VACUUM thought that reltuples represents the total number of tuples in
the relation, while ANALYZE counted only live tuples. This can cause
"flapping" in the value when background vacuums and analyzes happen
separately. The planner's use of reltuples essentially assumes that
it's the count of live (visible) tuples, so let's standardize on having
it mean live tuples.
Another issue is that the definition of "live tuple" isn't totally clear;
what should be done with INSERT_IN_PROGRESS or DELETE_IN_PROGRESS tuples?
ANALYZE's choices in this regard are made on the assumption that if the
originating transaction commits at all, it will happen after ANALYZE
finishes, so we should ignore the effects of the in-progress transaction
--- unless it is our own transaction, and then we should count it.
Let's propagate this definition into VACUUM, too.
Likewise propagate this definition into CREATE INDEX, and into
contrib/pgstattuple's pgstattuple_approx() function.
Tomas Vondra, reviewed by Haribabu Kommi, some corrections by me
Discussion: https://postgr.es/m/16db4468-edfa-830a-f921-39a50498e77e@2ndquadrant.com
The existing logic for updating pg_class.reltuples trusted the sampling
results only for the pages ANALYZE actually visited, preferring to
believe the previous tuple density estimate for all the unvisited pages.
While there's some rationale for doing that for VACUUM (first that
VACUUM is likely to visit a very nonrandom subset of pages, and second
that we know for sure that the unvisited pages did not change), there's
no such rationale for ANALYZE: by assumption, it's looked at an unbiased
random sample of the table's pages. Furthermore, in a very large table
ANALYZE will have examined only a tiny fraction of the table's pages,
meaning it cannot slew the overall density estimate very far at all.
In a table that is physically growing, this causes reltuples to increase
nearly proportionally to the change in relpages, regardless of what is
actually happening in the table. This has been observed to cause reltuples
to become so much larger than reality that it effectively shuts off
autovacuum, whose threshold for doing anything is a fraction of reltuples.
(Getting to the point where that would happen seems to require some
additional, not well understood, conditions. But it's undeniable that if
reltuples is seriously off in a large table, ANALYZE alone will not fix it
in any reasonable number of iterations, especially not if the table is
continuing to grow.)
Hence, restrict the use of vac_estimate_reltuples() to VACUUM alone,
and in ANALYZE, just extrapolate from the sample pages on the assumption
that they provide an accurate model of the whole table. If, by very bad
luck, they don't, at least another ANALYZE will fix it; in the old logic
a single bad estimate could cause problems indefinitely.
In HEAD, let's remove vac_estimate_reltuples' is_analyze argument
altogether; it was never used for anything and now it's totally pointless.
But keep it in the back branches, in case any third-party code is calling
this function.
Per bug #15005. Back-patch to all supported branches.
David Gould, reviewed by Alexander Kuzmenkov, cosmetic changes by me
Discussion: https://postgr.es/m/20180117164916.3fdcf2e9@engels
The previous commit has shown that the sanity checks around freezing
aren't strong enough. Strengthening them seems especially important
because the existance of the bug has caused corruption that we don't
want to make even worse during future vacuum cycles.
The errors are emitted with ereport rather than elog, despite being
"should never happen" messages, so a proper error code is emitted. To
avoid superflous translations, mark messages as internal.
Author: Andres Freund and Alvaro Herrera
Reviewed-By: Alvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20171102112019.33wb7g5wp4zpjelu@alap3.anarazel.de
Backpatch: 9.3-
It turns out we misdiagnosed what the real problem was. Revert the
previous changes, because they may have worse consequences going
forward. A better fix is forthcoming.
The simplistic test case is kept, though disabled.
Discussion: https://postgr.es/m/20171102112019.33wb7g5wp4zpjelu@alap3.anarazel.de
Vacuum calls page-level HOT prune to remove dead HOT tuples before doing
liveness checks (HeapTupleSatisfiesVacuum) on the remaining tuples. But
concurrent transaction commit/abort may turn DEAD some of the HOT tuples
that survived the prune, before HeapTupleSatisfiesVacuum tests them.
This happens to activate the code that decides to freeze the tuple ...
which resuscitates it, duplicating data.
(This is especially bad if there's any unique constraints, because those
are now internally violated due to the duplicate entries, though you
won't know until you try to REINDEX or dump/restore the table.)
One possible fix would be to simply skip doing anything to the tuple,
and hope that the next HOT prune would remove it. But there is a
problem: if the tuple is older than freeze horizon, this would leave an
unfrozen XID behind, and if no HOT prune happens to clean it up before
the containing pg_clog segment is truncated away, it'd later cause an
error when the XID is looked up.
Fix the problem by having the tuple freezing routines cope with the
situation: don't freeze the tuple (and keep it dead). In the cases that
the XID is older than the freeze age, set the HEAP_XMAX_COMMITTED flag
so that there is no need to look up the XID in pg_clog later on.
An isolation test is included, authored by Michael Paquier, loosely
based on Daniel Wood's original reproducer. It only tests one
particular scenario, though, not all the possible ways for this problem
to surface; it be good to have a more reliable way to test this more
fully, but it'd require more work.
In message https://postgr.es/m/20170911140103.5akxptyrwgpc25bw@alvherre.pgsql
I outlined another test case (more closely matching Dan Wood's) that
exposed a few more ways for the problem to occur.
Backpatch all the way back to 9.3, where this problem was introduced by
multixact juggling. In branches 9.3 and 9.4, this includes a backpatch
of commit e5ff9fefcd50 (of 9.5 era), since the original is not
correctable without matching the coding pattern in 9.5 up.
Reported-by: Daniel Wood
Diagnosed-by: Daniel Wood
Reviewed-by: Yi Wen Wong, Michaël Paquier
Discussion: https://postgr.es/m/E5711E62-8FDF-4DCA-A888-C200BF6B5742@amazon.com
Don't move parenthesized lines to the left, even if that means they
flow past the right margin.
By default, BSD indent lines up statement continuation lines that are
within parentheses so that they start just to the right of the preceding
left parenthesis. However, traditionally, if that resulted in the
continuation line extending to the right of the desired right margin,
then indent would push it left just far enough to not overrun the margin,
if it could do so without making the continuation line start to the left of
the current statement indent. That makes for a weird mix of indentations
unless one has been completely rigid about never violating the 80-column
limit.
This behavior has been pretty universally panned by Postgres developers.
Hence, disable it with indent's new -lpl switch, so that parenthesized
lines are always lined up with the preceding left paren.
This patch is much less interesting than the first round of indent
changes, but also bulkier, so I thought it best to separate the effects.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
Change pg_bsd_indent to follow upstream rules for placement of comments
to the right of code, and remove pgindent hack that caused comments
following #endif to not obey the general rule.
Commit e3860ffa4d wasn't actually using
the published version of pg_bsd_indent, but a hacked-up version that
tried to minimize the amount of movement of comments to the right of
code. The situation of interest is where such a comment has to be
moved to the right of its default placement at column 33 because there's
code there. BSD indent has always moved right in units of tab stops
in such cases --- but in the previous incarnation, indent was working
in 8-space tab stops, while now it knows we use 4-space tabs. So the
net result is that in about half the cases, such comments are placed
one tab stop left of before. This is better all around: it leaves
more room on the line for comment text, and it means that in such
cases the comment uniformly starts at the next 4-space tab stop after
the code, rather than sometimes one and sometimes two tabs after.
Also, ensure that comments following #endif are indented the same
as comments following other preprocessor commands such as #else.
That inconsistency turns out to have been self-inflicted damage
from a poorly-thought-through post-indent "fixup" in pgindent.
This patch is much less interesting than the first round of indent
changes, but also bulkier, so I thought it best to separate the effects.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
The new indent version includes numerous fixes thanks to Piotr Stefaniak.
The main changes visible in this commit are:
* Nicer formatting of function-pointer declarations.
* No longer unexpectedly removes spaces in expressions using casts,
sizeof, or offsetof.
* No longer wants to add a space in "struct structname *varname", as
well as some similar cases for const- or volatile-qualified pointers.
* Declarations using PG_USED_FOR_ASSERTS_ONLY are formatted more nicely.
* Fixes bug where comments following declarations were sometimes placed
with no space separating them from the code.
* Fixes some odd decisions for comments following case labels.
* Fixes some cases where comments following code were indented to less
than the expected column 33.
On the less good side, it now tends to put more whitespace around typedef
names that are not listed in typedefs.list. This might encourage us to
put more effort into typedef name collection; it's not really a bug in
indent itself.
There are more changes coming after this round, having to do with comment
indentation and alignment of lines appearing within parentheses. I wanted
to limit the size of the diffs to something that could be reviewed without
one's eyes completely glazing over, so it seemed better to split up the
changes as much as practical.
Discussion: https://postgr.es/m/E1dAmxK-0006EE-1r@gemulon.postgresql.org
Discussion: https://postgr.es/m/30527.1495162840@sss.pgh.pa.us
Previously manual VACUUM did not report the number of skipped frozen
pages even when VERBOSE option is specified. But this information is
helpful to monitor the VACUUM activity, and also autovacuum reports that
number in the log file when the condition of log_autovacuum_min_duration
is met.
This commit changes VACUUM VERBOSE so that it reports the number
of frozen pages that it skips.
Author: Masahiko Sawada
Reviewed-by: Yugo Nagata and Jim Nasby
Discussion: http://postgr.es/m/CAD21AoDZQKCxo0L39Mrq08cONNkXQKXuh=2DP1Q8ebmt35SoaA@mail.gmail.com
presence of page pins, which leads to serious estimation errors in the
planner. This particularly affects small heavily-accessed tables,
especially where locking (e.g. from FK constraints) forces frequent
vacuums for mxid cleanup.
Fix by keeping separate track of pages whose live tuples were actually
counted vs. pages that were only scanned for freezing purposes. Thus,
reltuples can only be set to 0 if all pages of the relation were
actually counted.
Backpatch to all supported versions.
Per bug #14057 from Nicolas Baccelli, analyzed by me.
Discussion: https://postgr.es/m/20160331103739.8956.94469@wrigleys.postgresql.org
Allow VACUUM and Autovacuum to report the oldestxmin value they
used while cleaning tables, helping to make better sense out of
the other statistics we report in various cases.
Vacuum truncation scan can be sped up on rotating media by prefetching
blocks in forward direction. That makes the blocks already present in
memory by the time they are needed, while also letting OS read-ahead
kick in.
The truncate scan has been measured to be five times faster than without
this patch (that was on a slow disk, but it shouldn't hurt on fast
disks.)
Author: Álvaro Herrera, loosely based on a submission by Claudio Freire
Discussion: https://postgr.es/m/CAGTBQpa6NFGO_6g_y_7zQx8L9GcHDSQKYdo1tGuh791z6PYgEg@mail.gmail.com
lazy_truncate_heap() was waiting for
VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL, but in microseconds
not milliseconds as originally intended.
Found by code inspection.
Simon Riggs
Since a892234 & fd31cd265 the visibilitymap's freeze bit is used to
avoid vacuuming the whole relation in anti-wraparound vacuums. Doing so
correctly relies on not adding xids to the heap without also unsetting
the visibilitymap flag. Tuple locking related code has not done so.
To allow selectively resetting all-frozen - to avoid pessimizing
heap_lock_tuple - allow to selectively reset the all-frozen with
visibilitymap_clear(). To avoid having to use
visibilitymap_get_status (e.g. via VM_ALL_FROZEN) inside a critical
section, have visibilitymap_clear() return whether any bits have been
reset.
There's a remaining issue (denoted by XXX): After the PageIsAllVisible()
check in heap_lock_tuple() and heap_lock_updated_tuple_rec() the page
status could theoretically change. Practically that currently seems
impossible, because updaters will hold a page level pin already. Due to
the next beta coming up, it seems better to get the required WAL magic
bump done before resolving this issue.
The added flags field fields to xl_heap_lock and xl_heap_lock_updated
require bumping the WAL magic. Since there's already been a catversion
bump since the last beta, that's not an issue.
Reviewed-By: Robert Haas, Amit Kapila and Andres Freund
Author: Masahiko Sawada, heavily revised by Andres Freund
Discussion: CAEepm=3fWAbWryVW9swHyLTY4sXVf0xbLvXqOwUoDiNCx9mBjQ@mail.gmail.com
Backpatch: -
If you really want to vacuum every single page in the relation,
regardless of apparent visibility status or anything else, you can use
this option. In previous releases, this behavior could be achieved
using VACUUM (FREEZE), but because we can now recognize all-frozen
pages as not needing to be frozen again, that no longer works. There
should be no need for routine use of this option, but maybe bugs or
disaster recovery will necessitate its use.
Patch by me, reviewed by Andres Freund.
Commit a892234f83 added a new bit per
page to the visibility map fork indicating whether the page is
all-frozen, but incorrectly assumed that if lazy_scan_heap chose to
freeze a tuple then that tuple would not need to later be frozen
again. This turns out to be false, because xmin and xmax (and
conceivably xvac, if dealing with tuples from very old releases) could
be frozen at separate times.
Thanks to Andres Freund for help in uncovering and tracking down this
issue.
Per post-commit review comments from Andres Freund, improve variable
names, comments, and in one place, slightly improve the code structure.
Masahiko Sawada
The reverted changes were intended to force a choice of whether any
newly-added BufferGetPage() calls needed to be accompanied by a
test of the snapshot age, to support the "snapshot too old"
feature. Such an accompanying test is needed in about 7% of the
cases, where the page is being used as part of a scan rather than
positioning for other purposes (such as DML or vacuuming). The
additional effort required for back-patching, and the doubt whether
the intended benefit would really be there, have indicated it is
best just to rely on developers to do the right thing based on
comments and existing usage, as we do with many other conventions.
This change should have little or no effect on generated executable
code.
Motivated by the back-patching pain of Tom Lane and Robert Haas
This feature is controlled by a new old_snapshot_threshold GUC. A
value of -1 disables the feature, and that is the default. The
value of 0 is just intended for testing. Above that it is the
number of minutes a snapshot can reach before pruning and vacuum
are allowed to remove dead tuples which the snapshot would
otherwise protect. The xmin associated with a transaction ID does
still protect dead tuples. A connection which is using an "old"
snapshot does not get an error unless it accesses a page modified
recently enough that it might not be able to produce accurate
results.
This is similar to the Oracle feature, and we use the same SQLSTATE
and error message for compatibility.
This patch is a no-op patch which is intended to reduce the chances
of failures of omission once the functional part of the "snapshot
too old" patch goes in. It adds parameters for snapshot, relation,
and an enum to specify whether the snapshot age check needs to be
done for the page at this point. This initial patch passes NULL
for the first two new parameters and BGP_NO_SNAPSHOT_TEST for the
third. The follow-on patch will change the places where the test
needs to be made.
Vinayak Pokale provided a patch for a copy-and-paste error in a
comment. I noticed that I'd use the word "automatically" nearby where
I meant to talk about things being "atomic". Rahila Syed spotted a
misplaced counter update. Fix all that stuff.
There's a lot more that could be done here yet - in particular, this
reports only very coarse-grained information about the index vacuuming
phase - but even as it stands, the new pg_stat_progress_vacuum can
tell you quite a bit about what a long-running vacuum is actually
doing.
Amit Langote and Robert Haas, based on earlier work by Vinayak Pokale
and Rahila Syed.
Commit a892234f83 gave us enough
infrastructure to avoid vacuuming pages where every tuple on the
page is already frozen. So, replace the notion of a scan_all or
whole-table vacuum with the less onerous notion of an "aggressive"
vacuum, which will pages that are all-visible, but still skip those
that are all-frozen.
This should greatly reduce the cost of anti-wraparound vacuuming
on large clusters where the majority of data is never touched
between one cycle and the next, because we'll no longer have to
read all of those pages only to find out that we don't need to
do anything with them.
Patch by me, reviewed by Masahiko Sawada.
Using this facility, any utility command can report the target relation
upon which it is operating, if there is one, and up to 10 64-bit
counters; the intent of this is that users should be able to figure out
what a utility command is doing without having to resort to ugly hacks
like attaching strace to a backend.
As a demonstration, this adds very crude reporting to lazy vacuum; we
just report the target relation and nothing else. A forthcoming patch
will make VACUUM report a bunch of additional data that will make this
much more interesting. But this gets the basic framework in place.
Vinayak Pokale, Rahila Syed, Amit Langote, Robert Haas, reviewed by
Kyotaro Horiguchi, Jim Nasby, Thom Brown, Masahiko Sawada, Fujii Masao,
and Masanori Oyama.
Commit a892234f83 added a second bit per
page to the visibility map, which still seems like a good idea, but it
also added a second page-level bit alongside PD_ALL_VISIBLE to track
whether the visibility map bit was set. That no longer seems like a
clever plan, because we don't really need that bit for anything. We
always clear both bits when the page is modified anyway.
Patch by me, reviewed by Kyotaro Horiguchi and Masahiko Sawada.
The new bit indicates whether every tuple on the page is already frozen.
It is cleared only when the all-visible bit is cleared, and it can be
set only when we vacuum a page and find that every tuple on that page is
both visible to every transaction and in no need of any future
vacuuming.
A future commit will use this new bit to optimize away full-table scans
that would otherwise be triggered by XID wraparound considerations. A
page which is merely all-visible must still be scanned in that case, but
a page which is all-frozen need not be. This commit does not attempt
that optimization, although that optimization is the goal here. It
seems better to get the basic infrastructure in place first.
Per discussion, it's very desirable for pg_upgrade to automatically
migrate existing VM forks from the old format to the new format. That,
too, will be handled in a follow-on patch.
Masahiko Sawada, reviewed by Kyotaro Horiguchi, Fujii Masao, Amit
Kapila, Simon Riggs, Andres Freund, and others, and substantially
revised by me.
In the extreme edge case where contended pages are the only ones that
escape being scanned, the previous commit would have allowed us to think
that relfrozenxid could be advanced, which is exactly wrong.
VACUUM can skip heap pages altogether when there's a run of consecutive
pages that are all-visible according to the visibility map. This causes it
to not update its nonempty_pages count, just as if those pages were empty,
which means that at the end we will think they are candidates for deletion.
Thus, we may take the table's AccessExclusive lock only to find that no
pages are really truncatable. This usually causes no real problems on a
master server, thanks to the lock being acquired only conditionally; but on
hot-standby servers, the same lock must be acquired unconditionally which
can result in unnecessary query cancellations.
To improve matters, force examination of the table's last page whenever
we reach there with a nonempty_pages count that would allow a truncation
attempt. If it's not empty, we'll advance nonempty_pages and thereby
prevent the truncation attempt.
If we are unable to acquire cleanup lock on that page, there's no need to
force it, unless we're doing an anti-wraparound vacuum. We can just check
for tuples with a shared buffer lock and then give up. (When we are doing
an anti-wraparound vacuum, and decide it's okay to skip the page because it
contains no freezable tuples, this patch still improves matters because
nonempty_pages is properly updated, which it was not before.)
Since only the last page is special-cased in this way, we might attempt a
truncation that will release many fewer pages than the normal heuristic
would suggest; at worst, only one page would be truncated. But that seems
all right, because the situation won't repeat during the next vacuum.
The real problem with the old logic is that the useless truncation attempt
happens every time we vacuum, so long as the state of the last few dozen
pages doesn't change.
This is a longstanding deficiency, but since the consequences aren't very
severe in most scenarios, I'm not going to risk a back-patch.
Jeff Janes and Tom Lane
This is useful to control autovacuum log volume, for situations where
monitoring only a set of tables is necessary.
Author: Michael Paquier
Reviewed by: A team led by Naoya Anzai (also including Akira Kurosawa,
Taiki Kondo, Huong Dangminh), Fujii Masao.
We were involving the parser too much in setting up initial vacuuming
parameters. This patch moves that responsibility elsewhere to simplify
code, and also to make future additions easier. To do this, create a
new struct VacuumParams which is filled just prior to vacuum execution,
instead of at parse time; for user-invoked vacuuming this is set up in a
new function ExecVacuum, while autovacuum sets it up by itself.
While at it, add a new member VACOPT_SKIPTOAST to enum VacuumOption,
only set by autovacuum, which is used to disable vacuuming of the toast
table instead of the old do_toast parameter; this relieves the argument
list of vacuum() and some callees a bit. This partially makes up for
having added more arguments in an effort to avoid having autovacuum from
constructing a VacuumStmt parse node.
Author: Michael Paquier. Some tweaks by Álvaro
Reviewed by: Robert Haas, Stephen Frost, Álvaro Herrera
Tom Lane
Bruce Momjian
Andres Freund
Robert Haas
Alvaro Herrera
Peter Eisentraut
Fujii Masao
Andrew Gierth
Simon Riggs
Kevin Grittner
Heikki Linnakangas