This results in reconstructing each row with the dropped column replaced by a null value. The default (and recommended) setting of constraint_exclusion is actually neither on nor off, but an intermediate setting called partition, which causes the technique to be applied only to queries that are likely to be working on partitioned tables. your experience with the particular feature or requires further clarification, When the planner can prove this, it excludes the partition from the query plan. The choice of how to partition a table should be made carefully as the performance of query planning and execution can be negatively affected by poor design. Automatically drop objects that depend on the dropped column or constraint (for example, views referencing the column), and in turn all objects that depend on those objects (see Section 5.14). Ensure that the constraint_exclusion configuration parameter is not disabled in postgresql.conf. As we can see, a complex partitioning scheme could require a substantial amount of DDL. It is not necessary to create table constraints describing partition boundary condition for partitions. Scanning a large table to verify a new foreign key or check constraint can take a long time, and other updates to the table are locked out until the ALTER TABLE ADD CONSTRAINT command is committed. To alter the owner, you must also be a direct or indirect member of the new owning role, and that role must have CREATE privilege on the table's schema. Removal of unwanted data is also a factor to consider when planning your partitioning strategy. To remove old data quickly, simply drop the partition that is no longer necessary: To remove the partition from the partitioned table but retain access to it as a table in its own right: To add a new partition to handle new data, create an empty partition just as the original partitions were created above: Alternatively, one may want to create the new table outside the partition structure, and make it a partition after the data is loaded, checked, and transformed. This is particularly true for the UPDATE and DELETE commands. This configuration is ignored for ON SELECT rules, which are always applied in order to keep views working even if the current session is in a non-default replication role. Therefore it isn't necessary to define indexes on the key columns. We want our application to be able to say INSERT INTO measurement ... and have the data be redirected into the appropriate partition table. If it is, queries will not be optimized as desired. This form resets one or more storage parameters to their defaults. Just as with declarative partitioning, these partitions are in every way normal PostgreSQL tables (or foreign tables). See Section 68.2 for more information. Similarly we can add a new partition to handle new data. A disabled trigger is still known to the system, but is not executed when its triggering event occurs. A USING clause must be provided if there is no implicit or assignment cast from old to new type. 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. To implement partitioning using inheritance, use the following steps: Create the “master” table, from which all of the partitions will inherit. Because of this flexibility, the USING 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. In all other cases, this is a fast operation. The index cannot have expression columns nor be a partial index. Currently supported partitioning methods include range and list, where each partition is assigned a range of keys and a list of keys, respectively. This form changes the table from unlogged to logged or vice-versa (see UNLOGGED). This form attaches an existing table (which might itself be partitioned) as a partition of the target table. PostgreSQL offers built-in support for the following forms of partitioning: The table is partitioned into “ranges” defined by a key column or set of columns, with no overlap between the ranges of values assigned to different partitions. Refer to CREATE TABLE for a further description of valid parameters. This form removes the target table from the list of children of the specified parent table. If data will be added only to the latest partition, we can use a very simple trigger function: After creating the function, we create a trigger which calls the trigger function: We must redefine the trigger function each month so that it always points to the current partition. When a table has a default partition, defining a new partition changes the partition constraint for the default partition. All tables in the current database in a tablespace can be moved by using the ALL IN TABLESPACE form, which will lock all tables to be moved first and then move each one. The table to be attached must have all the same columns as the target table and no more; moreover, the column types must also match. Do not throw an error if the table does not exist. This form moves the table into another schema. If disabled (the default) then row level security will not be applied when the user is the table owner. If IF NOT EXISTS is specified and a column already exists with this name, no error is thrown. 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. Do not define any check constraints on this table, unless you intend them to be applied equally to all partitions. An UPDATE that attempts to do that will fail because of the CHECK constraints. The user name of the new owner of the table. It is possible to specify a tablespace and storage parameters for each partition separately. If you see anything in the documentation that is not correct, does not match The table's list of column names and types must precisely match that of the composite type. We can arrange that by attaching a suitable trigger function to the master table. The columns must have matching data types, and if they have NOT NULL constraints in the parent then they must also have NOT NULL constraints in the child. That requires a full table scan to verify the column(s) contain no nulls. A nonrecursive DROP COLUMN (i.e., ALTER TABLE ONLY ... DROP COLUMN) never removes any descendant columns, but instead marks them as independently defined rather than inherited. Simply enabled triggers (the default) will fire when the replication role is “origin” (the default) or “local”. An ACCESS EXCLUSIVE lock is acquired unless explicitly noted. As an example: Without constraint exclusion, the above query would scan each of the partitions of the measurement table. However, it is possible to add a regular or partitioned table containing data as a partition of a partitioned table, or remove a partition from a partitioned table turning it into a standalone table; see ALTER TABLE to learn more about the ATTACH PARTITION and DETACH PARTITION sub-commands. This section describes why and how to implement partitioning as part of your database design. As an exception, when changing the type of an existing column, if the USING 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. The DROP COLUMN form does not physically remove the column, but simply makes it invisible to SQL operations. Be aware that COPY ignores rules. 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. Copyright © 1996-2021 The PostgreSQL Global Development Group, PostgreSQL 13.2, 12.6, 11.11, 10.16, 9.6.21, & 9.5.25 Released. Subsequently, queries against the parent will include records of the target table. 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 regular ADD PRIMARY KEY or ADD UNIQUE command. Ensure that the constraints guarantee that there is no overlap between the key values permitted in different partitions. This controls whether this column is held inline or in a secondary TOAST table, and whether the data should be compressed or not. This form removes the most recently used CLUSTER index specification from the table. For example, data inserted into the partitioned table is not routed to foreign table partitions. All rows inserted into a partitioned table will be routed to one of the partitions based on the value of the partition key. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2. For example, it is possible to add several columns and/or alter the type of several columns in a single command. Never assume that more partitions are better than fewer partitions and vice-versa. CHECK constraints that are marked NO INHERIT are not allowed to be created on partitioned tables. Indexes must be created separately for each partition. However, dividing the table into too many partitions can also cause issues. That way, the system will be able to skip the scan to validate the implicit partition constraint. To add a foreign key constraint to a table: To add a foreign key constraint to a table with the least impact on other work: To add a (multicolumn) unique constraint to a table: To add an automatically named primary key constraint to a table, noting that a table can only ever have one primary key: To move a table to a different tablespace: To recreate a primary key constraint, without blocking updates while the index is rebuilt: To attach a partition to a range-partitioned table: To attach a partition to a list-partitioned table: To attach a partition to a hash-partitioned table: To attach a default partition to a partitioned table: To detach a partition from a partitioned table: The forms ADD (without USING INDEX), DROP [COLUMN], DROP IDENTITY, RESTART, SET DEFAULT, SET DATA TYPE (without USING), SET GENERATED, and SET sequence_option conform with the SQL standard. This is the default behavior. It cannot be applied to a temporary table. Also, because selecting from the parent also selects from its descendants, a constraint on the parent cannot be marked valid unless it is also marked valid for those descendants. It is possible to avoid this scan by adding a valid CHECK constraint to the table that allows only rows satisfying the desired partition constraint before running this command. After that, a VALIDATE CONSTRAINT command can be issued to verify that existing rows satisfy the constraint. Declarative Partitioning Best Practices. The query planner is generally able to handle partition hierarchies with up to a few hundred partitions. Keep the partitioning constraints simple, else the planner may not be able to prove that partitions don't need to be visited. To drop the NOT NULL constraint from all the partitions, perform DROP NOT NULL on the parent table. If the new partition is a regular table, a full table scan is performed to check that existing rows in the table do not violate the partition constraint. partition_column_name is the name of a partition column. Guide to PostgreSQL Commands. Similarly, when attaching a new partition it may be scanned to verify that existing rows meet the partition constraint. That can be done with VACUUM FULL, CLUSTER or one of the forms of ALTER TABLE that forces a table rewrite. (The key index is not strictly necessary, but in most scenarios it is helpful. If your application needs to use other forms of partitioning not listed above, alternative methods such as inheritance and UNION ALL views can be used instead. Since primary keys are not supported on partitioned tables, foreign keys referencing partitioned tables are not supported, nor are foreign key references from a partitioned table to some other table. This form sets the storage mode for a column. Like SET DEFAULT, these forms only affect the behavior of subsequent INSERT and UPDATE commands; they do not cause rows already in the table to change. Triggers may be complicated to write, and will be much slower than the tuple routing performed internally by declarative partitioning. (That restriction does not apply to index-based constraints, however.) The main purpose of the NOT VALID constraint option is to reduce the impact of adding a constraint on concurrent updates. If a constraint name is provided then the index will be renamed to match the constraint name. Note that SET STORAGE doesn't itself change anything in the table, it just sets the strategy to be pursued during future table updates. The other forms are PostgreSQL extensions of the SQL standard. For a deferred trigger, the enable status is checked when the event occurs, not when the trigger function is actually executed. Sub-partitioning can be useful to further divide partitions that are expected to become larger than other partitions, although excessive sub-partitioning can easily lead to large numbers of partitions and can cause the same problems mentioned in the preceding paragraph. Once partitions exist, using ONLY will result in an error as adding or dropping constraints on only the partitioned table, when partitions exist, is not supported. this form In all of these cases, ALTER TABLE ONLY will be rejected. Instead, constraints can be added or dropped, when they are not present in the parent table, directly on the partitions. This form dissociates a typed table from its type. Partitions thus created are in every way normal PostgreSQL tables (or, possibly, foreign tables). Normally the set of partitions established when initially defining the table is not intended to remain static. You cannot drop the NOT NULL constraint on a partition's column if the constraint is present in the parent table. It would be better to instead create partitions as follows: For each partition, create an index on the key column(s), as well as any other indexes you might want. In the above example we would be creating a new partition each month, so it might be wise to write a script that generates the required DDL automatically. This form sets the per-column statistics-gathering target for subsequent ANALYZE operations. However, a superuser can alter ownership of any table anyway.) See also CREATE TABLESPACE. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2. FULL records the old values of all columns in the row. It is common to want to remove old partitions of data and periodically add new partitions for new data. A nonrecursive DROP COLUMN command will fail for a partitioned table, because all partitions of a table must have the same columns as the partitioning root. For example, if a user_name or host_name value in an account name is legal as an unquoted identifier, you need not quote it. This form adds a new column to the table, using the same syntax as CREATE TABLE. See CREATE TABLE for more details on creating partitioned tables and partitions. (These restrictions enforce that altering the owner doesn't do anything you couldn't do by dropping and recreating the table. Adding a CHECK or NOT NULL constraint requires scanning the table to verify that existing rows meet the constraint, but does not require a table rewrite. ATTACH PARTITION only if their columns exactly match the parent, including any oid column. 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. Declarative partitioning only supports list and range partitioning, whereas table inheritance allows data to be divided in a manner of the user's choosing. Partitions cannot have columns that are not present in the parent. If you were to try to run "SELECT FROM table WHERE column IS NULL", then Postgresql wouldn't use the index anyway, because it would be faster to just read sequentially through the entire table and filter out the 1% that don't match. SHARE UPDATE EXCLUSIVE lock will be taken for fillfactor, toast and autovacuum storage parameters, as well as the planner parameter parallel_workers. Suppose we are constructing a database for a large ice cream company. The optional USING 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. For planner related parameters, changes will take effect from the next time the table is locked so currently executing queries will not be affected. For example, it is possible to add several columns and/or alter the type of several columns in a single command. Changing any part of a system catalog table is not permitted. All the forms of ALTER TABLE that act on a single table, except RENAME, SET SCHEMA, ATTACH PARTITION, and DETACH PARTITION can be combined into a list of multiple alterations to be applied together. If you see anything in the documentation that is not correct, does not match We might want to insert data and have the server automatically locate the partition into which the row should be added. The on setting causes the planner to examine CHECK constraints in all queries, even simple ones that are unlikely to benefit. Currently, the only defined per-attribute options are n_distinct and n_distinct_inherited, which override the number-of-distinct-values estimates made by subsequent ANALYZE operations. The partition bound specification for a new partition. Disable or enable all triggers belonging to the table. All the forms of ALTER TABLE that act on a single table, except RENAME, SET SCHEMA, ATTACH PARTITION, and DETACH PARTITION can be combined into a list of multiple alterations to be applied together. It is safer to create code that generates partitions and creates and/or modifies associated objects than to write each by hand. Whether an index needs to be created for a given partition depends on whether you expect that queries that scan the partition will generally scan a large part of the partition or just a small part. You can use the EXPLAIN command to show the difference between a plan with constraint_exclusion on and a plan with it off. A notice is issued in this case. This affects future cluster operations that don't specify an index. Note that system catalogs are not moved by this command; use ALTER DATABASE or explicit ALTER TABLE invocations instead if desired. A different approach to redirecting inserts into the appropriate partition table is to set up rules, instead of a trigger, on the master table. Of course, this will often result in a larger number of partitions, each of which is individually smaller. PostgreSQL offers a way to specify how to divide a table into pieces called partitions. If enabled, row level security policies will be applied when the user is the table owner. table_references indicates the table or tables from which to retrieve rows. You must own the table to use ALTER TABLE. In this case a notice is issued instead. These forms configure the firing of rewrite rules belonging to the table. Hence, if the partitioned table is permanent, so must be its partitions and likewise if the partitioned table is temporary. If DROP IDENTITY IF EXISTS is specified and the column is not an identity column, no error is thrown. (If the constraint is a foreign key then a ROW SHARE lock is also required on the table referenced by the constraint.) As oid system columns cannot be added anymore, this never has an effect. One of the most critical design decisions will be the column or columns by which you partition your data. This is useful because if a trigger is used on the origin to propagate data between tables, then the replication system will also replicate the propagated data, and the trigger should not fire a second time on the replica, because that would lead to duplication. For example, one might partition by date ranges, or by ranges of identifiers for particular business objects. external_table_option_list specifies a list of options for creating the external table. To add a column or alter a column type or use the OF clause, you must also have USAGE privilege on the data type. The specification consists of the partitioning method and a list of columns or expressions to be used as the partition key. Also, to attach a table as a new partition of the table, you must own the table being attached. Note that each IF test must exactly match the CHECK constraint for its partition. Fig. to report a documentation issue. It might also be a useful time to aggregate data into smaller formats, perform other data manipulations, or run reports. Name of a single trigger to disable or enable. There must also be matching child-table constraints for all CHECK constraints of the parent, except those marked non-inheritable (that is, created with ALTER TABLE ... ADD CONSTRAINT ... NO INHERIT) in the parent, which are ignored; all child-table constraints matched must not be marked non-inheritable. COPY does fire triggers, so you can use it normally if you use the trigger approach. A command like: INSERT statements with ON CONFLICT clauses are unlikely to work as expected, as the ON CONFLICT action is only taken in case of unique violations on the specified target relation, not its child relations. This form alters the attributes of a constraint that was previously created. To change the schema or tablespace of a table, you must also have CREATE privilege on the new schema or tablespace. In such cases, drop the default with DROP DEFAULT, perform the ALTER TYPE, and then use SET DEFAULT to add a suitable new default. An UPDATE that causes a row to move from one partition to another fails, because the new value of the row fails to satisfy the implicit partition constraint of the original partition. The following caveats apply to constraint exclusion, which is used by both inheritance and partitioned tables: Constraint exclusion only works when the query's WHERE clause contains constants (or externally supplied parameters). SET NOT NULL may only be applied to a column provided none of the records in the table contain a NULL value for the column. Seldom-used data can be migrated to cheaper and slower storage media. A common mistake is to set up range constraints like: This is wrong since it is not clear which partition the key value 200 belongs in. Queries against the parent table will no longer include records drawn from the target table. This command acquires a SHARE ROW EXCLUSIVE lock. 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. Another option that is often preferable is to remove the partition from the partitioned table but retain access to it as a table in its own right: This allows further operations to be performed on the data before it is dropped. A good rule of thumb is that partitioning constraints should contain only comparisons of the partitioning column(s) to constants using B-tree-indexable operators, which applies even to partitioned tables, because only B-tree-indexable column(s) are allowed in the partition key. The actions for identity columns (ADD GENERATED, SET etc., DROP IDENTITY), as well as the actions TRIGGER, CLUSTER, OWNER, and TABLESPACE never recurse to descendant tables; that is, they always act as though ONLY were specified. Indexes on the table, if any, are not moved; but they can be moved separately with additional SET TABLESPACE commands. The table can be attached as a partition for specific values using FOR VALUES or as a default partition by using DEFAULT. Constraint exclusion is a query optimization technique that improves performance for partitioned tables defined in the fashion described above (both declaratively partitioned tables and those implemented using inheritance). You will need to say CASCADE if anything outside the table depends on the column, for example, foreign key references or views. For example, consider a table range partitioned using columns lastname and firstname (in that order) as the partition key. EXTERNAL is for external, uncompressed data, and EXTENDED is for external, compressed data. This table will contain no data. 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. See also CREATE POLICY. For simplicity we have shown the trigger's tests in the same order as in other parts of this example. A recursive DROP 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. In this case a notice is issued instead. Using ONLY to add or drop a constraint on only the partitioned table is supported when there are no partitions. The simplest option for removing old data is to drop the partition that is no longer necessary: This can very quickly delete millions of records because it doesn't have to individually delete every record. PostgreSQL SET command used to change the runtime configuration parameter from database server, there are multiple parameter which we have change during runtime using set command in PostgreSQL. Add non-overlapping table constraints to the partition tables to define the allowed key values in each partition. This form changes one or more storage parameters for the table. The USING option of SET 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 is the default for system tables.) Each partition has a subset of the data defined by its partition bounds. The following caveats apply to partitioned tables implemented using inheritance: There is no automatic way to verify that all of the CHECK constraints are mutually exclusive. If DROP EXPRESSION IF EXISTS is specified and the column is not a stored generated column, no error is thrown. Note that specifying bounds such that the new partition's values will overlap with those in one or more existing partitions will cause an error. Partitioning can provide several benefits: Query performance can be improved dramatically in certain situations, particularly when most of the heavily accessed rows of the table are in a single partition or a small number of partitions. We use the non-partitioned measurement table above. When queries or updates access a large percentage of a single partition, performance can be improved by taking advantage of sequential scan of that partition instead of using an index and random access reads scattered across the whole table. When we enable constraint exclusion, we get a significantly cheaper plan that will deliver the same answer: Note that constraint exclusion is driven only by CHECK constraints, not by the presence of indexes. Include this field if your external data uses a hive-partitioned layout. (Note, however, that if constraint exclusion is unable to prune partitions effectively, query performance will be very poor.). SET STATISTICS acquires a SHARE UPDATE EXCLUSIVE lock. Using the ON CONFLICT clause with partitioned tables will cause an error, because unique or exclusion constraints can only be created on individual partitions. There is no support for enforcing uniqueness (or an exclusion constraint) across an entire partitioning hierarchy. See Storage Parameters in the CREATE TABLE documentation for details on the available parameters. Table inheritance allows for multiple inheritance. For example, a partition cannot have any parents other than the partitioned table it is a partition of, nor can a regular table inherit from a partitioned table making the latter its parent. The expression must return a single value. this form 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 effects are the same as if the two sub-commands had been issued in separate ALTER TABLE commands. Some operations require a stronger lock when using declarative partitioning than when using table inheritance. Simulations of the intended workload are often beneficial for optimizing the partitioning strategy. Another reason to be concerned about having a large number of partitions is that the server's memory consumption may grow significantly over a period of time, especially if many sessions touch large numbers of partitions. It may be desired to drop the redundant CHECK constraint after ATTACH PARTITION is finished. In particular, dropping the constraint will make the index disappear too. There is no point in defining any indexes or unique constraints on it, either. The space will be reclaimed over time as existing rows are updated. The table must not inherit from any other table. In this case a notice is issued instead. To force immediate reclamation of space occupied by a dropped column, you can execute one of the forms of ALTER TABLE that performs a rewrite of the whole table. When choosing how to partition your table, it's also important to consider what changes may occur in the future. These restrictions ensure that the index is equivalent to one that would be built by a regular ADD PRIMARY KEY or ADD UNIQUE command.