1. 分批次读取
1.1 BatchSize如何设置
源码分析：
sql/colfetcher/colbatch_scan.go
// NewColBatchScan creates a new ColBatchScan operator.
func NewColBatchScan(
ctx context.Context,
allocator *colmem.Allocator,
flowCtx *execinfra.FlowCtx,
evalCtx *tree.EvalContext,
spec *execinfrapb.TableReaderSpec,
post *execinfrapb.PostProcessSpec,
) (*ColBatchScan, error) {
...
limitHint := execinfra.LimitHint(spec.LimitHint, post)
...
s := colBatchScanPool.Get().(*ColBatchScan)
spans := s.spans[:0]
for i := range spec.Spans {
spans = append(spans, spec.Spans[i].Span)
}
*s = ColBatchScan{
ctx: ctx,
spans: spans,
flowCtx: flowCtx,
rf: fetcher,
limitHint: limitHint, // 初始化limitHint
// Parallelize shouldn't be set when there's a limit hint, but double-check
// just in case.
parallelize: spec.Parallelize && limitHint == 0,
ResultTypes: typs,
}
return s, nil
}开始Scan时设置的limitHint
// Init initializes a ColBatchScan.
func (s *ColBatchScan) Init() {
s.init = true
limitBatches := !s.parallelize
if err := s.rf.StartScan(
s.ctx, s.flowCtx.Txn, s.spans,
limitBatches, s.limitHint, // 传入limitHint
s.flowCtx.TraceKV,
); err != nil {
colexecerror.InternalError(err)
}
}sql/colfetcher/cfetcher.go
// StartScan initializes and starts the key-value scan. Can be used multiple
// times.
func (rf *cFetcher) StartScan(
ctx context.Context,
txn *kv.Txn,
spans roachpb.Spans,
limitBatches bool,
limitHint int64,
traceKV bool,
) error {
...
firstBatchLimit := limitHint
if firstBatchLimit != 0 {
// The limitHint is a row limit, but each row could be made up
// of more than one key. We take the maximum possible keys
// per row out of all the table rows we could potentially
// scan over.
firstBatchLimit = limitHint * int64(rf.maxKeysPerRow)
// We need an extra key to make sure we form the last row.
firstBatchLimit++ // 自增１
}
// Note that we pass a nil memMonitor here, because the cfetcher does its own
// memory accounting.
f, err := row.NewKVFetcher(
txn,
spans,
rf.reverse,
limitBatches,
firstBatchLimit, // 传入firstBatchLimit
rf.lockStrength,
rf.lockWaitPolicy,
nil, /* memMonitor */
)
...
}sql/row/kv_fetcher.go
// NewKVFetcher creates a new KVFetcher.
// If mon is non-nil, this fetcher will track its fetches and must be Closed.
func NewKVFetcher(
txn *kv.Txn,
spans roachpb.Spans,
reverse bool,
useBatchLimit bool,
firstBatchLimit int64,
lockStrength descpb.ScanLockingStrength,
lockWaitPolicy descpb.ScanLockingWaitPolicy,
mon *mon.BytesMonitor,
) (*KVFetcher, error) {
kvBatchFetcher, err := makeKVBatchFetcher(
txn, spans, reverse, useBatchLimit, firstBatchLimit, lockStrength, lockWaitPolicy, mon,
) //参数传递
return newKVFetcher(&kvBatchFetcher), err
}
func makeKVBatchFetcher(
txn *kv.Txn,
spans roachpb.Spans,
reverse bool,
useBatchLimit bool,
firstBatchLimit int64,
lockStrength descpb.ScanLockingStrength,
lockWaitPolicy descpb.ScanLockingWaitPolicy,
mon *mon.BytesMonitor,
) (txnKVFetcher, error) {
sendFn := func(ctx context.Context, ba roachpb.BatchRequest) (*roachpb.BatchResponse, error) {
res, err := txn.Send(ctx, ba)
if err != nil {
return nil, err.GoError()
}
return res, nil
}
return makeKVBatchFetcherWithSendFunc(
sendFn, spans, reverse, useBatchLimit, firstBatchLimit, lockStrength, lockWaitPolicy, mon,
) //参数传递
}
// makeKVBatchFetcherWithSendFunc is like makeKVBatchFetcher but uses a custom
// send function.
func makeKVBatchFetcherWithSendFunc(
sendFn sendFunc,
spans roachpb.Spans,
reverse bool,
useBatchLimit bool,
firstBatchLimit int64,
lockStrength descpb.ScanLockingStrength,
lockWaitPolicy descpb.ScanLockingWaitPolicy,
mon *mon.BytesMonitor,
) (txnKVFetcher, error) {
if firstBatchLimit < 0 || (!useBatchLimit && firstBatchLimit != 0) {
return txnKVFetcher{}, errors.Errorf("invalid batch limit %d (useBatchLimit: %t)",
firstBatchLimit, useBatchLimit)
}
if useBatchLimit {
// Verify the spans are ordered if a batch limit is used.
for i := 1; i < len(spans); i++ {
if spans[i].Key.Compare(spans[i-1].EndKey) < 0 {
return txnKVFetcher{}, errors.Errorf("unordered spans (%s %s)", spans[i-1], spans[i])
}
}
} else if util.RaceEnabled {
// Otherwise, just verify the spans don't contain consecutive overlapping
// spans.
for i := 1; i < len(spans); i++ {
if spans[i].Key.Compare(spans[i-1].EndKey) >= 0 {
// Current span's start key is greater than or equal to the last span's
// end key - we're good.
continue
} else if spans[i].EndKey.Compare(spans[i-1].Key) < 0 {
// Current span's end key is less than or equal to the last span's start
// key - also good.
continue
}
// Otherwise, the two spans overlap, which isn't allowed - it leaves us at
// risk of incorrect results, since the row fetcher can't distinguish
// between identical rows in two different batches.
return txnKVFetcher{}, errors.Errorf("overlapping neighbor spans (%s %s)", spans[i-1], spans[i])
}
}
// Make a copy of the spans because we update them.
copySpans := make(roachpb.Spans, len(spans))
for i := range spans {
if reverse {
// Reverse scans receive the spans in decreasing order.
copySpans[len(spans)-i-1] = spans[i]
} else {
copySpans[i] = spans[i]
}
}
return txnKVFetcher{
sendFn: sendFn,
spans: copySpans,
reverse: reverse,
useBatchLimit: useBatchLimit,
firstBatchLimit: firstBatchLimit, //参数传递
lockStrength: lockStrength,
lockWaitPolicy: lockWaitPolicy,
mon: mon,
acc: mon.MakeBoundAccount(),
}, nil
}sql/row/kv_batch_fetcher.go
// fetch retrieves spans from the kv layer.
func (f *txnKVFetcher) fetch(ctx context.Context) error {
var ba roachpb.BatchRequest
ba.Header.WaitPolicy = f.getWaitPolicy()
ba.Header.MaxSpanRequestKeys = f.getBatchSize() //参数传递
...
}// getBatchSize returns the max size of the next batch.
func (f *txnKVFetcher) getBatchSize() int64 {
return f.getBatchSizeForIdx(f.batchIdx)
}
func (f *txnKVFetcher) getBatchSizeForIdx(batchIdx int) int64 {
if !f.useBatchLimit {
return 0
}
if f.firstBatchLimit == 0 || f.firstBatchLimit >= kvBatchSize {
return kvBatchSize
}
// We grab the first batch according to the limit. If it turns out that we
// need another batch, we grab a bigger batch. If that's still not enough,
// we revert to the default batch size.
switch batchIdx {
case 0:
return f.firstBatchLimit
case 1:
// Make the second batch 10 times larger (but at most the default batch
// size and at least 1/10 of the default batch size). Sample
// progressions of batch sizes:
//
// First batch | Second batch | Subsequent batches
// -----------------------------------------------
// 1 | 1,000 | 10,000
// 100 | 1,000 | 10,000
// 500 | 5,000 | 10,000
// 1000 | 10,000 | 10,000
secondBatch := f.firstBatchLimit * 10
switch {
case secondBatch < kvBatchSize/10:
return kvBatchSize / 10
case secondBatch > kvBatchSize:
return kvBatchSize
default:
return secondBatch
}
default:
return kvBatchSize
}
}