Improved comments on the cell-overwrite optimization code.

FossilOrigin-Name: a4fe966da2fc479b18bf521ff596000410af3a611f7d8723d126795e595ccf22

manifest

@@ -1,5 +1,5 @@
-C On\san\sUPDATE,\stry\sto\soverwrite\san\sexisting\sbtree\scell\swith\sthe\smodified\ncontent,\sif\sthe\sold\sand\snew\scell\sare\sthe\ssame\ssize.\s\sUse\smemcmp()\sfirst\nto\savoid\sdirtying\spages\sthat\sare\sunchanged.
-D 2018-05-07T11:48:22.641
+C Improved\scomments\son\sthe\scell-overwrite\soptimization\scode.
+D 2018-05-07T17:27:04.995
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F Makefile.in 5ce9343cba9c189046f1afe6d2bcc1f68079439febc05267b98aec6ecc752439
@@ -434,7 +434,7 @@ F src/auth.c 6277d63837357549fe14e723490d6dc1a38768d71c795c5eb5c0f8a99f918f73
 F src/backup.c faf17e60b43233c214aae6a8179d24503a61e83b
 F src/bitvec.c 17ea48eff8ba979f1f5b04cc484c7bb2be632f33
 F src/btmutex.c 8acc2f464ee76324bf13310df5692a262b801808984c1b79defb2503bbafadb6
-F src/btree.c 6be0267a7da8ca04c466094ca9d9a8a26333e175cff794afe5138aeec0cdb7c2
+F src/btree.c 7b21976dc09778f05af0e085c40b735467ffdcea64bd07dace3f12ca0a2defce
 F src/btree.h 0866c0a08255142ea0e754aabd211c843cab32045c978a592a43152405ed0c84
 F src/btreeInt.h 620ab4c7235f43572cf3ac2ac8723cbdf68073be4d29da24897c7b77dda5fd96
 F src/build.c 0c2be5839f22aa2938f217c6c6c2120d9fc96872a546a37541a8271541cb355e
@@ -1727,8 +1727,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 374d8e264487b0437a8d995ced1bc026a92d495a2d0568f65f033e9ebe11d0e2 3e11dc3183bc3e8ec49af244a8e8b3e07d12f7a2e59028b2bf64ce0ab589a91f
-R 891ce8c2c78f1f487f80460b8c248fd0
-T +closed 3e11dc3183bc3e8ec49af244a8e8b3e07d12f7a2e59028b2bf64ce0ab589a91f
+P 5887d8beb502ad62689d31b850f46ab50831a1e9db36adf20d55ad45619d207e
+R e6599f47435d4296a22afaa3f1649f4a
 U drh
-Z 7955ef5d032aea210d99e8bc24e906bc
+Z 8048adc03c51022109ca997ec38e8ad8

manifest.uuid

@@ -1 +1 @@
-5887d8beb502ad62689d31b850f46ab50831a1e9db36adf20d55ad45619d207e
+a4fe966da2fc479b18bf521ff596000410af3a611f7d8723d126795e595ccf22

src/btree.c

@@ -8332,43 +8332,70 @@ int sqlite3BtreeInsert(
     invalidateIncrblobCursors(p, pCur->pgnoRoot, pX->nKey, 0);
 
     /* If BTREE_SAVEPOSITION is set, the cursor must already be pointing 
-    ** to a row with the same key as the new entry being inserted.  */
-    assert( (flags & BTREE_SAVEPOSITION)==0 || 
-            ((pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey) );
+    ** to a row with the same key as the new entry being inserted.
+    */
+#ifdef SQLITE_DEBUG
+    if( flags & BTREE_SAVEPOSITION ){
+      assert( pCur->curFlags & BTCF_ValidNKey );
+      assert( pX->nKey==pCur->info.nKey );
+      assert( pCur->info.nSize!=0 );
+      assert( loc==0 );
+    }
+#endif
 
-    /* If the cursor is currently on the last row and we are appending a
-    ** new row onto the end, set the "loc" to avoid an unnecessary
-    ** btreeMoveto() call */
+    /* On the other hand, BTREE_SAVEPOSITION==0 does not imply
+    ** that the cursor is not pointing to a row to be overwritten.
+    ** So do a complete check.
+    */
     if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){
-      /* The current is currently pointing to the entry that is to be
+      /* The cursor is pointing to the entry that is to be
       ** overwritten */
       assert( pX->nData>=0 && pX->nZero>=0 );
       if( pCur->info.nSize!=0
        && pCur->info.nPayload==(u32)pX->nData+pX->nZero
       ){
+        /* New entry is the same size as the old.  Do an overwrite */
         return btreeOverwriteCell(pCur, pX);
       }
-      loc = 0;
+      assert( loc==0 );
     }else if( loc==0 ){
+      /* The cursor is *not* pointing to the cell to be overwritten, nor
+      ** to an adjacent cell.  Move the cursor so that it is pointing either
+      ** to the cell to be overwritten or an adjacent cell.
+      */
       rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);
       if( rc ) return rc;
     }
-  }else if( loc==0 && (flags & BTREE_SAVEPOSITION)==0 ){
-    if( pX->nMem ){
-      UnpackedRecord r;
-      r.pKeyInfo = pCur->pKeyInfo;
-      r.aMem = pX->aMem;
-      r.nField = pX->nMem;
-      r.default_rc = 0;
-      r.errCode = 0;
-      r.r1 = 0;
-      r.r2 = 0;
-      r.eqSeen = 0;
-      rc = sqlite3BtreeMovetoUnpacked(pCur, &r, 0, flags!=0, &loc);
-    }else{
-      rc = btreeMoveto(pCur, pX->pKey, pX->nKey, flags!=0, &loc);
+  }else{
+    /* This is an index or a WITHOUT ROWID table */
+
+    /* If BTREE_SAVEPOSITION is set, the cursor must already be pointing 
+    ** to a row with the same key as the new entry being inserted.
+    */
+    assert( (flags & BTREE_SAVEPOSITION)==0 || loc==0 );
+
+    /* If the cursor is not already pointing either to the cell to be
+    ** overwritten, or if a new cell is being inserted, if the cursor is
+    ** not pointing to an immediately adjacent cell, then move the cursor
+    ** so that it does.
+    */
+    if( loc==0 && (flags & BTREE_SAVEPOSITION)==0 ){
+      if( pX->nMem ){
+        UnpackedRecord r;
+        r.pKeyInfo = pCur->pKeyInfo;
+        r.aMem = pX->aMem;
+        r.nField = pX->nMem;
+        r.default_rc = 0;
+        r.errCode = 0;
+        r.r1 = 0;
+        r.r2 = 0;
+        r.eqSeen = 0;
+        rc = sqlite3BtreeMovetoUnpacked(pCur, &r, 0, flags!=0, &loc);
+      }else{
+        rc = btreeMoveto(pCur, pX->pKey, pX->nKey, flags!=0, &loc);
+      }
+      if( rc ) return rc;
     }
-    if( rc ) return rc;
   }
   assert( pCur->eState==CURSOR_VALID || (pCur->eState==CURSOR_INVALID && loc) );