diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 341fea524a..6b02590971 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -359,8 +359,8 @@
behaves similarly. In Read Committed mode, each row proposed for insertion
will either insert or update. Unless there are unrelated errors, one of
those two outcomes is guaranteed. If a conflict originates in another
- transaction whose effects are not yet visible to the INSERT
- , the UPDATE clause will affect that row,
+ transaction whose effects are not yet visible to the INSERT,
+ the UPDATE clause will affect that row,
even though possibly no version of that row is
conventionally visible to the command.
@@ -373,6 +373,37 @@
the case in Read Committed mode.
+
+ MERGE allows the user to specify various
+ combinations of INSERT, UPDATE
+ and DELETE subcommands. A MERGE
+ command with both INSERT and UPDATE
+ subcommands looks similar to INSERT with an
+ ON CONFLICT DO UPDATE clause but does not
+ guarantee that either INSERT or
+ UPDATE will occur.
+ If MERGE attempts an UPDATE or
+ DELETE and the row is concurrently updated but
+ the join condition still passes for the current target and the
+ current source tuple, then MERGE will behave
+ the same as the UPDATE or
+ DELETE commands and perform its action on the
+ updated version of the row. However, because MERGE
+ can specify several actions and they can be conditional, the
+ conditions for each action are re-evaluated on the updated version of
+ the row, starting from the first action, even if the action that had
+ originally matched appears later in the list of actions.
+ On the other hand, if the row is concurrently updated or deleted so
+ that the join condition fails, then MERGE will
+ evaluate the condition's NOT MATCHED actions next,
+ and execute the first one that succeeds.
+ If MERGE attempts an INSERT
+ and a unique index is present and a duplicate row is concurrently
+ inserted, then a uniqueness violation error is raised;
+ MERGE does not attempt to avoid such
+ errors by evaluating MATCHED conditions.
+
+
Because of the above rules, it is possible for an updating command to see
an inconsistent snapshot: it can see the effects of concurrent updating
@@ -422,37 +453,6 @@ COMMIT;
11, which no longer matches the criteria.
-
- MERGE allows the user to specify various
- combinations of INSERT, UPDATE
- or DELETE subcommands. A MERGE
- command with both INSERT and UPDATE
- subcommands looks similar to INSERT with an
- ON CONFLICT DO UPDATE clause but does not
- guarantee that either INSERT or
- UPDATE will occur.
- If MERGE attempts an UPDATE or
- DELETE and the row is concurrently updated but
- the join condition still passes for the current target and the
- current source tuple, then MERGE will behave
- the same as the UPDATE or
- DELETE commands and perform its action on the
- updated version of the row. However, because MERGE
- can specify several actions and they can be conditional, the
- conditions for each action are re-evaluated on the updated version of
- the row, starting from the first action, even if the action that had
- originally matched appears later in the list of actions.
- On the other hand, if the row is concurrently updated or deleted so
- that the join condition fails, then MERGE will
- evaluate the condition's NOT MATCHED actions next,
- and execute the first one that succeeds.
- If MERGE attempts an INSERT
- and a unique index is present and a duplicate row is concurrently
- inserted, then a uniqueness violation is raised.
- MERGE does not attempt to avoid the
- error by executing an UPDATE.
-
-
Because Read Committed mode starts each command with a new snapshot
that includes all transactions committed up to that instant,
@@ -516,8 +516,9 @@ COMMIT;
- UPDATE, DELETE, SELECT
- FOR UPDATE, and SELECT FOR SHARE commands
+ UPDATE, DELETE,
+ MERGE, SELECT FOR UPDATE,
+ and SELECT FOR SHARE commands
behave the same as SELECT
in terms of searching for target rows: they will only find target rows
that were committed as of the transaction start time. However, such a
diff --git a/doc/src/sgml/ref/merge.sgml b/doc/src/sgml/ref/merge.sgml
index f68aa09736..f0745d01c7 100644
--- a/doc/src/sgml/ref/merge.sgml
+++ b/doc/src/sgml/ref/merge.sgml
@@ -539,6 +539,10 @@ MERGE total_count
+ When MERGE is run concurrently with other commands
+ that modify the target table, the usual transaction isolation rules
+ apply; see for an explanation
+ on the behavior at each isolation level.
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