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PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

It is clearly impossible for readers and inserters to deadlock, and in
fact this algorithm allows them a very high degree of concurrency.
(The exclusive metapage lock taken to update the tuple count is stronger
than necessary, since readers do not care about the tuple count, but the
lock is held for such a short time that this is probably not an issue.)

When an inserter cannot find space in any existing page of a bucket, it
must obtain an overflow page and add that page to the bucket's chain.
Details of that part of the algorithm appear later.

The page split algorithm is entered whenever an inserter observes that the
index is overfull (has a higher-than-wanted ratio of tuples to buckets).
The algorithm attempts, but does not necessarily succeed, to split one
existing bucket in two, thereby lowering the fill ratio:

	exclusive-lock page 0 (assert the right to begin a split)
	read/exclusive-lock meta page
	check split still needed
	if split not needed anymore, drop locks and exit
	decide which bucket to split
	Attempt to X-lock old bucket number (definitely could fail)
	Attempt to X-lock new bucket number (shouldn't fail, but...)
	if above fail, drop locks and exit
	update meta page to reflect new number of buckets
	write/release meta page
	release X-lock on page 0
	-- now, accesses to all other buckets can proceed.
	Perform actual split of bucket, moving tuples as needed
	>> see below about acquiring needed extra space
	Release X-locks of old and new buckets

Note the page zero and metapage locks are not held while the actual tuple
rearrangement is performed, so accesses to other buckets can proceed in
parallel; in fact, it's possible for multiple bucket splits to proceed
in parallel.

Split's attempt to X-lock the old bucket number could fail if another
process holds S-lock on it.  We do not want to wait if that happens, first
because we don't want to wait while holding the metapage exclusive-lock,
and second because it could very easily result in deadlock.  (The other
process might be out of the hash AM altogether, and could do something
that blocks on another lock this process holds; so even if the hash
algorithm itself is deadlock-free, a user-induced deadlock could occur.)
So, this is a conditional LockAcquire operation, and if it fails we just
abandon the attempt to split.  This is all right since the index is
overfull but perfectly functional.  Every subsequent inserter will try to
split, and eventually one will succeed.  If multiple inserters failed to
split, the index might still be overfull, but eventually, the index will
not be overfull and split attempts will stop.  (We could make a successful
splitter loop to see if the index is still overfull, but it seems better to
distribute the split overhead across successive insertions.)

A problem is that if a split fails partway through (eg due to insufficient
disk space) the index is left corrupt.  The probability of that could be
made quite low if we grab a free page or two before we update the meta
page, but the only real solution is to treat a split as a WAL-loggable,
must-complete action.  I'm not planning to teach hash about WAL in this
go-round.

The fourth operation is garbage collection (bulk deletion):

	next bucket := 0
	read/sharelock meta page
	fetch current max bucket number
	release meta page
	while next bucket <= max bucket do
		Acquire X lock on target bucket
		Scan and remove tuples, compact free space as needed
		Release X lock
		next bucket ++
	end loop
	exclusive-lock meta page
	check if number of buckets changed
	if so, release lock and return to for-each-bucket loop
	else update metapage tuple count
	write/release meta page

Note that this is designed to allow concurrent splits.  If a split occurs,
tuples relocated into the new bucket will be visited twice by the scan,
but that does no harm.  (We must however be careful about the statistics
reported by the VACUUM operation.  What we can do is count the number of
tuples scanned, and believe this in preference to the stored tuple count
if the stored tuple count and number of buckets did *not* change at any
time during the scan.  This provides a way of correcting the stored tuple
count if it gets out of sync for some reason.  But if a split or insertion
does occur concurrently, the scan count is untrustworthy; instead,
subtract the number of tuples deleted from the stored tuple count and
use that.)

The exclusive lock request could deadlock in some strange scenarios, but
we can just error out without any great harm being done.


Free space management
---------------------

Free space management consists of two sub-algorithms, one for reserving
an overflow page to add to a bucket chain, and one for returning an empty
overflow page to the free pool.

Obtaining an overflow page:

	read/exclusive-lock meta page
	determine next bitmap page number; if none, exit loop
	release meta page lock
	read/exclusive-lock bitmap page
	search for a free page (zero bit in bitmap)
	if found:
		set bit in bitmap
		write/release bitmap page
		read/exclusive-lock meta page
		if first-free-bit value did not change,
			update it and write meta page
		release meta page
		return page number
	else (not found):
	release bitmap page
	loop back to try next bitmap page, if any
-- here when we have checked all bitmap pages; we hold meta excl. lock
	extend index to add another overflow page; update meta information
	write/release meta page
	return page number

It is slightly annoying to release and reacquire the metapage lock
multiple times, but it seems best to do it that way to minimize loss of
concurrency against processes just entering the index.  We don't want
to hold the metapage exclusive lock while reading in a bitmap page.
(We can at least avoid repeated buffer pin/unpin here.)

The normal path for extending the index does not require doing I/O while
holding the metapage lock.  We do have to do I/O when the extension
requires adding a new bitmap page as well as the required overflow page
... but that is an infrequent case, so the loss of concurrency seems
acceptable.

The portion of tuple insertion that calls the above subroutine looks
like this:

	-- having determined that no space is free in the target bucket:
	remember last page of bucket, drop write lock on it
	call free-page-acquire routine
	re-write-lock last page of bucket
	if it is not last anymore, step to the last page
	update (former) last page to point to new page
	write-lock and initialize new page, with back link to former last page
	write and release former last page
	insert tuple into new page
	-- etc.

Notice this handles the case where two concurrent inserters try to extend
the same bucket.  They will end up with a valid, though perhaps
space-inefficient, configuration: two overflow pages will be added to the
bucket, each containing one tuple.

The last part of this violates the rule about holding write lock on two
pages concurrently, but it should be okay to write-lock the previously
free page; there can be no other process holding lock on it.

Bucket splitting uses a similar algorithm if it has to extend the new
bucket, but it need not worry about concurrent extension since it has
exclusive lock on the new bucket.

Freeing an overflow page is done by garbage collection and by bucket
splitting (the old bucket may contain no-longer-needed overflow pages).
In both cases, the process holds exclusive lock on the containing bucket,
so need not worry about other accessors of pages in the bucket.  The
algorithm is:

	delink overflow page from bucket chain
	(this requires read/update/write/release of fore and aft siblings)
	read/share-lock meta page
	determine which bitmap page contains the free space bit for page
	release meta page
	read/exclusive-lock bitmap page
	update bitmap bit
	write/release bitmap page
	if page number is less than what we saw as first-free-bit in meta:
	read/exclusive-lock meta page
	if page number is still less than first-free-bit,
		update first-free-bit field and write meta page
	release meta page

We have to do it this way because we must clear the bitmap bit before
changing the first-free-bit field (hashm_firstfree).  It is possible that
we set first-free-bit too small (because someone has already reused the
page we just freed), but that is okay; the only cost is the next overflow
page acquirer will scan more bitmap bits than he needs to.  What must be
avoided is having first-free-bit greater than the actual first free bit,
because then that free page would never be found by searchers.

All the freespace operations should be called while holding no buffer
locks.  Since they need no lmgr locks, deadlock is not possible.


Other notes
-----------

All the shenanigans with locking prevent a split occurring while *another*
process is stopped in a given bucket.  They do not ensure that one of
our *own* backend's scans is not stopped in the bucket, because lmgr
doesn't consider a process's own locks to conflict.  So the Split
algorithm must check for that case separately before deciding it can go
ahead with the split.  VACUUM does not have this problem since nothing
else can be happening within the vacuuming backend.

Should we instead try to fix the state of any conflicting local scan?
Seems mighty ugly --- got to move the held bucket S-lock as well as lots
of other messiness.  For now, just punt and don't split.