Avoid unnecessary streaming of transactions during logical replication.

After restart, we don't perform streaming of an in-progress transaction if
it was previously decoded and confirmed by the client. To achieve that we
were comparing the END location of the WAL record being decoded with the
WAL location we have already decoded and confirmed by the client. While
decoding the commit record, to decide whether to process and send the
complete transaction, we compare its START location with the WAL location
we have already decoded and confirmed by the client. Now, if we need to
queue some change in the transaction while decoding the commit record
(e.g. snapshot), it is possible that we decide to stream the transaction
but later commit processing decides to skip it. In such a case, we would
needlessly send the changes and later when we decide to skip it, we will
send stream abort.

We also sometimes decide to stream the changes when we actually just need
to process them locally like a change for invalidations. This will lead us
to send empty streams. To avoid this, while queuing each change for
decoding, we remember whether the transaction has any change that actually
needs to be sent downstream and use that information later to decide
whether to stream the transaction or not.

Note, we can't avoid all cases where we have to send empty streams like
the case where the plugin later decides that the change is not
publishable. However, we will no longer need to send stream_abort when we
skip sending a particular transaction.

Author: Dilip Kumar
Reviewed-by: Hou Zhijie, Ashutosh Bapat, Shi yu, Amit Kapila
Discussion: https://postgr.es/m/CAFiTN-tHK=7LzfrPs8fbT2ksrOJGQbzywcgXst2bM9-rJJAAUg@mail.gmail.com

1 files changed, 15 insertions, 8 deletions

diff --git a/src/include/replication/reorderbuffer.h b/src/include/replication/reorderbuffer.h
index b23d8cc4f9f..c700b55b1c0 100644
--- a/src/include/replication/reorderbuffer.h
+++ b/src/include/replication/reorderbuffer.h
@@ -168,14 +168,15 @@ typedef struct ReorderBufferChange
 } ReorderBufferChange;
 
 /* ReorderBufferTXN txn_flags */
-#define RBTXN_HAS_CATALOG_CHANGES 0x0001
-#define RBTXN_IS_SUBXACT          0x0002
-#define RBTXN_IS_SERIALIZED       0x0004
-#define RBTXN_IS_SERIALIZED_CLEAR 0x0008
-#define RBTXN_IS_STREAMED         0x0010
-#define RBTXN_HAS_PARTIAL_CHANGE  0x0020
-#define RBTXN_PREPARE             0x0040
-#define RBTXN_SKIPPED_PREPARE	  0x0080
+#define RBTXN_HAS_CATALOG_CHANGES 	0x0001
+#define RBTXN_IS_SUBXACT          	0x0002
+#define RBTXN_IS_SERIALIZED       	0x0004
+#define RBTXN_IS_SERIALIZED_CLEAR 	0x0008
+#define RBTXN_IS_STREAMED         	0x0010
+#define RBTXN_HAS_PARTIAL_CHANGE  	0x0020
+#define RBTXN_PREPARE             	0x0040
+#define RBTXN_SKIPPED_PREPARE	  	0x0080
+#define RBTXN_HAS_STREAMABLE_CHANGE	0x0100
 
 /* Does the transaction have catalog changes? */
 #define rbtxn_has_catalog_changes(txn) \
@@ -207,6 +208,12 @@ typedef struct ReorderBufferChange
 	((txn)->txn_flags & RBTXN_HAS_PARTIAL_CHANGE) != 0 \
 )
 
+/* Does this transaction contain streamable changes? */
+#define rbtxn_has_streamable_change(txn) \
+( \
+	((txn)->txn_flags & RBTXN_HAS_STREAMABLE_CHANGE) != 0 \
+)
+
 /*
  * Has this transaction been streamed to downstream?
  *