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perf(iterator): pre-size iters slice in Txn.NewIterator #2290

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perf(iterator): pre-size iters slice in Txn.NewIterator #2290

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18 changes: 15 additions & 3 deletions iterator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -473,9 +473,21 @@ func (txn *Txn) NewIterator(opt IteratorOptions) *Iterator {
tables, decr := txn.db.getMemTables()
defer decr()
txn.db.vlog.incrIteratorCount()
var iters []y.Iterator
if itr := txn.newPendingWritesIterator(opt.Reverse); itr != nil {
iters = append(iters, itr)
// Pre-size for the deterministic memtable-pass plus the optional
// pending-writes iterator. appendIterators may still grow the slice
// for level iterators, but eliminating the per-memtable growslice
// path is a measurable win in dgraph's posting list rollup, where
// len(tables) is typically 6 and the original append-from-nil
// pattern hit growslice 3 times (0→1→2→4→8) just to fit the
// memtable iterators.
pendingItr := txn.newPendingWritesIterator(opt.Reverse)
itersCap := len(tables)
if pendingItr != nil {
itersCap++
}
iters := make([]y.Iterator, 0, itersCap)
if pendingItr != nil {
iters = append(iters, pendingItr)
}
for i := 0; i < len(tables); i++ {
iters = append(iters, tables[i].sl.NewUniIterator(opt.Reverse))
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