func (scan *HeapScan) getBuffer(blockNum system.BlockNumber) (storage.Buffer, system.BlockNumber, error) {
// release previous scan buffer, if any
if scan.cBuf.IsValid() {
scan.bufMgr.ReleaseBuffer(scan.cBuf)
scan.cBuf = storage.InvalidBuffer()
}
// read page
buf, err := scan.bufMgr.ReadBuffer(scan.rel.RelNode, blockNum)
if err != nil {
return storage.InvalidBuffer(), system.InvalidBlockNumber, err
}
return buf, blockNum, nil
}
func (scan *HeapScan) Next() (Tuple, error) {
var lineOff system.OffsetNumber
var cBlock system.BlockNumber = system.InvalidBlockNumber
tuple := scan.cTuple
if !scan.inited {
// return immediately if relation is empty
if scan.nBlocks == 0 {
tuple.SetData(nil, system.InvalidItemPointer)
return nil, nil
}
cBlock = scan.startBlock
if buf, block, err := scan.getBuffer(cBlock); err != nil {
return nil, err
} else {
scan.cBuf, scan.cBlock = buf, block
}
lineOff = system.FirstOffsetNumber
scan.inited = true
} else {
// continue from previously returned page/tuple
cBlock = scan.cBlock
lineOff = tuple.self.OffsetNumber().Next()
}
scan.cBuf.RLock()
page := scan.cBuf.GetPage()
nLines := page.MaxOffsetNumber()
linesLeft := nLines - lineOff + 1
itemId := page.ItemId(lineOff)
for {
for linesLeft > 0 {
if itemId.IsNormal() {
tid := system.MakeItemPointer(cBlock, lineOff)
tuple.SetData(page.Item(itemId), tid)
// TODO: valid = HeapTupleSatisfyiesVisibility()
scan.cBuf.Unlock()
}
// otherwise move to the next item on the page
linesLeft--
lineOff++
itemId = page.ItemId(lineOff)
}
// if we get here, it means we've exhausted the items on this page and
// it's time to move to the next.
scan.cBuf.Unlock()
cBlock++
if cBlock >= scan.nBlocks {
cBlock = 0
}
finished := cBlock == scan.startBlock
if finished {
if scan.cBuf.IsValid() {
scan.bufMgr.ReleaseBuffer(scan.cBuf)
}
scan.cBuf = storage.InvalidBuffer()
scan.cBlock = system.InvalidBlockNumber
tuple.SetData(nil, system.InvalidItemPointer)
scan.inited = false
return nil, nil
}
if buf, block, err := scan.getBuffer(cBlock); err != nil {
return nil, err
} else {
scan.cBuf, scan.cBlock = buf, block
}
scan.cBuf.RLock()
page = scan.cBuf.GetPage()
nLines = page.MaxOffsetNumber()
linesLeft = nLines
lineOff = system.FirstOffsetNumber
itemId = page.ItemId(lineOff)
}
}