Implement lazy XID allocation: transactions that do not modify any database

rows will normally never obtain an XID at all.  We already did things this way
for subtransactions, but this patch extends the concept to top-level
transactions.  In applications where there are lots of short read-only
transactions, this should improve performance noticeably; not so much from
removal of the actual XID-assignments, as from reduction of overhead that's
driven by the rate of XID consumption.  We add a concept of a "virtual
transaction ID" so that active transactions can be uniquely identified even
if they don't have a regular XID.  This is a much lighter-weight concept:
uniqueness of VXIDs is only guaranteed over the short term, and no on-disk
record is made about them.

Florian Pflug, with some editorialization by Tom.

1 files changed, 1 insertions, 28 deletions

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 358e9a5ad9..87cf57daec 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.355 2007/08/13 19:08:26 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.356 2007/09/05 18:10:47 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2601,14 +2601,6 @@ repair_frag(VRelStats *vacrelstats, Relation onerel,
 				PageSetLSN(page, recptr);
 				PageSetTLI(page, ThisTimeLineID);
 			}
-			else
-			{
-				/*
-				 * No XLOG record, but still need to flag that XID exists on
-				 * disk
-				 */
-				MyXactMadeTempRelUpdate = true;
-			}
 
 			END_CRIT_SECTION();
 
@@ -2761,13 +2753,6 @@ move_chain_tuple(Relation rel,
 		PageSetLSN(dst_page, recptr);
 		PageSetTLI(dst_page, ThisTimeLineID);
 	}
-	else
-	{
-		/*
-		 * No XLOG record, but still need to flag that XID exists on disk
-		 */
-		MyXactMadeTempRelUpdate = true;
-	}
 
 	END_CRIT_SECTION();
 
@@ -2868,13 +2853,6 @@ move_plain_tuple(Relation rel,
 		PageSetLSN(dst_page, recptr);
 		PageSetTLI(dst_page, ThisTimeLineID);
 	}
-	else
-	{
-		/*
-		 * No XLOG record, but still need to flag that XID exists on disk
-		 */
-		MyXactMadeTempRelUpdate = true;
-	}
 
 	END_CRIT_SECTION();
 
@@ -3070,11 +3048,6 @@ vacuum_page(Relation onerel, Buffer buffer, VacPage vacpage)
 		PageSetLSN(page, recptr);
 		PageSetTLI(page, ThisTimeLineID);
 	}
-	else
-	{
-		/* No XLOG record, but still need to flag that XID exists on disk */
-		MyXactMadeTempRelUpdate = true;
-	}
 
 	END_CRIT_SECTION();
 }