From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Tue, 16 Nov 2010 09:02:11 +0000 (+0200)
Subject: The GiST scan algorithm uses LSNs to detect concurrent pages splits, but
X-Git-Tag: REL9_0_2~18
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=821bb177227be9287f8fd50c2398c7bc5c3929e3;p=postgresql

The GiST scan algorithm uses LSNs to detect concurrent pages splits, but
temporary indexes are not WAL-logged. We used a constant LSN for temporary
indexes, on the assumption that we don't need to worry about concurrent page
splits in temporary indexes because they're only visible to the current
session. But that assumption is wrong, it's possible to insert rows and
split pages in the same session, while a scan is in progress. For example,
by opening a cursor and fetching some rows, and INSERTing new rows before
fetching some more.

Fix by generating fake increasing LSNs, used in place of real LSNs in
temporary GiST indexes.
---

diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
index cec08c7226..4a5d4bf6f1 100644
--- a/src/backend/access/gist/gist.c
+++ b/src/backend/access/gist/gist.c
@@ -22,8 +22,6 @@
 #include "storage/indexfsm.h"
 #include "utils/memutils.h"
 
-const XLogRecPtr XLogRecPtrForTemp = {1, 1};
-
 /* Working state for gistbuild and its callback */
 typedef struct
 {
@@ -132,7 +130,7 @@ gistbuild(PG_FUNCTION_ARGS)
 		PageSetTLI(page, ThisTimeLineID);
 	}
 	else
-		PageSetLSN(page, XLogRecPtrForTemp);
+		PageSetLSN(page, GetXLogRecPtrForTemp());
 
 	UnlockReleaseBuffer(buffer);
 
@@ -423,7 +421,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 		{
 			for (ptr = dist; ptr; ptr = ptr->next)
 			{
-				PageSetLSN(ptr->page, XLogRecPtrForTemp);
+				PageSetLSN(ptr->page, GetXLogRecPtrForTemp());
 			}
 		}
 
@@ -491,7 +489,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 			PageSetTLI(state->stack->page, ThisTimeLineID);
 		}
 		else
-			PageSetLSN(state->stack->page, XLogRecPtrForTemp);
+			PageSetLSN(state->stack->page, GetXLogRecPtrForTemp());
 
 		if (state->stack->blkno == GIST_ROOT_BLKNO)
 			state->needInsertComplete = false;
@@ -1027,7 +1025,7 @@ gistnewroot(Relation r, Buffer buffer, IndexTuple *itup, int len, ItemPointer ke
 		PageSetTLI(page, ThisTimeLineID);
 	}
 	else
-		PageSetLSN(page, XLogRecPtrForTemp);
+		PageSetLSN(page, GetXLogRecPtrForTemp());
 
 	END_CRIT_SECTION();
 }
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 03c5773d4d..a23047642f 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -677,3 +677,24 @@ gistoptions(PG_FUNCTION_ARGS)
 		PG_RETURN_BYTEA_P(result);
 	PG_RETURN_NULL();
 }
+
+/*
+ * Temporary GiST indexes are not WAL-logged, but we need LSNs to detect
+ * concurrent page splits anyway. GetXLogRecPtrForTemp() provides a fake
+ * sequence of LSNs for that purpose. Each call generates an LSN that is
+ * greater than any previous value returned by this function in the same
+ * session.
+ */
+XLogRecPtr
+GetXLogRecPtrForTemp(void)
+{
+	static XLogRecPtr counter = {0, 1};
+
+	counter.xrecoff++;
+	if (counter.xrecoff == 0)
+	{
+		counter.xlogid++;
+		counter.xrecoff++;
+	}
+	return counter;
+}
diff --git a/src/backend/access/gist/gistvacuum.c b/src/backend/access/gist/gistvacuum.c
index abd3d99956..33f7c8c76a 100644
--- a/src/backend/access/gist/gistvacuum.c
+++ b/src/backend/access/gist/gistvacuum.c
@@ -268,7 +268,7 @@ gistbulkdelete(PG_FUNCTION_ARGS)
 					pfree(rdata);
 				}
 				else
-					PageSetLSN(page, XLogRecPtrForTemp);
+					PageSetLSN(page, GetXLogRecPtrForTemp());
 
 				END_CRIT_SECTION();
 			}
diff --git a/src/include/access/gist_private.h b/src/include/access/gist_private.h
index a85da662ce..4df5fed116 100644
--- a/src/include/access/gist_private.h
+++ b/src/include/access/gist_private.h
@@ -87,7 +87,6 @@ typedef struct GISTScanOpaqueData
 typedef GISTScanOpaqueData *GISTScanOpaque;
 
 /* XLog stuff */
-extern const XLogRecPtr XLogRecPtrForTemp;
 
 #define XLOG_GIST_PAGE_UPDATE		0x00
 #define XLOG_GIST_NEW_ROOT			0x20
@@ -326,6 +325,8 @@ extern void gistMakeUnionKey(GISTSTATE *giststate, int attno,
 				 GISTENTRY *entry2, bool isnull2,
 				 Datum *dst, bool *dstisnull);
 
+extern XLogRecPtr GetXLogRecPtrForTemp(void);
+
 /* gistvacuum.c */
 extern Datum gistbulkdelete(PG_FUNCTION_ARGS);
 extern Datum gistvacuumcleanup(PG_FUNCTION_ARGS);