQL operations. Subsequent insert and update operations in the table will store a null value for the column. Thus, dropping a column is quick but it will not immediately reduce the on-disk size of your table, as the space occupied by the dropped column is not reclaimed. The space will be reclaimed over time as existing rows are updated. (These statements do not apply when dropping the systemoid column; that is done with an immediate rewrite.)
To force an immediate rewrite of the table, you can useVACUUM FULL,CLUSTER or one of the forms of ALTER TABLE that forces a rewrite. This results in no semantically-visible change in the table, but gets rid of no-longer-useful data.
TheUSING option ofSET DATA TYPE can actually specify any expression involving the old values of the row; that is, it can refer to other columns as well as the one being converted. This allows very general conversions to be done with theSET DATA TYPE syntax. Because of this flexibility, theUSING expression is not applied to the column's default value (if any); the result might not be a constant expression as required for a default. This means that when there is no implicit or assignment cast from old to new type,SET DATA TYPE might fail to convert the default even though aUSING clause is supplied. In such cases, drop the default withDROP DEFAULT, perform theALTER TYPE, and then useSET DEFAULT to add a suitable new default. Similar considerations apply to indexes and constraints involving the column.
If a table has any descendant tables, it is not permitted to add, rename, or change the type of a column, or rename an inherited constraint in the parent table without doing the same to the descendants. That is,ALTER TABLE ONLY will be rejected. This ensures that the descendants always have columns matching the parent.
A recursiveDROP COLUMN operation will remove a descendant table's column only if the descendant does not inherit that column from any other parents and never had an independent definition of the column. A nonrecursiveDROP COLUMN (i.e.,ALTER TABLE ONLY ... DROP COLUMN) never removes any descendant columns, but instead marks them as independently defined rather than inherited.
TheTRIGGER,CLUSTER,OWNER, andTABLESPACE actions never recurse to descendant tables; that is, they always act as thoughONLY were specified. Adding a constraint recurses only forCHECK constraints that are not markedNO INHERIT.
Changing any part of a system catalog table is not permitted.
Refer toCREATE TABLE for a further description of valid parameters.Chapter 5 has further information on inheritance.
Examples
To add a column of typevarchar to a table:
ALTER TABLE distributors ADD COLUMN address varchar(30);
To drop a column from a table:
ALTER TABLE distributors DROP COLUMN address RESTRICT;
To change the types of two existing columns in one operation:
ALTER TABLE distributors ALTER COLUMN address TYPE varchar(80), ALTER COLUMN name TYPE varchar(100);
To change an integer column containing UNIX timestamps totimestamp with time zone via aUSING clause:
ALTER TABLE foo ALTER COLUMN foo_timestamp SET DATA TYPE timestamp with time zone USING timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';
The same, when the column has a default expression that won't automatically cast to the new data type:
ALTER TABLE foo ALTER COLUMN foo_timestamp DROP DEFAULT, ALTER COLUMN foo_timestamp TYPE timestamp with time zone USING timestamp with time zone 'epoch' + foo_timestamp * interval '1 second', ALTER COLUMN foo_timestamp SET DEFAULT now();
To rename an existing column:
ALTER TABLE distributors RENAME COLUMN address TO city;
To rename an existing table:
ALTER TABLE distributors RENAME TO suppliers;
To rename an existing constraint:
ALTER TABLE distributors RENAME CONSTRAINT zipchk TO zip_check;
To add a not-null constraint to a column:
ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
To remove a not-null constraint from a column:
ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
To add a check constraint to a table and all its children:
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
To add a check constraint only to a table and not to its children:
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5) NO INHERIT;
(The check constraint will not be inherited by future children, either.)
To remove a check constraint from a table and all its children:
ALTER TABLE distributors DROP CONSTRAINT zipchk;
To remove a check constraint from one table only:
ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk;
(The check constraint remains in place for any child tables.)
To add a foreign key constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address);
To add a foreign key constraint to a table with the least impact on other work:
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID;ALTER TABLE distributors VALIDATE CONSTRAINT distfk;
To add a (multicolumn) unique constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
To add an automatically named primary key constraint to a table, noting that a table can only ever have one primary key:
ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
To move a table to a different tablespace:
ALTER TABLE distributors SET TABLESPACE fasttablespace;
To move a table to a different schema:
ALTER TABLE myschema.distributors SET SCHEMA yourschema;
To recreate a primary key constraint, without blocking updates while the index is rebuilt:
CREATE UNIQUE INDEX CONCURRENTLY dist_id_temp_idx ON distributors (dist_id);ALTER TABLE distributors DROP CONSTRAINT distributors_pkey, ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx;
Compatibility
The formsADD (withoutUSING INDEX),DROP,SET DEFAULT, andSET DATA TYPE (withoutUSING) conform with the SQL standard. The other forms arePostgreSQL extensions of the SQL standard. Also, the ability to specify more than one manipulation in a singleALTER TABLE command is an extension.
ALTER TABLE DROP COLUMN can be used to drop the only column of a table, leaving a zero-column table. This is an extension of SQL, which disallows zero-column tables.