fix(DQL): optimize query for has function with offset.

[minhaj-shakeel]

Fixes DGRAPH-574.

This PR reduces the latency of the has function slightly. For example the given query:-

{
    me(func: has(actor.film), first: 10, offset: 300000){
       name@en
    }
}

when run on the 21million dataset takes around 80 milliseconds to execute as compared to earlier around 93 milliseconds.

Machine Specs:-

Model Name:	MacBook Pro
  Model Identifier:	MacBookPro16,1
  Processor Name:	6-Core Intel Core i7
  Processor Speed:	2.6 GHz
  Number of Processors:	1
  Total Number of Cores:	6
  L2 Cache (per Core):	256 KB
  L3 Cache:	12 MB
  Memory:	16 GB

Note:-
This does not optimize for has function on predicates with @lang tag.

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This change is 

[pawanrawal]

The change looks good. I have a couple of comments.

1. Are there any tests that already cover this or else could we add some?
2. Also add some benchmark numbers based on your testing in the description. Add a comment saying that this doesn't improve latency for predicates with lang tag.

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @minhaj-shakeel, and @vvbalaji-dgraph)

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protos/pb.proto, line 77 at r1 (raw file):

	int32 first = 15; // used to limit the number of result. Typically, the count is value of first
	// field. Now, It's been used only for has query.
	int32 offset = 16;

add a comment that first and offset help fetch lesser results for the has query when there is no filter and order.

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query/query.go, line 2291 at r1 (raw file):

	if len(sg.Params.Order) == 0 && len(sg.Params.FacetsOrder) == 0 {
		// There is no ordering. Just apply pagination and return.
		if !(len(sg.Filters) == 0 && sg.SrcFunc != nil && sg.SrcFunc.Name == "has") {

Add a comment here that for has function when there is no filtering and sorting we fetch correct paginated results from disk, so no need to apply pagination again.

[pawanrawal]

Reviewed 3 of 3 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @minhaj-shakeel, and @vvbalaji-dgraph)

[minhaj-shakeel]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @pawanrawal, and @vvbalaji-dgraph)

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protos/pb.proto, line 77 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

add a comment that first and offset help fetch lesser results for the has query when there is no filter and order.

Done.

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query/query.go, line 2291 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

Add a comment here that for has function when there is no filtering and sorting we fetch correct paginated results from disk, so no need to apply pagination again.

Done.