Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

1 files changed, 0 insertions, 36 deletions

diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index b379c76e273..d89021f9187 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -250,8 +250,6 @@ extern void AbortBufferIO(Buffer buffer);
 
 extern bool BgBufferSync(struct WritebackContext *wb_context);
 
-extern void TestForOldSnapshot_impl(Snapshot snapshot, Relation relation);
-
 /* in buf_init.c */
 extern void InitBufferPool(void);
 extern Size BufferShmemSize(void);
@@ -347,9 +345,6 @@ BufferGetPageSize(Buffer buffer)
 /*
  * BufferGetPage
  *		Returns the page associated with a buffer.
- *
- * When this is called as part of a scan, there may be a need for a nearby
- * call to TestForOldSnapshot().  See the definition of that for details.
  */
 static inline Page
 BufferGetPage(Buffer buffer)
@@ -357,37 +352,6 @@ BufferGetPage(Buffer buffer)
 	return (Page) BufferGetBlock(buffer);
 }
 
-/*
- * Check whether the given snapshot is too old to have safely read the given
- * page from the given table.  If so, throw a "snapshot too old" error.
- *
- * This test generally needs to be performed after every BufferGetPage() call
- * that is executed as part of a scan.  It is not needed for calls made for
- * modifying the page (for example, to position to the right place to insert a
- * new index tuple or for vacuuming).  It may also be omitted where calls to
- * lower-level functions will have already performed the test.
- *
- * Note that a NULL snapshot argument is allowed and causes a fast return
- * without error; this is to support call sites which can be called from
- * either scans or index modification areas.
- *
- * For best performance, keep the tests that are fastest and/or most likely to
- * exclude a page from old snapshot testing near the front.
- */
-static inline void
-TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
-{
-	Assert(relation != NULL);
-
-	if (old_snapshot_threshold >= 0
-		&& (snapshot) != NULL
-		&& ((snapshot)->snapshot_type == SNAPSHOT_MVCC
-			|| (snapshot)->snapshot_type == SNAPSHOT_TOAST)
-		&& !XLogRecPtrIsInvalid((snapshot)->lsn)
-		&& PageGetLSN(page) > (snapshot)->lsn)
-		TestForOldSnapshot_impl(snapshot, relation);
-}
-
 #endif							/* FRONTEND */
 
 #endif							/* BUFMGR_H */