MERGE

Description

MERGE performs actions that modify rows in the target table identified as target_table_name, using the data_source. MERGE provides a single SQL statement that can conditionally INSERT, UPDATE or DELETE rows, a task that would otherwise require multiple procedural language statements.

First, the MERGE command performs a join from data_source to the target table producing zero or more candidate change rows. For each candidate change row, the status of MATCHED or NOT MATCHED is set just once, after which WHEN clauses are evaluated in the order specified. For each candidate change row, the first clause to evaluate as true is executed. No more than one WHEN clause is executed for any candidate change row.

MERGE actions have the same effect as regular UPDATE, INSERT, or DELETE commands of the same names. The syntax of those commands is different, notably that there is no WHERE clause and no table name is specified. All actions refer to the target table, though modifications to other tables may be made using triggers.

When DO NOTHING is specified, the source row is skipped. Since actions are evaluated in their specified order, DO NOTHING can be handy to skip non-interesting source rows before more fine-grained handling.

There is no separate MERGE privilege. If you specify an update action, you must have the UPDATE privilege on the column(s) of the target table that are referred to in the SET clause. If you specify an insert action, you must have the INSERT privilege on the target table. If you specify a delete action, you must have the DELETE privilege on the target table. If you specify a DO NOTHING action, you must have the SELECT privilege on at least one column of the target table. You will also need SELECT privilege on any column(s) of the data_source and of the target table referred to in any condition (including join_condition) or expression. Privileges are tested once at statement start and are checked whether or not particular WHEN clauses are executed.

MERGE is not supported if the target table is a materialized view, foreign table, or if it has any rules defined on it.

Parameters

with_query

The WITH clause allows you to specify one or more subqueries that can be referenced by name in the MERGE query. See Section 7.8 and SELECT for details. Note that WITH RECURSIVE is not supported by MERGE.

target_table_name

The name (optionally schema-qualified) of the target table to merge into. If ONLY is specified before the table name, matching rows are updated or deleted in the named table only. If ONLY is not specified, matching rows are also updated or deleted in any tables inheriting from the named table. Optionally,  can be specified after the table name to explicitly indicate that descendant tables are included. The ONLY keyword and  option do not affect insert actions, which always insert into the named table only.

target_alias

A substitute name for the target table. When an alias is provided, it completely hides the actual name of the table. For example, given MERGE INTO foo AS f, the remainder of the MERGE statement must refer to this table as f not foo.

source_table_name

The name (optionally schema-qualified) of the source table, view, or transition table. If ONLY is specified before the table name, matching rows are included from the named table only. If ONLY is not specified, matching rows are also included from any tables inheriting from the named table. Optionally, ` can be specified after the table name to explicitly indicate that descendant tables are included.

source_query

A query (SELECT statement or VALUES statement) that supplies the rows to be merged into the target table. Refer to the SELECT statement or VALUES statement for a description of the syntax.

source_alias

A substitute name for the data source. When an alias is provided, it completely hides the actual name of the table or the fact that a query was issued.

join_condition

join_condition is an expression resulting in a value of type boolean (similar to a WHERE clause) that specifies which rows in the data_source match rows in the target table.

Warning

Only columns from the target table that attempt to match data_source rows should appear in join_condition. join_condition subexpressions that only reference the target table's columns can affect which action is taken, often in surprising ways.

when_clause

At least one WHEN clause is required.

If the WHEN clause specifies WHEN MATCHED and the candidate change row matches a row in the target table, the WHEN clause is executed if the condition is absent or it evaluates to true.

Conversely, if the WHEN clause specifies WHEN NOT MATCHED and the candidate change row does not match a row in the target table, the WHEN clause is executed if the condition is absent or it evaluates to true.

condition

An expression that returns a value of type boolean. If this expression for a WHEN clause returns true, then the action for that clause is executed for that row.

A condition on a WHEN MATCHED clause can refer to columns in both the source and the target relations. A condition on a WHEN NOT MATCHED clause can only refer to columns from the source relation, since by definition there is no matching target row. Only the system attributes from the target table are accessible.

merge_insert

The specification of an INSERT action that inserts one row into the target table. The target column names can be listed in any order. If no list of column names is given at all, the default is all the columns of the table in their declared order.

Each column not present in the explicit or implicit column list will be filled with a default value, either its declared default value or null if there is none.

If the target table is a partitioned table, each row is routed to the appropriate partition and inserted into it. If the target table is a partition, an error will occur if any input row violates the partition constraint.

Column names may not be specified more than once. INSERT actions cannot contain sub-selects.

Only one VALUES clause can be specified. The VALUES clause can only refer to columns from the source relation, since by definition there is no matching target row.

merge_update

The specification of an UPDATE action that updates the current row of the target table. Column names may not be specified more than once.

Neither a table name nor a WHERE clause are allowed.

merge_delete

Specifies a DELETE action that deletes the current row of the target table. Do not include the table name or any other clauses, as you would normally do with a DELETE command.

column_name

The name of a column in the target table. The column name can be qualified with a subfield name or array subscript, if needed. (Inserting into only some fields of a composite column leaves the other fields null.) Do not include the table's name in the specification of a target column.

OVERRIDING SYSTEM VALUE

Without this clause, it is an error to specify an explicit value (other than DEFAULT) for an identity column defined as GENERATED ALWAYS. This clause overrides that restriction.

OVERRIDING USER VALUE

If this clause is specified, then any values supplied for identity columns defined as GENERATED BY DEFAULT are ignored and the default sequence-generated values are applied.

DEFAULT VALUES

All columns will be filled with their default values. (An OVERRIDING clause is not permitted in this form.)

expression

An expression to assign to the column. If used in a WHEN MATCHED clause, the expression can use values from the original row in the target table, and values from the data_source row. If used in a WHEN NOT MATCHED clause, the expression can use values from the data_source row.

DEFAULT

Set the column to its default value (which will be NULL if no specific default expression has been assigned to it).

sub-SELECT

A SELECT sub-query that produces as many output columns as are listed in the parenthesized column list preceding it. The sub-query must yield no more than one row when executed. If it yields one row, its column values are assigned to the target columns; if it yields no rows, NULL values are assigned to the target columns. The sub-query can refer to values from the original row in the target table, and values from the data_source row.

Outputs

On successful completion, a MERGE command returns a command tag of the form

MERGE total_count

The total_count is the total number of rows changed (whether inserted, updated, or deleted). If total_count is 0, no rows were changed in any way.

Notes

The following steps take place during the execution of MERGE.

1. Perform any BEFORE STATEMENT triggers for all actions specified, whether or not their WHEN clauses match.

2. Perform a join from source to target table. The resulting query will be optimized normally and will produce a set of candidate change rows. For each candidate change row,

3. Evaluate whether each row is MATCHED or NOT MATCHED.

4. Test each WHEN condition in the order specified until one returns true.

5. When a condition returns true, perform the following actions:

6. Perform any BEFORE ROW triggers that fire for the action's event type.

7. Perform the specified action, invoking any check constraints on the target table.

8. Perform any AFTER ROW triggers that fire for the action's event type.

9. Perform any AFTER STATEMENT triggers for actions specified, whether or not they actually occur. This is similar to the behavior of an UPDATE statement that modifies no rows.

In summary, statement triggers for an event type (say, INSERT) will be fired whenever we specify an action of that kind. In contrast, row-level triggers will fire only for the specific event type being executed. So a MERGE command might fire statement triggers for both UPDATE and INSERT, even though only UPDATE row triggers were fired.

You should ensure that the join produces at most one candidate change row for each target row. In other words, a target row shouldn't join to more than one data source row. If it does, then only one of the candidate change rows will be used to modify the target row; later attempts to modify the row will cause an error. This can also occur if row triggers make changes to the target table and the rows so modified are then subsequently also modified by MERGE. If the repeated action is an INSERT, this will cause a uniqueness violation, while a repeated UPDATE or DELETE will cause a cardinality violation; the latter behavior is required by the SQL standard. This differs from historical PostgreSQL behavior of joins in UPDATE and DELETE statements where second and subsequent attempts to modify the same row are simply ignored.

If a WHEN clause omits an AND sub-clause, it becomes the final reachable clause of that kind (MATCHED or NOT MATCHED). If a later WHEN clause of that kind is specified it would be provably unreachable and an error is raised. If no final reachable clause is specified of either kind, it is possible that no action will be taken for a candidate change row.

The order in which rows are generated from the data source is indeterminate by default. A source_query can be used to specify a consistent ordering, if required, which might be needed to avoid deadlocks between concurrent transactions.

There is no RETURNING clause with MERGE. Actions of INSERT, UPDATE and DELETE cannot contain RETURNING or WITH clauses.

When MERGE is run concurrently with other commands that modify the target table, the usual transaction isolation rules apply. You may also wish to consider using INSERT ... ON CONFLICT as an alternative statement which offers the ability to run an UPDATE if a concurrent INSERT occurs. There are a variety of differences and restrictions between the two statement types and they are not interchangeable.

Examples

Perform maintenance on customer_accounts based upon new recent_transactions.

MERGE INTO customer_account ca
USING recent_transactions t
ON t.customer_id = ca.customer_id
WHEN MATCHED THEN
 UPDATE SET balance = balance + transaction_value
WHEN NOT MATCHED THEN
 INSERT (customer_id, balance)
 VALUES (t.customer_id, t.transaction_value);

Notice that this would be exactly equivalent to the following statement because the MATCHED result does not change during execution.

MERGE INTO customer_account ca
USING (SELECT customer_id, transaction_value FROM recent_transactions) AS t
ON t.customer_id = ca.customer_id
WHEN MATCHED THEN
 UPDATE SET balance = balance + transaction_value
WHEN NOT MATCHED THEN
 INSERT (customer_id, balance)
 VALUES (t.customer_id, t.transaction_value);

Attempt to insert a new stock item along with the quantity of stock. If the item already exists, instead update the stock count of the existing item. Don't allow entries that have zero stock.

MERGE INTO wines w
USING wine_stock_changes s
ON s.winename = w.winename
WHEN NOT MATCHED AND s.stock_delta > 0 THEN
 INSERT VALUES(s.winename, s.stock_delta)
WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN
 UPDATE SET stock = w.stock + s.stock_delta
WHEN MATCHED THEN
 DELETE;

The wine_stock_changes table might be, for example, a temporary table recently loaded into the database.