$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.12 2008/10/20 19:18:18 alvherre Exp $

The Transaction System
======================

PostgreSQL's transaction system is a three-layer system.  The bottom layer
implements low-level transactions and subtransactions, on top of which rests
the mainloop's control code, which in turn implements user-visible
transactions and savepoints.

The middle layer of code is called by postgres.c before and after the
processing of each query, or after detecting an error:

		StartTransactionCommand
		CommitTransactionCommand
		AbortCurrentTransaction

Meanwhile, the user can alter the system's state by issuing the SQL commands
BEGIN, COMMIT, ROLLBACK, SAVEPOINT, ROLLBACK TO or RELEASE.  The traffic cop
redirects these calls to the toplevel routines

		BeginTransactionBlock
		EndTransactionBlock
		UserAbortTransactionBlock
		DefineSavepoint
		RollbackToSavepoint
		ReleaseSavepoint

respectively.  Depending on the current state of the system, these functions
call low level functions to activate the real transaction system:

		StartTransaction
		CommitTransaction
		AbortTransaction
		CleanupTransaction
		StartSubTransaction
		CommitSubTransaction
		AbortSubTransaction
		CleanupSubTransaction

Additionally, within a transaction, CommandCounterIncrement is called to
increment the command counter, which allows future commands to "see" the
effects of previous commands within the same transaction.  Note that this is
done automatically by CommitTransactionCommand after each query inside a
transaction block, but some utility functions also do it internally to allow
some operations (usually in the system catalogs) to be seen by future
operations in the same utility command.  (For example, in DefineRelation it is
done after creating the heap so the pg_class row is visible, to be able to
lock it.)


For example, consider the following sequence of user commands:

1)		BEGIN
2)		SELECT * FROM foo
3)		INSERT INTO foo VALUES (...)
4)		COMMIT

In the main processing loop, this results in the following function call
sequence:

	 /	StartTransactionCommand;
	/		StartTransaction;
1) <		ProcessUtility;				<< BEGIN
	\		BeginTransactionBlock;
	 \	CommitTransactionCommand;

	/	StartTransactionCommand;
2) /		ProcessQuery;				<< SELECT ...
   \		CommitTransactionCommand;
	\		CommandCounterIncrement;

	/	StartTransactionCommand;
3) /		ProcessQuery;				<< INSERT ...
   \		CommitTransactionCommand;
	\		CommandCounterIncrement;

	 /	StartTransactionCommand;
	/	ProcessUtility;				<< COMMIT
4) <			EndTransactionBlock;
	\	CommitTransactionCommand;
	 \		CommitTransaction;

The point of this example is to demonstrate the need for
StartTransactionCommand and CommitTransactionCommand to be state smart -- they
should call CommandCounterIncrement between the calls to BeginTransactionBlock
and EndTransactionBlock and outside these calls they need to do normal start,
commit or abort processing.

Furthermore, suppose the "SELECT * FROM foo" caused an abort condition.	In
this case AbortCurrentTransaction is called, and the transaction is put in
aborted state.  In this state, any user input is ignored except for
transaction-termination statements, or ROLLBACK TO <savepoint> commands.

Transaction aborts can occur in two ways:

1)	system dies from some internal cause  (syntax error, etc)
2)	user types ROLLBACK

The reason we have to distinguish them is illustrated by the following two
situations:

	case 1					case 2
	------					------
1) user types BEGIN			1) user types BEGIN
2) user does something			2) user does something
3) user does not like what		3) system aborts for some reason
   she sees and types ABORT		   (syntax error, etc)

In case 1, we want to abort the transaction and return to the default state.
In case 2, there may be more commands coming our way which are part of the
same transaction block; we have to ignore these commands until we see a COMMIT
or ROLLBACK.

Internal aborts are handled by AbortCurrentTransaction, while user aborts are
handled by UserAbortTransactionBlock.  Both of them rely on AbortTransaction
to do all the real work.  The only difference is what state we enter after
AbortTransaction does its work:

* AbortCurrentTransaction leaves us in TBLOCK_ABORT,
* UserAbortTransactionBlock leaves us in TBLOCK_ABORT_END

Low-level transaction abort handling is divided in two phases:
* AbortTransaction executes as soon as we realize the transaction has
  failed.  It should release all shared resources (locks etc) so that we do
  not delay other backends unnecessarily.
* CleanupTransaction executes when we finally see a user COMMIT
  or ROLLBACK command; it cleans things up and gets us out of the transaction
  completely.  In particular, we mustn't destroy TopTransactionContext until
  this point.

Also, note that when a transaction is committed, we don't close it right away.
Rather it's put in TBLOCK_END state, which means that when
CommitTransactionCommand is called after the query has finished processing,
the transaction has to be closed.  The distinction is subtle but important,
because it means that control will leave the xact.c code with the transaction
open, and the main loop will be able to keep processing inside the same
transaction.  So, in a sense, transaction commit is also handled in two
phases, the first at EndTransactionBlock and the second at
CommitTransactionCommand (which is where CommitTransaction is actually
called).

The rest of the code in xact.c are routines to support the creation and
finishing of transactions and subtransactions.  For example, AtStart_Memory
takes care of initializing the memory subsystem at main transaction start.


Subtransaction Handling
-----------------------

Subtransactions are implemented using a stack of TransactionState structures,
each of which has a pointer to its parent transaction's struct.  When a new
subtransaction is to be opened, PushTransaction is called, which creates a new
TransactionState, with its parent link pointing to the current transaction.
StartSubTransaction is in charge of initializing the new TransactionState to
sane values, and properly initializing other subsystems (AtSubStart routines).

When closing a subtransaction, either CommitSubTransaction has to be called
(if the subtransaction is committing), or AbortSubTransaction and
CleanupSubTransaction (if it's aborting).  In either case, PopTransaction is
called so the system returns to the parent transaction.

One important point regarding subtransaction handling is that several may need
to be closed in response to a single user command.  That's because savepoints
have names, and we allow to commit or rollback a savepoint by name, which is
not necessarily the one that was last opened.  Also a COMMIT or ROLLBACK
command must be able to close out the entire stack.  We handle this by having
the utility command subroutine mark all the state stack entries as commit-
pending or abort-pending, and then when the main loop reaches
CommitTransactionCommand, the real work is done.  The main point of doing
things this way is that if we get an error while popping state stack entries,
the remaining stack entries still show what we need to do to finish up.

In the case of ROLLBACK TO <savepoint>, we abort all the subtransactions up
through the one identified by the savepoint name, and then re-create that
subtransaction level with the same name.  So it's a completely new
subtransaction as far as the internals are concerned.

Other subsystems are allowed to start "internal" subtransactions, which are
handled by BeginInternalSubtransaction.  This is to allow implementing
exception handling, e.g. in PL/pgSQL.  ReleaseCurrentSubTransaction and
RollbackAndReleaseCurrentSubTransaction allows the subsystem to close said
subtransactions.  The main difference between this and the savepoint/release
path is that we execute the complete state transition immediately in each
subroutine, rather than deferring some work until CommitTransactionCommand.
Another difference is that BeginInternalSubtransaction is allowed when no
explicit transaction block has been established, while DefineSavepoint is not.


Transaction and Subtransaction Numbering
----------------------------------------

Transactions and subtransactions are assigned permanent XIDs only when/if
they first do something that requires one --- typically, insert/update/delete
a tuple, though there are a few other places that need an XID assigned.
If a subtransaction requires an XID, we always first assign one to its
parent.  This maintains the invariant that child transactions have XIDs later
than their parents, which is assumed in a number of places.

The subsidiary actions of obtaining a lock on the XID and and entering it into
pg_subtrans and PG_PROC are done at the time it is assigned.

A transaction that has no XID still needs to be identified for various
purposes, notably holding locks.  For this purpose we assign a "virtual
transaction ID" or VXID to each top-level transaction.  VXIDs are formed from
two fields, the backendID and a backend-local counter; this arrangement allows
assignment of a new VXID at transaction start without any contention for
shared memory.  To ensure that a VXID isn't re-used too soon after backend
exit, we store the last local counter value into shared memory at backend
exit, and initialize it from the previous value for the same backendID slot
at backend start.  All these counters go back to zero at shared memory
re-initialization, but that's OK because VXIDs never appear anywhere on-disk.

Internally, a backend needs a way to identify subtransactions whether or not
they have XIDs; but this need only lasts as long as the parent top transaction
endures.  Therefore, we have SubTransactionId, which is somewhat like
CommandId in that it's generated from a counter that we reset at the start of
each top transaction.  The top-level transaction itself has SubTransactionId 1,
and subtransactions have IDs 2 and up.  (Zero is reserved for
InvalidSubTransactionId.)  Note that subtransactions do not have their
own VXIDs; they use the parent top transaction's VXID.


Interlocking Transaction Begin, Transaction End, and Snapshots
--------------------------------------------------------------

We try hard to minimize the amount of overhead and lock contention involved
in the frequent activities of beginning/ending a transaction and taking a
snapshot.  Unfortunately, we must have some interlocking for this, because
we must ensure consistency about the commit order of transactions.
For example, suppose an UPDATE in xact A is blocked by xact B's prior
update of the same row, and xact B is doing commit while xact C gets a
snapshot.  Xact A can complete and commit as soon as B releases its locks.
If xact C's GetSnapshotData sees xact B as still running, then it had
better see xact A as still running as well, or it will be able to see two
tuple versions - one deleted by xact B and one inserted by xact A.  Another
reason why this would be bad is that C would see (in the row inserted by A)
earlier changes by B, and it would be inconsistent for C not to see any
of B's changes elsewhere in the database.

Formally, the correctness requirement is "if a snapshot A considers
transaction X as committed, and any of transaction X's snapshots considered
transaction Y as committed, then snapshot A must consider transaction Y as
committed".

What we actually enforce is strict serialization of commits and rollbacks
with snapshot-taking: we do not allow any transaction to exit the set of
running transactions while a snapshot is being taken.  (This rule is
stronger than necessary for consistency, but is relatively simple to
enforce, and it assists with some other issues as explained below.)  The
implementation of this is that GetSnapshotData takes the ProcArrayLock in
shared mode (so that multiple backends can take snapshots in parallel),
but ProcArrayEndTransaction must take the ProcArrayLock in exclusive mode
while clearing MyProc->xid at transaction end (either commit or abort).

ProcArrayEndTransaction also holds the lock while advancing the shared
latestCompletedXid variable.  This allows GetSnapshotData to use
latestCompletedXid + 1 as xmax for its snapshot: there can be no
transaction >= this xid value that the snapshot needs to consider as
completed.

In short, then, the rule is that no transaction may exit the set of
currently-running transactions between the time we fetch latestCompletedXid
and the time we finish building our snapshot.  However, this restriction
only applies to transactions that have an XID --- read-only transactions
can end without acquiring ProcArrayLock, since they don't affect anyone
else's snapshot nor latestCompletedXid.

Transaction start, per se, doesn't have any interlocking with these
considerations, since we no longer assign an XID immediately at transaction
start.  But when we do decide to allocate an XID, GetNewTransactionId must
store the new XID into the shared ProcArray before releasing XidGenLock.
This ensures that all top-level XIDs <= latestCompletedXid are either
present in the ProcArray, or not running anymore.  (This guarantee doesn't
apply to subtransaction XIDs, because of the possibility that there's not
room for them in the subxid array; instead we guarantee that they are
present or the overflow flag is set.)  If a backend released XidGenLock
before storing its XID into MyProc, then it would be possible for another
backend to allocate and commit a later XID, causing latestCompletedXid to
pass the first backend's XID, before that value became visible in the
ProcArray.  That would break GetOldestXmin, as discussed below.

We allow GetNewTransactionId to store the XID into MyProc->xid (or the
subxid array) without taking ProcArrayLock.  This was once necessary to
avoid deadlock; while that is no longer the case, it's still beneficial for
performance.  We are thereby relying on fetch/store of an XID to be atomic,
else other backends might see a partially-set XID.  This also means that
readers of the ProcArray xid fields must be careful to fetch a value only
once, rather than assume they can read it multiple times and get the same
answer each time.  (Use volatile-qualified pointers when doing this, to
ensure that the C compiler does exactly what you tell it to.)

Another important activity that uses the shared ProcArray is GetOldestXmin,
which must determine a lower bound for the oldest xmin of any active MVCC
snapshot, system-wide.  Each individual backend advertises the smallest
xmin of its own snapshots in MyProc->xmin, or zero if it currently has no
live snapshots (eg, if it's between transactions or hasn't yet set a
snapshot for a new transaction).  GetOldestXmin takes the MIN() of the
valid xmin fields.  It does this with only shared lock on ProcArrayLock,
which means there is a potential race condition against other backends
doing GetSnapshotData concurrently: we must be certain that a concurrent
backend that is about to set its xmin does not compute an xmin less than
what GetOldestXmin returns.  We ensure that by including all the active
XIDs into the MIN() calculation, along with the valid xmins.  The rule that
transactions can't exit without taking exclusive ProcArrayLock ensures that
concurrent holders of shared ProcArrayLock will compute the same minimum of
currently-active XIDs: no xact, in particular not the oldest, can exit
while we hold shared ProcArrayLock.  So GetOldestXmin's view of the minimum
active XID will be the same as that of any concurrent GetSnapshotData, and
so it can't produce an overestimate.  If there is no active transaction at
all, GetOldestXmin returns latestCompletedXid + 1, which is a lower bound
for the xmin that might be computed by concurrent or later GetSnapshotData
calls.  (We know that no XID less than this could be about to appear in
the ProcArray, because of the XidGenLock interlock discussed above.)

GetSnapshotData also performs an oldest-xmin calculation (which had better
match GetOldestXmin's) and stores that into RecentGlobalXmin, which is used
for some tuple age cutoff checks where a fresh call of GetOldestXmin seems
too expensive.  Note that while it is certain that two concurrent
executions of GetSnapshotData will compute the same xmin for their own
snapshots, as argued above, it is not certain that they will arrive at the
same estimate of RecentGlobalXmin.  This is because we allow XID-less
transactions to clear their MyProc->xmin asynchronously (without taking
ProcArrayLock), so one execution might see what had been the oldest xmin,
and another not.  This is OK since RecentGlobalXmin need only be a valid
lower bound.  As noted above, we are already assuming that fetch/store
of the xid fields is atomic, so assuming it for xmin as well is no extra
risk.


pg_clog and pg_subtrans
-----------------------

pg_clog and pg_subtrans are permanent (on-disk) storage of transaction related
information.  There is a limited number of pages of each kept in memory, so
in many cases there is no need to actually read from disk.  However, if
there's a long running transaction or a backend sitting idle with an open
transaction, it may be necessary to be able to read and write this information
from disk.  They also allow information to be permanent across server restarts.

pg_clog records the commit status for each transaction that has been assigned
an XID.  A transaction can be in progress, committed, aborted, or
"sub-committed".  This last state means that it's a subtransaction that's no
longer running, but its parent has not updated its state yet.  It is not
necessary to update a subtransaction's transaction status to subcommit, so we
can just defer it until main transaction commit.  The main role of marking
transactions as sub-committed is to provide an atomic commit protocol when
transaction status is spread across multiple clog pages. As a result, whenever
transaction status spreads across multiple pages we must use a two-phase commit
protocol: the first phase is to mark the subtransactions as sub-committed, then
we mark the top level transaction and all its subtransactions committed (in
that order).  Thus, subtransactions that have not aborted appear as in-progress
even when they have already finished, and the subcommit status appears as a
very short transitory state during main transaction commit.  Subtransaction
abort is always marked in clog as soon as it occurs.  When the transaction
status all fit in a single CLOG page, we atomically mark them all as committed
without bothering with the intermediate sub-commit state.

Savepoints are implemented using subtransactions.  A subtransaction is a
transaction inside a transaction; its commit or abort status is not only
dependent on whether it committed itself, but also whether its parent
transaction committed.  To implement multiple savepoints in a transaction we
allow unlimited transaction nesting depth, so any particular subtransaction's
commit state is dependent on the commit status of each and every ancestor
transaction.

The "subtransaction parent" (pg_subtrans) mechanism records, for each
transaction with an XID, the TransactionId of its parent transaction.  This
information is stored as soon as the subtransaction is assigned an XID.
Top-level transactions do not have a parent, so they leave their pg_subtrans
entries set to the default value of zero (InvalidTransactionId).

pg_subtrans is used to check whether the transaction in question is still
running --- the main Xid of a transaction is recorded in the PGPROC struct,
but since we allow arbitrary nesting of subtransactions, we can't fit all Xids
in shared memory, so we have to store them on disk.  Note, however, that for
each transaction we keep a "cache" of Xids that are known to be part of the
transaction tree, so we can skip looking at pg_subtrans unless we know the
cache has been overflowed.  See storage/ipc/procarray.c for the gory details.

slru.c is the supporting mechanism for both pg_clog and pg_subtrans.  It
implements the LRU policy for in-memory buffer pages.  The high-level routines
for pg_clog are implemented in transam.c, while the low-level functions are in
clog.c.  pg_subtrans is contained completely in subtrans.c.


Write-Ahead Log Coding
----------------------

The WAL subsystem (also called XLOG in the code) exists to guarantee crash
recovery.  It can also be used to provide point-in-time recovery, as well as
hot-standby replication via log shipping.  Here are some notes about
non-obvious aspects of its design.

A basic assumption of a write AHEAD log is that log entries must reach stable
storage before the data-page changes they describe.  This ensures that
replaying the log to its end will bring us to a consistent state where there
are no partially-performed transactions.  To guarantee this, each data page
(either heap or index) is marked with the LSN (log sequence number --- in
practice, a WAL file location) of the latest XLOG record affecting the page.
Before the bufmgr can write out a dirty page, it must ensure that xlog has
been flushed to disk at least up to the page's LSN.  This low-level
interaction improves performance by not waiting for XLOG I/O until necessary.
The LSN check exists only in the shared-buffer manager, not in the local
buffer manager used for temp tables; hence operations on temp tables must not
be WAL-logged.

During WAL replay, we can check the LSN of a page to detect whether the change
recorded by the current log entry is already applied (it has been, if the page
LSN is >= the log entry's WAL location).

Usually, log entries contain just enough information to redo a single
incremental update on a page (or small group of pages).  This will work only
if the filesystem and hardware implement data page writes as atomic actions,
so that a page is never left in a corrupt partly-written state.  Since that's
often an untenable assumption in practice, we log additional information to
allow complete reconstruction of modified pages.  The first WAL record
affecting a given page after a checkpoint is made to contain a copy of the
entire page, and we implement replay by restoring that page copy instead of
redoing the update.  (This is more reliable than the data storage itself would
be because we can check the validity of the WAL record's CRC.)  We can detect
the "first change after checkpoint" by noting whether the page's old LSN
precedes the end of WAL as of the last checkpoint (the RedoRecPtr).

The general schema for executing a WAL-logged action is

1. Pin and exclusive-lock the shared buffer(s) containing the data page(s)
to be modified.

2. START_CRIT_SECTION()  (Any error during the next three steps must cause a
PANIC because the shared buffers will contain unlogged changes, which we
have to ensure don't get to disk.  Obviously, you should check conditions
such as whether there's enough free space on the page before you start the
critical section.)

3. Apply the required changes to the shared buffer(s).

4. Mark the shared buffer(s) as dirty with MarkBufferDirty().  (This must
happen before the WAL record is inserted; see notes in SyncOneBuffer().)

5. Build a WAL log record and pass it to XLogInsert(); then update the page's
LSN and TLI using the returned XLOG location.  For instance,

		recptr = XLogInsert(rmgr_id, info, rdata);

		PageSetLSN(dp, recptr);
		PageSetTLI(dp, ThisTimeLineID);

6. END_CRIT_SECTION()

7. Unlock and unpin the buffer(s).

XLogInsert's "rdata" argument is an array of pointer/size items identifying
chunks of data to be written in the XLOG record, plus optional shared-buffer
IDs for chunks that are in shared buffers rather than temporary variables.
The "rdata" array must mention (at least once) each of the shared buffers
being modified, unless the action is such that the WAL replay routine can
reconstruct the entire page contents.  XLogInsert includes the logic that
tests to see whether a shared buffer has been modified since the last
checkpoint.  If not, the entire page contents are logged rather than just the
portion(s) pointed to by "rdata".

Because XLogInsert drops the rdata components associated with buffers it
chooses to log in full, the WAL replay routines normally need to test to see
which buffers were handled that way --- otherwise they may be misled about
what the XLOG record actually contains.  XLOG records that describe multi-page
changes therefore require some care to design: you must be certain that you
know what data is indicated by each "BKP" bit.  An example of the trickiness
is that in a HEAP_UPDATE record, BKP(1) normally is associated with the source
page and BKP(2) is associated with the destination page --- but if these are
the same page, only BKP(1) would have been set.

For this reason as well as the risk of deadlocking on buffer locks, it's best
to design WAL records so that they reflect small atomic actions involving just
one or a few pages.  The current XLOG infrastructure cannot handle WAL records
involving references to more than three shared buffers, anyway.

In the case where the WAL record contains enough information to re-generate
the entire contents of a page, do *not* show that page's buffer ID in the
rdata array, even if some of the rdata items point into the buffer.  This is
because you don't want XLogInsert to log the whole page contents.  The
standard replay-routine pattern for this case is

	reln = XLogOpenRelation(rnode);
	buffer = XLogReadBuffer(reln, blkno, true);
	Assert(BufferIsValid(buffer));
	page = (Page) BufferGetPage(buffer);

	... initialize the page ...

	PageSetLSN(page, lsn);
	PageSetTLI(page, ThisTimeLineID);
	MarkBufferDirty(buffer);
	UnlockReleaseBuffer(buffer);

In the case where the WAL record provides only enough information to
incrementally update the page, the rdata array *must* mention the buffer
ID at least once; otherwise there is no defense against torn-page problems.
The standard replay-routine pattern for this case is

	if (record->xl_info & XLR_BKP_BLOCK_n)
		<< do nothing, page was rewritten from logged copy >>;

	reln = XLogOpenRelation(rnode);
	buffer = XLogReadBuffer(reln, blkno, false);
	if (!BufferIsValid(buffer))
		<< do nothing, page has been deleted >>;
	page = (Page) BufferGetPage(buffer);

	if (XLByteLE(lsn, PageGetLSN(page)))
	{
		/* changes are already applied */
		UnlockReleaseBuffer(buffer);
		return;
	}

	... apply the change ...

	PageSetLSN(page, lsn);
	PageSetTLI(page, ThisTimeLineID);
	MarkBufferDirty(buffer);
	UnlockReleaseBuffer(buffer);

As noted above, for a multi-page update you need to be able to determine
which XLR_BKP_BLOCK_n flag applies to each page.  If a WAL record reflects
a combination of fully-rewritable and incremental updates, then the rewritable
pages don't count for the XLR_BKP_BLOCK_n numbering.  (XLR_BKP_BLOCK_n is
associated with the n'th distinct buffer ID seen in the "rdata" array, and
per the above discussion, fully-rewritable buffers shouldn't be mentioned in
"rdata".)

Due to all these constraints, complex changes (such as a multilevel index
insertion) normally need to be described by a series of atomic-action WAL
records.  What do you do if the intermediate states are not self-consistent?
The answer is that the WAL replay logic has to be able to fix things up.
In btree indexes, for example, a page split requires insertion of a new key in
the parent btree level, but for locking reasons this has to be reflected by
two separate WAL records.  The replay code has to remember "unfinished" split
operations, and match them up to subsequent insertions in the parent level.
If no matching insert has been found by the time the WAL replay ends, the
replay code has to do the insertion on its own to restore the index to
consistency.  Such insertions occur after WAL is operational, so they can
and should write WAL records for the additional generated actions.


Asynchronous Commit
-------------------

As of PostgreSQL 8.3 it is possible to perform asynchronous commits - i.e.,
we don't wait while the WAL record for the commit is fsync'ed.
We perform an asynchronous commit when synchronous_commit = off.  Instead
of performing an XLogFlush() up to the LSN of the commit, we merely note
the LSN in shared memory.  The backend then continues with other work.
We record the LSN only for an asynchronous commit, not an abort; there's
never any need to flush an abort record, since the presumption after a
crash would be that the transaction aborted anyway.

We always force synchronous commit when the transaction is deleting
relations, to ensure the commit record is down to disk before the relations
are removed from the filesystem.  Also, certain utility commands that have
non-roll-backable side effects (such as filesystem changes) force sync
commit to minimize the window in which the filesystem change has been made
but the transaction isn't guaranteed committed.

Every wal_writer_delay milliseconds, the walwriter process performs an
XLogBackgroundFlush().  This checks the location of the last completely
filled WAL page.  If that has moved forwards, then we write all the changed
buffers up to that point, so that under full load we write only whole
buffers.  If there has been a break in activity and the current WAL page is
the same as before, then we find out the LSN of the most recent
asynchronous commit, and flush up to that point, if required (i.e.,
if it's in the current WAL page).  This arrangement in itself would
guarantee that an async commit record reaches disk during at worst the
second walwriter cycle after the transaction completes.  However, we also
allow XLogFlush to flush full buffers "flexibly" (ie, not wrapping around
at the end of the circular WAL buffer area), so as to minimize the number
of writes issued under high load when multiple WAL pages are filled per
walwriter cycle.  This makes the worst-case delay three walwriter cycles.

There are some other subtle points to consider with asynchronous commits.
First, for each page of CLOG we must remember the LSN of the latest commit
affecting the page, so that we can enforce the same flush-WAL-before-write
rule that we do for ordinary relation pages.  Otherwise the record of the
commit might reach disk before the WAL record does.  Again, abort records
need not factor into this consideration.

In fact, we store more than one LSN for each clog page.  This relates to
the way we set transaction status hint bits during visibility tests.
We must not set a transaction-committed hint bit on a relation page and
have that record make it to disk prior to the WAL record of the commit.
Since visibility tests are normally made while holding buffer share locks,
we do not have the option of changing the page's LSN to guarantee WAL
synchronization.  Instead, we defer the setting of the hint bit if we have
not yet flushed WAL as far as the LSN associated with the transaction.
This requires tracking the LSN of each unflushed async commit.  It is
convenient to associate this data with clog buffers: because we will flush
WAL before writing a clog page, we know that we do not need to remember a
transaction's LSN longer than the clog page holding its commit status
remains in memory.  However, the naive approach of storing an LSN for each
clog position is unattractive: the LSNs are 32x bigger than the two-bit
commit status fields, and so we'd need 256K of additional shared memory for
each 8K clog buffer page.  We choose instead to store a smaller number of
LSNs per page, where each LSN is the highest LSN associated with any
transaction commit in a contiguous range of transaction IDs on that page.
This saves storage at the price of some possibly-unnecessary delay in
setting transaction hint bits.

How many transactions should share the same cached LSN (N)?  If the
system's workload consists only of small async-commit transactions, then
it's reasonable to have N similar to the number of transactions per
walwriter cycle, since that is the granularity with which transactions will
become truly committed (and thus hintable) anyway.  The worst case is where
a sync-commit xact shares a cached LSN with an async-commit xact that
commits a bit later; even though we paid to sync the first xact to disk,
we won't be able to hint its outputs until the second xact is sync'd, up to
three walwriter cycles later.  This argues for keeping N (the group size)
as small as possible.  For the moment we are setting the group size to 32,
which makes the LSN cache space the same size as the actual clog buffer
space (independently of BLCKSZ).

It is useful that we can run both synchronous and asynchronous commit
transactions concurrently, but the safety of this is perhaps not
immediately obvious.  Assume we have two transactions, T1 and T2.  The Log
Sequence Number (LSN) is the point in the WAL sequence where a transaction
commit is recorded, so LSN1 and LSN2 are the commit records of those
transactions.  If T2 can see changes made by T1 then when T2 commits it
must be true that LSN2 follows LSN1.  Thus when T2 commits it is certain
that all of the changes made by T1 are also now recorded in the WAL.  This
is true whether T1 was asynchronous or synchronous.  As a result, it is
safe for asynchronous commits and synchronous commits to work concurrently
without endangering data written by synchronous commits.  Sub-transactions
are not important here since the final write to disk only occurs at the
commit of the top level transaction.

Changes to data blocks cannot reach disk unless WAL is flushed up to the
point of the LSN of the data blocks.  Any attempt to write unsafe data to
disk will trigger a write which ensures the safety of all data written by
that and prior transactions.  Data blocks and clog pages are both protected
by LSNs.

Changes to a temp table are not WAL-logged, hence could reach disk in
advance of T1's commit, but we don't care since temp table contents don't
survive crashes anyway.

Database writes made via any of the paths we have introduced to avoid WAL
overhead for bulk updates are also safe.  In these cases it's entirely
possible for the data to reach disk before T1's commit, because T1 will
fsync it down to disk without any sort of interlock, as soon as it finishes
the bulk update.  However, all these paths are designed to write data that
no other transaction can see until after T1 commits.  The situation is thus
not different from ordinary WAL-logged updates.
