Modified files for MERGE

1 files changed, 10 insertions, 0 deletions

diff --git a/src/backend/executor/README b/src/backend/executor/README
index 0d7cd552eb6..05769772b77 100644
--- a/src/backend/executor/README
+++ b/src/backend/executor/README
@@ -37,6 +37,16 @@ the plan tree returns the computed tuples to be updated, plus a "junk"
 one.  For DELETE, the plan tree need only deliver a CTID column, and the
 ModifyTable node visits each of those rows and marks the row deleted.
 
+MERGE runs one generic plan that returns candidate target rows. Each row
+consists of a super-row that contains all the columns needed by any of the
+individual actions, plus a CTID and a TABLEOID junk columns. The CTID column is
+required to know if a matching target row was found or not and the TABLEOID
+column is needed to find the underlying target partition, in case when the
+target table is a partition table. If the CTID column is set we attempt to
+activate WHEN MATCHED actions, or if it is NULL then we will attempt to
+activate WHEN NOT MATCHED actions. Once we know which action is activated we
+form the final result row and apply only those changes.
+
 XXX a great deal more documentation needs to be written here...