Implement Overflow pages

[kawasin73]

calculate the thresholds in lockBtree().

  /* maxLocal is the maximum amount of payload to store locally for
  ** a cell.  Make sure it is small enough so that at least minFanout
  ** cells can will fit on one page.  We assume a 10-byte page header.
  ** Besides the payload, the cell must store:
  **     2-byte pointer to the cell
  **     4-byte child pointer
  **     9-byte nKey value
  **     4-byte nData value
  **     4-byte overflow page pointer
  ** So a cell consists of a 2-byte pointer, a header which is as much as
  ** 17 bytes long, 0 to N bytes of payload, and an optional 4 byte overflow
  ** page pointer.
  */
  pBt->maxLocal = (u16)((pBt->usableSize-12)*64/255 - 23);
  pBt->minLocal = (u16)((pBt->usableSize-12)*32/255 - 23);
  pBt->maxLeaf = (u16)(pBt->usableSize - 35);
  pBt->minLeaf = (u16)((pBt->usableSize-12)*32/255 - 23);

[kawasin73]

• btreePayloadToLocal()

  • check the payload is overflowed

• sqlite3BtreeTransferRow()

  • copy the all payload to another cursor

    nIn = pSrc->info.nLocal;
    aIn = pSrc->info.pPayload;

• BtreePayload is used to pass data on insert.

/* An instance of the BtreePayload object describes the content of a single
** entry in either an index or table btree.
**
** Index btrees (used for indexes and also WITHOUT ROWID tables) contain
** an arbitrary key and no data.  These btrees have pKey,nKey set to the
** key and the pData,nData,nZero fields are uninitialized.  The aMem,nMem
** fields give an array of Mem objects that are a decomposition of the key.
** The nMem field might be zero, indicating that no decomposition is available.
**
** Table btrees (used for rowid tables) contain an integer rowid used as
** the key and passed in the nKey field.  The pKey field is zero.  
** pData,nData hold the content of the new entry.  nZero extra zero bytes
** are appended to the end of the content when constructing the entry.
** The aMem,nMem fields are uninitialized for table btrees.
**
** Field usage summary:
**
**               Table BTrees                   Index Btrees
**
**   pKey        always NULL                    encoded key
**   nKey        the ROWID                      length of pKey
**   pData       data                           not used
**   aMem        not used                       decomposed key value
**   nMem        not used                       entries in aMem
**   nData       length of pData                not used
**   nZero       extra zeros after pData        not used
**
** This object is used to pass information into sqlite3BtreeInsert().  The
** same information used to be passed as five separate parameters.  But placing
** the information into this object helps to keep the interface more 
** organized and understandable, and it also helps the resulting code to
** run a little faster by using fewer registers for parameter passing.
*/
struct BtreePayload {
  const void *pKey;       /* Key content for indexes.  NULL for tables */
  sqlite3_int64 nKey;     /* Size of pKey for indexes.  PRIMARY KEY for tabs */
  const void *pData;      /* Data for tables. */
  sqlite3_value *aMem;    /* First of nMem value in the unpacked pKey */
  u16 nMem;               /* Number of aMem[] value.  Might be zero */
  int nData;              /* Size of pData.  0 if none. */
  int nZero;              /* Extra zero data appended after pData,nData */
};

• To read payload, BtCursor is used.

int sqlite3BtreePayload(BtCursor*, u32 offset, u32 amt, void*);
const void *sqlite3BtreePayloadFetch(BtCursor*, u32 *pAmt);
u32 sqlite3BtreePayloadSize(BtCursor*);

• sqlite3BtreePayloadFetch returns the contents on the cache directly (for non overflow data).

/*
** For the entry that cursor pCur is point to, return as
** many bytes of the key or data as are available on the local
** b-tree page.  Write the number of available bytes into *pAmt.
**
** The pointer returned is ephemeral.  The key/data may move
** or be destroyed on the next call to any Btree routine,
** including calls from other threads against the same cache.
** Hence, a mutex on the BtShared should be held prior to calling
** this routine.
**
** These routines is used to get quick access to key and data
** in the common case where no overflow pages are used.
*/
const void *sqlite3BtreePayloadFetch(BtCursor *pCur, u32 *pAmt){
  return fetchPayload(pCur, pAmt);
}

• sqlite3BtreePayload copies the data (mainly for overflow data).

/*
** Read part of the payload for the row at which that cursor pCur is currently
** pointing.  "amt" bytes will be transferred into pBuf[].  The transfer
** begins at "offset".
**
** pCur can be pointing to either a table or an index b-tree.
** If pointing to a table btree, then the content section is read.  If
** pCur is pointing to an index b-tree then the key section is read.
**
** For sqlite3BtreePayload(), the caller must ensure that pCur is pointing
** to a valid row in the table.  For sqlite3BtreePayloadChecked(), the
** cursor might be invalid or might need to be restored before being read.
**
** Return SQLITE_OK on success or an error code if anything goes
** wrong.  An error is returned if "offset+amt" is larger than
** the available payload.
*/
int sqlite3BtreePayload(BtCursor *pCur, u32 offset, u32 amt, void *pBuf){
  assert( cursorHoldsMutex(pCur) );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->iPage>=0 && pCur->pPage );
  return accessPayload(pCur, offset, amt, (unsigned char*)pBuf, 0);
}