Date: Wed, 19 Dec 2018 00:39:45 -0800
From: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@postgresql.org, Amit Kapila <amit.kapila16@gmail.com>,
	Robert Haas <robertmhaas@gmail.com>
Subject: Why are we PageInit'ing buffers in RelationAddExtraBlocks()?
Message-ID: <20181219083945.6khtgm36mivonhva@alap3.anarazel.de>
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Hi,

The zheap patchset, even after being based on pluggable storage,
currently has the following condition in RelationAddExtraBlocks():
		if (RelationStorageIsZHeap(relation))
		{
			Assert(BufferGetBlockNumber(buffer) != ZHEAP_METAPAGE);
			ZheapInitPage(page, BufferGetPageSize(buffer));
			freespace = PageGetZHeapFreeSpace(page);
		}
		else
		{
			PageInit(page, BufferGetPageSize(buffer), 0);
			freespace = PageGetHeapFreeSpace(page);
		}

I.e. it initializes the page differently when zheap is used versus
heap.

Thinking about whether it's worth to allow to extend that function in an
extensible manner made me wonder:  Is it actually a good idea to
initialize the page at that point, including marking it dirty?

As far as I can tell that that has several downsides:
- Dirtying the buffer for initialization will cause potentially
  superfluous IO, with no interesting data in the write except for a
  newly initialized page.
- As there's no sort of interlock, it's entirely possible that, after a
  crash, the blocks will come up empty, but with the FSM returning it as
  as empty, so that path would be good to support anyway.
- It adds heap specific code to a routine that otherwise could be
  generic for different table access methods

It seems to me, this could be optimized by *not* initializing the page,
and having a PageIsNew(), check at the places that check whether the
page is new, and initialize it in that case.

Right now we'll just ignore such pages when we encounter them
(e.g. after a crash) until vacuum initializes them. But given that we've
accepted that we'll potentially create empty pages anyway, I don't see
what advantages that provides?  The warnings emitted by vacuum are
pretty scary, and can happen in a lot of legitimate cases, so removing
them seems like a good idea anyway.

I'm pretty tired, so I might be missing something large and obvious
here.

Greetings,

Andres Freund