fix(DQL): optimize query for has function with offset. (#7727)

[all-seeing-code]

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 the has function on predicates with the @lang tag.

(cherry picked from commit 9ba15b7)

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✅ anurags92
❌ minhaj-shakeel
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[MichelDiz]

@anurags92 this needs to clean up the hidden comments.

[mangalaman93]

Is there a case possible where this if evaluates to false but we still need to perform pagination like before?

[akon-dey]

This should probably be:
return int32(count), int32(sg.Params.Offset)
There is a case where one might want all the results after skipping the first few (denoted by the Offset).

[matthewmcneely]

@akon-dey Actually no, I think that in this case worker/task.go needs pb.query.Offset to in fact be 0 in order to function correctly.

[all-seeing-code]

This PR has a failing test, which needs to be properly understood. I am marking this as draft for now.

[coveralls]

Coverage decreased (-0.007%) to 37.175% when pulling 5e6f44c on cherry-pick-7727 into a4729b3 on main.

[all-seeing-code]

I don't see a significant performance improvement on my end.

main

Dgraph version   : local
Dgraph codename  : dgraph
Dgraph SHA-256   : 8525cf5541e0d9d63398304254cc9638d92a94b4e0dece17a1b0212d509aac81
Commit SHA-1     : a4729b3ca
Commit timestamp : 2022-11-30 17:57:07 -0300
Branch           : main
Go version       : go1.19.2
jemalloc enabled : true

Benchmark results:

goos: linux
goarch: amd64
pkg: github.com/dgraph-io/test
cpu: 12th Gen Intel(R) Core(TM) i7-12700H
BenchmarkQuery100_0-20               403           2635436 ns/op
BenchmarkQuery100_20-20              444           3041838 ns/op
BenchmarkQuery100_200-20             336           3470780 ns/op
BenchmarkQuery100_20000-20           134           7816592 ns/op
PASS
ok      github.com/dgraph-io/test       7.107s

cherry-pick-7727

Dgraph version   : local
Dgraph codename  : dgraph
Dgraph SHA-256   : 23a5ae17dfef9391a7131d1953595e30f5a919f5db61ba9635ac6a1bd7383432
Commit SHA-1     : 5e6f44c03
Commit timestamp : 2022-12-01 17:31:05 +0530
Branch           : cherry-pick-7727
Go version       : go1.19.2
jemalloc enabled : true

Benchmark results:

goos: linux
goarch: amd64
pkg: github.com/dgraph-io/test
cpu: 12th Gen Intel(R) Core(TM) i7-12700H
BenchmarkQuery100_0-20               391           3201947 ns/op
BenchmarkQuery100_20-20              363           3034455 ns/op
BenchmarkQuery100_200-20             481           2211461 ns/op
BenchmarkQuery100_20000-20           229           8433493 ns/op
PASS
ok      github.com/dgraph-io/test       6.790s

Here's the code I used to get the results. It expects a local dgraph running with ldbc-sf.03 dataset inserted.

[all-seeing-code]

This PR has a failing test, which needs to be properly understood. I am marking this as draft for now.

This was fixed. It had a test which required a different set of changes. They are being brought in via: #8471

[MichelDiz]

@anurags92 the description have some hidden comments. Please, remove it.

[all-seeing-code]

On a larger dataset (sf10).

main

alvis@alvis-alien-ubuntu:~/go/src/github.com/dgraph-io/test$ go test -run=XXX -bench=BenchmarkQuery
goos: linux
goarch: amd64
pkg: github.com/dgraph-io/test
cpu: 12th Gen Intel(R) Core(TM) i7-12700H
BenchmarkQuery100_0-20               502           2370298 ns/op
BenchmarkQuery100_20-20              696           2391215 ns/op
BenchmarkQuery100_200-20             552           2456952 ns/op
BenchmarkQuery100_20000-20           187           7336503 ns/op
PASS
ok      github.com/dgraph-io/test       8.886s

cherry-pick-7727

alvis@alvis-alien-ubuntu:~/go/src/github.com/dgraph-io/test$ go test -run=XXX -bench=BenchmarkQuery
goos: linux
goarch: amd64
pkg: github.com/dgraph-io/test
cpu: 12th Gen Intel(R) Core(TM) i7-12700H
BenchmarkQuery100_0-20               470           2335904 ns/op
BenchmarkQuery100_20-20              489           2371475 ns/op
BenchmarkQuery100_200-20             469           2380480 ns/op
BenchmarkQuery100_20000-20           166           6956870 ns/op
PASS
ok      github.com/dgraph-io/test       5.815s

Code to repro: https://go.dev/play/p/Mv6Y6YOG_BR