From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 11 Apr 2014 17:09:01 +0000 (-0400)
Subject: improve FreePageManager
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=5025cef95acbff3d64eec5853de22671f845b999;p=users%2Frhaas%2Fpostgres.git

improve FreePageManager

- fix a bug when the btree root is created from the last free page
- when the btree root is a leaf and is surrounded on either side by
  freespace, collapse it to a singleton range
---

diff --git a/src/backend/utils/mmgr/freepage.c b/src/backend/utils/mmgr/freepage.c
index a572fefb8c..f830149892 100644
--- a/src/backend/utils/mmgr/freepage.c
+++ b/src/backend/utils/mmgr/freepage.c
@@ -507,34 +507,68 @@ FreePageBtreeCleanup(FreePageManager *fpm)
 	{
 		FreePageBtree *root = relptr_access(base, fpm->btree_root);
 
-		/* Can't do anything if the root has multiple keys. */
-		if (root->hdr.nused > 1)
-			break;
-
-		/* Root should never be empty. */
-		Assert(root->hdr.nused == 1);
+		/* If the root contains only one key, reduce depth by one. */
+		if (root->hdr.nused == 1)
+		{
+			/* Shrink depth of tree by one. */
+			Assert(fpm->btree_depth > 0);
+			--fpm->btree_depth;
+			if (root->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+			{
+				/* If root is a leaf, convert only entry to singleton range. */
+				relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
+				fpm->singleton_first_page = root->u.leaf_key[0].first_page;
+				fpm->singleton_npages = root->u.leaf_key[0].npages;
+			}
+			else
+			{
+				FreePageBtree *newroot;
 
-		/* Shrink depth of tree by one. */
-		Assert(fpm->btree_depth > 0);
-		--fpm->btree_depth;
-		if (root->hdr.magic == FREE_PAGE_LEAF_MAGIC)
+				/* If root is an internal page, make only child the root. */
+				Assert(root->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
+				relptr_copy(fpm->btree_root, root->u.internal_key[0].child);
+				newroot = relptr_access(base, fpm->btree_root);
+				relptr_store(base, newroot->hdr.parent, (FreePageBtree *) NULL);
+			}
+			FreePageBtreeRecycle(fpm, fpm_pointer_to_page(base, root));
+		}
+		else if (root->hdr.nused == 2 &&
+				 root->hdr.magic == FREE_PAGE_LEAF_MAGIC)
 		{
-			/* If root is a leaf, convert only entry to singleton range. */
-			relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
-			fpm->singleton_first_page = root->u.leaf_key[0].first_page;
-			fpm->singleton_npages = root->u.leaf_key[0].npages;
+			Size	end_of_first;
+			Size	start_of_second;
+
+			end_of_first = root->u.leaf_key[0].first_page +
+				root->u.leaf_key[0].npages;
+			start_of_second = root->u.leaf_key[1].first_page;
+
+			if (end_of_first + 1 == start_of_second)
+			{
+				Size	root_page = fpm_pointer_to_page(base, root);
+
+				if (end_of_first == root_page)
+				{
+					FreePagePopSpanLeader(fpm, root->u.leaf_key[0].first_page);
+					FreePagePopSpanLeader(fpm, root->u.leaf_key[1].first_page);
+					fpm->singleton_first_page = root->u.leaf_key[0].first_page;
+					fpm->singleton_npages = root->u.leaf_key[0].npages +
+						root->u.leaf_key[1].npages + 1;
+					fpm->btree_depth = 0;
+					relptr_store(base, fpm->btree_root,
+								 (FreePageBtree *) NULL);
+					FreePagePushSpanLeader(fpm, fpm->singleton_first_page,
+										   fpm->singleton_npages);
+				}
+			}
+
+			/* Whether it worked or not, it's time to stop. */
+			break;
 		}
 		else
 		{
-			FreePageBtree *newroot;
-
-			/* If root is an internal page, make only child the root. */
-			Assert(root->hdr.magic == FREE_PAGE_INTERNAL_MAGIC);
-			relptr_copy(fpm->btree_root, root->u.internal_key[0].child);
-			newroot = relptr_access(base, fpm->btree_root);
-			relptr_store(base, newroot->hdr.parent, (FreePageBtree *) NULL);
+			/* Nothing more to do.  Stop. */
+			break;
 		}
-		FreePageBtreeRecycle(fpm, fpm_pointer_to_page(base, root));
 	}
 
 	/*
@@ -1401,6 +1435,19 @@ FreePageManagerPutInternal(FreePageManager *fpm, Size first_page, Size npages,
 			fpm->singleton_npages = 0;
 			fpm->btree_depth = 1;
 
+			/*
+			 * Corner case: it may be that the btree root took the very last
+			 * free page.  In that case, the sole btree entry covers a zero
+			 * page run, which is invalid.  Overwrite it with the entry we're
+			 * trying to insert and get out.
+			 */
+			if (root->u.leaf_key[0].npages == 0)
+			{
+				root->u.leaf_key[0].first_page = first_page;
+				root->u.leaf_key[0].npages = npages;
+				return npages;
+			}
+
 			/* Fall through to insert the new key. */
 		}
 	}