[HACKERS] GSoC 2017: weekly progress reports (week 5) and Proposal forpredicate locking in gin index

During this week, I read documentation and source code of gin index to find appropriate places to insert calls to existing functions.

Proposal 

As gin searches have to go all the way to leaf pages to determine whether there is a match or not, we just need a predicate lock on leaf pages. 

Also, gin index has a feature called fast update which postpones the insertion of tuples by temporarily storing them into pending list. The pending list is eventually flushed during vacuum. So this creates a problem because even if a scan acquires a page level predicate lock on the pending list, we will not be able to detect r-w conflict because a new insert will just append tuples on the pending list. So, I think if a fast update is enabled, we need a predicate lock on the entire relation.

->startscanentry()

before calling collectMatchBitmap()

>scanPostingTree()

after acquiring a shared lock on a leaf page

->ginentryinsert()

->gininsertitempointers()

in case of insertion in a posting tree

3. PredicateLockPageSplit()

->dataBeginPlacetoPageLeaf()

after calling dataPlacetoPageLeafSplit()