Don't read data posting lists in certain cases

[manishrjain]

To further #3695 , only read posting lists in certain cases. For e.g., when they are indexed, or when there's a delete operation or when it's a single UID predicate, etc. For the rest, don't read them. This further improves the performance of live data loading.

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This change is 

[martinmr]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @gitlw and @manishrjain)

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posting/oracle.go, line 80 at r1 (raw file):

}

func (txn *Txn) GetFromDelta(key []byte) (*List, error) {

add docstring

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worker/mutation.go, line 82 at r1 (raw file):

Quoted 6 lines of code…

	// The following is a performance optimization which allows us to not read a posting list from
	// disk. We calculate this based on how AddMutationWithIndex works. General idea is that if
	// we're not using the read posting list, we don't need to retrieve it. We need posting list if
	// we're doing indexing or count index or enforcing single UID, etc. In other cases, we can just
	// create a posting list facade in memory and use it to store the delta in Badger. Later, Rollup
	// operation would consolidate all these deltas into a posting list.

minor:

"The general idea"
"We need the posting list"
"Later, the Rollup operation"

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worker/mutation.go, line 85 at r1 (raw file):

	// create a posting list facade in memory and use it to store the delta in Badger. Later, Rollup
	// operation would consolidate all these deltas into a posting list.
	var fn func(key []byte) (*posting.List, error)

maybe call this getFn as fn is too generic?

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worker/mutation.go, line 99 at r1 (raw file):

		// Reverse index doesn't need the posting list to be read. We already covered count index,
		// single uid and delete all above.
		// Values, whether single or list, don't need to read.

minor: "to be read"

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worker/mutation.go, line 100 at r1 (raw file):

		// single uid and delete all above.
		// Values, whether single or list, don't need to read.
		// Uid list doesn't need to read.

minor: "to be read"

[gitlw]

after addressing Martin's comments.

Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @gitlw and @manishrjain)

[manishrjain]

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @martinmr)

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posting/oracle.go, line 80 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

add docstring

Done.

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worker/mutation.go, line 82 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

	// The following is a performance optimization which allows us to not read a posting list from
	// disk. We calculate this based on how AddMutationWithIndex works. General idea is that if
	// we're not using the read posting list, we don't need to retrieve it. We need posting list if
	// we're doing indexing or count index or enforcing single UID, etc. In other cases, we can just
	// create a posting list facade in memory and use it to store the delta in Badger. Later, Rollup
	// operation would consolidate all these deltas into a posting list.

minor:

"The general idea"
"We need the posting list"
"Later, the Rollup operation"

Done.

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worker/mutation.go, line 85 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

maybe call this getFn as fn is too generic?

Done.