Change split keys to have a different prefix.

[martinmr]

Currently ReadPostingList is skipping the iterator past the last possible key for
a multi-part list. The assumption was that the only keys in this range are the
main keys and the keys for each part. This is the case when a predicate has a
term and count index.

This PR changes the format of the split keys so that the first byte is different for
the keys holding parts of a multi-part list. This allows the stream framework to
skip over those keys during a rollup, eliminating the need for skipping keys.

This PR, also changes ReadPostingList to return a nil list when an individual part
is requested and the List struct so that all the operations on a nil list are a no-op
to safely handle cases where the list is being tried to accessed from the key of one
of its parts.

Fixes #4905. See that issue for more details.

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[manishrjain]

The split key code generation has issue. It overwrites over the count key. Please add tests.

Do not merge this change.

Reviewable status: 0 of 2 files reviewed, all discussions resolved (waiting on @manishrjain)

[manishrjain]

Good to go.

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @martinmr)

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posting/list.go, line 543 at r1 (raw file):

//  })
func (l *List) Iterate(readTs uint64, afterUid uint64, f func(obj *pb.Posting) error) error {
	if l == nil {

Let's remove all the l == nil.

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posting/mvcc.go, line 162 at r1 (raw file):

		// public methods of the List object are no-ops and the list is being already
		// accessed via the main key in the places where this code is reached (e.g rollups).
		return nil, nil

could return an error.

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x/keys.go, line 420 at r1 (raw file):

// GetSplitKey takes a key baseKey and generates the key of the list split that starts at startUid.
func GetSplitKey(baseKey []byte, startUid uint64) ([]byte, error) {

Remove the Get prefix?

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x/keys.go, line 426 at r1 (raw file):

	// Change the first byte (i.e the key prefix) to ByteSplit to signal this is an
	// individual part of a single list key.
	keyCopy[0] = ByteSplit

Maybe check that it has a defaultPrefix byte, and not another byte. Return error in that case.

[martinmr]

Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @manishrjain)

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posting/list.go, line 543 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Let's remove all the l == nil.

Done.

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posting/mvcc.go, line 162 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

could return an error.

Done.

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x/keys.go, line 420 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Remove the Get prefix?

Done.

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x/keys.go, line 426 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Maybe check that it has a defaultPrefix byte, and not another byte. Return error in that case.

Done.