MERGE post-commit review

Review comments from Andres Freund

* Consolidate code into AfterTriggerGetTransitionTable()
* Rename nodeMerge.c to execMerge.c
* Rename nodeMerge.h to execMerge.h
* Move MERGE handling in ExecInitModifyTable()
  into a execMerge.c ExecInitMerge()
* Move mt_merge_subcommands flags into execMerge.h
* Rename opt_and_condition to opt_merge_when_and_condition
* Wordsmith various comments

Author: Pavan Deolasee
Reviewer: Simon Riggs

1 files changed, 6 insertions, 5 deletions

diff --git a/src/backend/executor/README b/src/backend/executor/README
index 05769772b77..7882ce44e78 100644
--- a/src/backend/executor/README
+++ b/src/backend/executor/README
@@ -39,13 +39,14 @@ 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
+individual actions, plus CTID and 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.
+target table is a partition table. When a matching target tuple is found, the
+CTID column identifies the matching target tuple and we attempt to activate
+WHEN MATCHED actions. If a matching tuple is not found, then CTID column is
+NULL and we 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...