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On an UPDATE, try to overwrite an existing btree cell with the modified

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content, if the old and new cell are the same size.  Use memcmp() first
to avoid dirtying pages that are unchanged.

FossilOrigin-Name: 5887d8beb502ad62689d31b850f46ab50831a1e9db36adf20d55ad45619d207e
This commit is contained in:
drh
2018-05-07 11:48:22 +00:00
parent e2188f0b0f 30f7a25325
commit e3c05a5597
4 changed files with 106 additions and 9 deletions
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96
src/btree.c
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@@ -8154,6 +8154,94 @@ static int balance(BtCursor *pCur){
return rc;
}
/* Overwrite content from pX into pDest. Only do the write if the
** content is different from what is already there.
*/
static int btreeOverwriteContent(
MemPage *pPage, /* MemPage on which writing will occur */
u8 *pDest, /* Pointer to the place to start writing */
const BtreePayload *pX, /* Source of data to write */
int iOffset, /* Offset of first byte to write */
int iAmt /* Number of bytes to be written */
){
int nData = pX->nData - iOffset;
if( nData<=0 ){
/* Overwritting with zeros */
int i;
for(i=0; i<iAmt && pDest[i]==0; i++){}
if( i<iAmt ){
int rc = sqlite3PagerWrite(pPage->pDbPage);
if( rc ) return rc;
memset(pDest + i, 0, iAmt - i);
}
}else{
if( nData<iAmt ){
/* Mixed read data and zeros at the end. Make a recursive call
** to write the zeros then fall through to write the real data */
int rc = btreeOverwriteContent(pPage, pDest+nData, pX, iOffset+nData,
iAmt-nData);
if( rc ) return rc;
iAmt = nData;
}
if( memcmp(pDest, ((u8*)pX->pData) + iOffset, iAmt)!=0 ){
int rc = sqlite3PagerWrite(pPage->pDbPage);
if( rc ) return rc;
memcpy(pDest, ((u8*)pX->pData) + iOffset, iAmt);
}
}
return SQLITE_OK;
}
/*
** Overwrite the cell that cursor pCur is pointing to with fresh content
** contained in pX.
*/
static int btreeOverwriteCell(BtCursor *pCur, const BtreePayload *pX){
int iOffset; /* Next byte of pX->pData to write */
int nTotal = pX->nData + pX->nZero; /* Total bytes of to write */
int rc; /* Return code */
MemPage *pPage = pCur->pPage; /* Page being written */
BtShared *pBt; /* Btree */
Pgno ovflPgno; /* Next overflow page to write */
u32 ovflPageSize; /* Size to write on overflow page */
if( pCur->info.pPayload + pCur->info.nLocal > pPage->aDataEnd ){
return SQLITE_CORRUPT_BKPT;
}
/* Overwrite the local portion first */
rc = btreeOverwriteContent(pPage, pCur->info.pPayload, pX,
0, pCur->info.nLocal);
if( rc ) return rc;
if( pCur->info.nLocal==nTotal ) return SQLITE_OK;
/* Now overwrite the overflow pages */
iOffset = pCur->info.nLocal;
assert( nTotal>=0 );
assert( iOffset>=0 );
ovflPgno = get4byte(pCur->info.pPayload + iOffset);
pBt = pPage->pBt;
ovflPageSize = pBt->usableSize - 4;
do{
rc = btreeGetPage(pBt, ovflPgno, &pPage, 0);
if( rc ) return rc;
if( sqlite3PagerPageRefcount(pPage->pDbPage)!=1 ){
rc = SQLITE_CORRUPT_BKPT;
}else{
if( iOffset+ovflPageSize<(u32)nTotal ){
ovflPgno = get4byte(pPage->aData);
}else{
ovflPageSize = nTotal - iOffset;
}
rc = btreeOverwriteContent(pPage, pPage->aData+4, pX,
iOffset, ovflPageSize);
}
sqlite3PagerUnref(pPage->pDbPage);
if( rc ) return rc;
iOffset += ovflPageSize;
}while( iOffset<nTotal );
return SQLITE_OK;
}
/*
** Insert a new record into the BTree. The content of the new record
@@ -8252,6 +8340,14 @@ int sqlite3BtreeInsert(
** new row onto the end, set the "loc" to avoid an unnecessary
** btreeMoveto() call */
if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){
/* The current is currently 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
){
return btreeOverwriteCell(pCur, pX);
}
loc = 0;
}else if( loc==0 ){
rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);