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perf(query): pre-size mergedList in merge() #9721

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perf(query): pre-size mergedList in merge() #9721

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11 changes: 9 additions & 2 deletions query/outputnode.go
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Original file line number Diff line number Diff line change
Expand Up @@ -888,8 +888,15 @@ func (enc *encoder) merge(parent, child []fastJsonNode) ([]fastJsonNode, error)
return child, nil
}

// Here we merge two slices of maps.
mergedList := make([]fastJsonNode, 0)
// Here we merge two slices of maps. The final size is len(parent) * len(child),
// but the loop below bails out once cnt exceeds LimitNormalizeNode, so cap the
// pre-allocation at that limit to avoid a huge (or overflowing) up-front alloc
// for queries that are going to be rejected anyway.
c := len(parent) * len(child)
if lim := x.Config.LimitNormalizeNode; lim > 0 && c > lim {
c = lim
}
mergedList := make([]fastJsonNode, 0, c)
cnt := 0
for _, pa := range parent {
for _, ca := range child {
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