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Add the SQLITE_DIRECT_OVERFLOW_READ compile time option.

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FossilOrigin-Name: 2ab14a846727857175eac7961b7338c4d3b417ca
This commit is contained in:
dan
2011-10-08 14:57:07 +00:00
parent 08ccfaa1b1
commit f4ba109356
7 changed files with 72 additions and 27 deletions
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60
src/btree.c
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@@ -3938,21 +3938,55 @@ static int accessPayload(
/* Need to read this page properly. It contains some of the
** range of data that is being read (eOp==0) or written (eOp!=0).
*/
DbPage *pDbPage;
#ifdef SQLITE_DIRECT_OVERFLOW_READ
sqlite3_file *fd;
#endif
int a = amt;
rc = sqlite3PagerGet(pBt->pPager, nextPage, &pDbPage);
if( rc==SQLITE_OK ){
aPayload = sqlite3PagerGetData(pDbPage);
nextPage = get4byte(aPayload);
if( a + offset > ovflSize ){
a = ovflSize - offset;
}
rc = copyPayload(&aPayload[offset+4], pBuf, a, eOp, pDbPage);
sqlite3PagerUnref(pDbPage);
offset = 0;
amt -= a;
pBuf += a;
if( a + offset > ovflSize ){
a = ovflSize - offset;
}
#ifdef SQLITE_DIRECT_OVERFLOW_READ
/* If all the following are true:
**
** 1) this is a read operation, and
** 2) data is required from the start of this overflow page, and
** 3) the database is file-backed, and
** 4) there is no open write-transaction, and
** 5) the database is not a WAL database,
**
** then data can be read directly from the database file into the
** output buffer, bypassing the page-cache altogether. This speeds
** up loading large records that span many overflow pages.
*/
if( eOp==0 /* (1) */
&& offset==0 /* (2) */
&& pBt->inTransaction==TRANS_READ /* (4) */
&& (fd = sqlite3PagerFile(pBt->pPager))->pMethods /* (3) */
&& pBt->pPage1->aData[19]==0x01 /* (5) */
){
u8 aSave[4];
u8 *aWrite = &pBuf[-4];
memcpy(aSave, aWrite, 4);
rc = sqlite3OsRead(fd, aWrite, a+4, pBt->pageSize * (nextPage-1));
nextPage = get4byte(aWrite);
memcpy(aWrite, aSave, 4);
}else
#endif
{
DbPage *pDbPage;
rc = sqlite3PagerGet(pBt->pPager, nextPage, &pDbPage);
if( rc==SQLITE_OK ){
aPayload = sqlite3PagerGetData(pDbPage);
nextPage = get4byte(aPayload);
rc = copyPayload(&aPayload[offset+4], pBuf, a, eOp, pDbPage);
sqlite3PagerUnref(pDbPage);
offset = 0;
}
}
amt -= a;
pBuf += a;
}
}
}