Suppress useless searches for unused line pointers in PageAddItem. To do

this, add a 16-bit "flags" field to page headers by stealing some bits from
pd_tli.  We use one flag bit as a hint to indicate whether there are any
unused line pointers; the remaining 15 are available for future use.

This is a cut-down form of an idea proposed by Hiroki Kataoka in July 2005.
At the time it was rejected because the original patch increased the size of
page headers and it wasn't clear that the benefit outweighed the distributed
cost.  The flag-bit approach gets most of the benefit without requiring an
increase in the page header size.

Heikki Linnakangas and Tom Lane

1 files changed, 29 insertions, 8 deletions

diff --git a/src/backend/storage/page/bufpage.c b/src/backend/storage/page/bufpage.c
index 8299f550d6e..b246b0afeb8 100644
--- a/src/backend/storage/page/bufpage.c
+++ b/src/backend/storage/page/bufpage.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/page/bufpage.c,v 1.71 2007/02/21 20:02:17 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/page/bufpage.c,v 1.72 2007/03/02 00:48:44 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -39,6 +39,7 @@ PageInit(Page page, Size pageSize, Size specialSize)
 	/* Make sure all fields of page are zero, as well as unused space */
 	MemSet(p, 0, pageSize);
 
+	/* p->pd_flags = 0; 					done by above MemSet */
 	p->pd_lower = SizeOfPageHeaderData;
 	p->pd_upper = pageSize - specialSize;
 	p->pd_special = pageSize - specialSize;
@@ -73,6 +74,7 @@ PageHeaderIsValid(PageHeader page)
 	/* Check normal case */
 	if (PageGetPageSize(page) == BLCKSZ &&
 		PageGetPageLayoutVersion(page) == PG_PAGE_LAYOUT_VERSION &&
+		(page->pd_flags & ~PD_VALID_FLAG_BITS) == 0 &&
 		page->pd_lower >= SizeOfPageHeaderData &&
 		page->pd_lower <= page->pd_upper &&
 		page->pd_upper <= page->pd_special &&
@@ -165,14 +167,27 @@ PageAddItem(Page page,
 	else
 	{
 		/* offsetNumber was not passed in, so find a free slot */
-		/* look for "recyclable" (unused & deallocated) ItemId */
-		for (offsetNumber = 1; offsetNumber < limit; offsetNumber++)
+		/* if no free slot, we'll put it at limit (1st open slot) */
+		if (PageHasFreeLinePointers(phdr))
+		{
+			/* look for "recyclable" (unused & deallocated) ItemId */
+			for (offsetNumber = 1; offsetNumber < limit; offsetNumber++)
+			{
+				itemId = PageGetItemId(phdr, offsetNumber);
+				if (!ItemIdIsUsed(itemId) && ItemIdGetLength(itemId) == 0)
+					break;
+			}
+			if (offsetNumber >= limit)
+			{
+				/* the hint is wrong, so reset it */
+				PageClearHasFreeLinePointers(phdr);
+			}
+		}
+		else
 		{
-			itemId = PageGetItemId(phdr, offsetNumber);
-			if (!ItemIdIsUsed(itemId) && ItemIdGetLength(itemId) == 0)
-				break;
+			/* don't bother searching if hint says there's no free slot */
+			offsetNumber = limit;
 		}
-		/* if no free slot, we'll put it at limit (1st open slot) */
 	}
 
 	if (offsetNumber > limit)
@@ -413,13 +428,19 @@ PageRepairFragmentation(Page page, OffsetNumber *unused)
 		pfree(itemidbase);
 	}
 
+	/* Set hint bit for PageAddItem */
+	if (nused < nline)
+		PageSetHasFreeLinePointers(page);
+	else
+		PageClearHasFreeLinePointers(page);
+
 	return (nline - nused);
 }
 
 /*
  * PageGetFreeSpace
  *		Returns the size of the free (allocatable) space on a page,
- *		deducted by the space needed for a new line pointer.
+ *		reduced by the space needed for a new line pointer.
  */
 Size
 PageGetFreeSpace(Page page)