UPDATE statements may be replicated as DELETE/INSERT pairs
This article describes that Update statements may be replicated as DELETE/INSERT pairs.
Original product version: SQL Server
Original KB number: 238254
If any column that is part of a unique constraint is updated, then SQL Server implements the update as a "deferred update", which means as a pair of
INSERT operations. This "deferred update" causes replication to send a pair of
INSERT statements to the subscribers. There are also other situations that might cause a deferred update. Therefore, any business logic that you implement in your
UPDATE triggers or custom stored procedures at the Subscriber should also be included in the
INSERT triggers or custom stored procedures.
The default behavior in transactional replication is to use
DELETE custom stored procedures to apply changes at the subscribers.
INSERT statements made at the Publisher are applied to subscribers through an
INSERT stored procedure call. Similarly, a
DELETE statement is applied through a
DELETE stored procedure call.
However, when an
UPDATE statement is executed as a "deferred update", the logreader agent places a pair of
INSERT stored procedure calls in the distribution database to be applied to the Subscribers rather than an update stored procedure call. For example, suppose you have a publishing table, named
TABLE1, with these three columns:
- col1 int
- col2 int
- col3 varchar(30)
The only unique constraint on
TABLE1 is defined on
col1 through a primary key constraint. Assume that you have one record (1,1,'Dallas').
When you execute this code:
UPDATE TABLE1 set col1 = 3 where col3 = 'Dallas'
UPDATE statement is implemented by SQL Server as a pair of
INSERT statements since you are updating
col1, which has a unique index defined. Thus, the logreader places a pair of
INSERT calls in the distribution database. This can impact any business logic that is present in the triggers or custom stored procedures at the Subscriber. You should incorporate the additional business logic in
INSERT triggers or stored procedures to handle this situation.
If you prefer to use single logic and you want all your
UPDATE commands replicated as
INSERT pairs, you can enable a trace flag.
Additionally, if you use a horizontal filter in your publication and if the updated row does not meet a filter condition, only a
DELETE procedure call is sent to the subscribers. If the updated row previously did not meet the filter condition but meets the condition after the update, only the
INSERT procedure call is sent through the replication process.
In the preceding example, assume that you also have a horizontal filter defined on
where col3 = 'Dallas'. If you execute this code:
UPDATE table1 set col3 = 'New York' where col1 = 3
the logreader agent only places a
DELETE stored procedure call to be applied to the subscribers since the updated row does not meet the horizontal filter criteria.
Now, if you execute this code:
UPDATE table1 set col3 = 'Dallas' where col1 = 3
the logreader generates only the
INSERT stored procedure call, since the row did not previously meet the filter condition.
UPDATE operation was performed at the Publisher, only the appropriate commands are applied at the Subscriber.