Try to avoid reading pages when moving overflow chains to the free-list. (CVS 3886)

FossilOrigin-Name: 8cccec68bd9073b2b19d3d31cf0b77b0ce76172e

manifest

@@ -1,5 +1,5 @@
-C Add\ssome\stests\s(and\s2\sresulting\sbug\sfixes)\sto\sincr\svacuum\smode.\s(CVS\s3885)
-D 2007-04-28T15:47:44
+C Try\sto\savoid\sreading\spages\swhen\smoving\soverflow\schains\sto\sthe\sfree-list.\s(CVS\s3886)
+D 2007-04-30T16:55:01
 F Makefile.in 8cab54f7c9f5af8f22fd97ddf1ecfd1e1860de62
 F Makefile.linux-gcc 2d8574d1ba75f129aba2019f0b959db380a90935
 F README 9c4e2d6706bdcc3efdd773ce752a8cdab4f90028
@@ -59,7 +59,7 @@ F src/alter.c 2c79ec40f65e33deaf90ca493422c74586e481a3
 F src/analyze.c 4bbf5ddf9680587c6d4917e02e378b6037be3651
 F src/attach.c a16ada4a4654a0d126b8223ec9494ebb81bc5c3c
 F src/auth.c 902f4722661c796b97f007d9606bd7529c02597f
-F src/btree.c 066ca57368d814ba8940d926a491f70c6866033f
+F src/btree.c abdbd0261d2ab4fed65332cc93f959906c185d22
 F src/btree.h 4c0b5855cef3e4e6627358aa69541d21a2015947
 F src/build.c 02e01ec7907c7d947ab3041fda0e81eaed05db42
 F src/callback.c 6414ed32d55859d0f65067aa5b88d2da27b3af9e
@@ -465,7 +465,7 @@ F www/tclsqlite.tcl bb0d1357328a42b1993d78573e587c6dcbc964b9
 F www/vdbe.tcl 87a31ace769f20d3627a64fa1fade7fed47b90d0
 F www/version3.tcl 890248cf7b70e60c383b0e84d77d5132b3ead42b
 F www/whentouse.tcl fc46eae081251c3c181bd79c5faef8195d7991a5
-P 9466367d65f43d58020e709428268dc2ff98aa35
-R 7cca5d0f43767198fb71376aeea99069
+P 89b1b3f897bda1fffceb9cf72fa4d42b809ccb8e
+R 25da30ea21a3fd3e2f878c36734a5bb9
 U danielk1977
-Z 03907aab221cf0da66dda4d095ab65a6
+Z f3a814360f6db57fb203458a6b31fb7b

manifest.uuid

@@ -1 +1 @@
-89b1b3f897bda1fffceb9cf72fa4d42b809ccb8e
+8cccec68bd9073b2b19d3d31cf0b77b0ce76172e

src/btree.c

@@ -9,7 +9,7 @@
 **    May you share freely, never taking more than you give.
 **
 *************************************************************************
-** $Id: btree.c,v 1.361 2007/04/28 15:47:44 danielk1977 Exp $
+** $Id: btree.c,v 1.362 2007/04/30 16:55:01 danielk1977 Exp $
 **
 ** This file implements a external (disk-based) database using BTrees.
 ** For a detailed discussion of BTrees, refer to
@@ -4046,6 +4046,60 @@ static int freePage(MemPage *pPage){
   return rc;
 }
 
+/*
+** Get a MemPage structure for overflow page number ovfl. If it
+** is not zero, the page number of the overflow page following the
+** one retrieved is written to *pPgnoNext.
+**
+** If it is possible to figure out the page-number of the next
+** overflow page without reading the data of page ovfl, then 
+** sqlite3PagerAcquire() is passed the "noContent" flag when
+** page ovfl is retrieved.
+*/
+static int getOverflowPage(
+  BtShared *pBt, 
+  Pgno ovfl, 
+  MemPage **ppPage, 
+  Pgno *pPgnoNext
+){
+  Pgno next = 0;
+  int rc;
+
+  if( !pPgnoNext ){
+    return getPage(pBt, ovfl, ppPage, 1);
+  }
+
+#ifndef SQLITE_OMIT_AUTOVACUUM
+  if( pBt->autoVacuum ){
+    Pgno pgno;
+    Pgno iGuess = ovfl+1;
+    u8 eType;
+
+    while( PTRMAP_ISPAGE(pBt, iGuess) || iGuess==PENDING_BYTE_PAGE(pBt) ){
+      iGuess++;
+    }
+
+    if( iGuess<sqlite3PagerPagecount(pBt->pPager) ){
+      rc = ptrmapGet(pBt, iGuess, &eType, &pgno);
+      if( rc!=SQLITE_OK ){
+        return rc;
+      }
+      if( eType==PTRMAP_OVERFLOW2 && pgno==ovfl ){
+        next = iGuess;
+      }
+    }
+  }
+#endif
+
+  rc = getPage(pBt, ovfl, ppPage, 0);
+  if( rc==SQLITE_OK ){
+    assert(next==0 || next==get4byte((*ppPage)->aData));
+    next = get4byte((*ppPage)->aData);
+  }
+  *pPgnoNext = next;
+  return rc;
+}
+
 /*
 ** Free any overflow pages associated with the given Cell.
 */
@@ -4070,11 +4124,9 @@ static int clearCell(MemPage *pPage, unsigned char *pCell){
     if( ovflPgno==0 || ovflPgno>sqlite3PagerPagecount(pBt->pPager) ){
       return SQLITE_CORRUPT_BKPT;
     }
-    rc = getPage(pBt, ovflPgno, &pOvfl, nOvfl==0);
+
+    rc = getOverflowPage(pBt, ovflPgno, &pOvfl, (nOvfl==0)?0:&ovflPgno);
     if( rc ) return rc;
-    if( nOvfl ){
-      ovflPgno = get4byte(pOvfl->aData);
-    }
     rc = freePage(pOvfl);
     sqlite3PagerUnref(pOvfl->pDbPage);
     if( rc ) return rc;