Notiziario settimanale PostgreSQL – 24 giugno 2012

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Questo notiziario settimanale PostgreSQL è stato realizzato da David Fetter; traduzione parziale in lingua italiana a cura di Carlo Ascani.

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Patch applicate

Tom Lane pushed:

– Change ON UPDATE SET NULL/SET DEFAULT referential actions to meet
SQL spec. Previously, when executing an ON UPDATE SET NULL or SET
DEFAULT action for a multicolumn MATCH SIMPLE foreign key
constraint, we would set only those referencing columns
corresponding to referenced columns that were changed. This is what
the SQL92 standard said to do — but more recent versions of the
standard say that all referencing columns should be set to null or
their default values, no matter exactly which referenced columns
changed. At least for SET DEFAULT, that is clearly saner behavior.
It’s somewhat debatable whether it’s an improvement for SET NULL,
but it appears that other RDBMS systems read the spec this way. So
let’s do it like that. This is a release-notable behavioral change,
although considering that our documentation already implied it was
done this way, the lack of complaints suggests few people use such

– Update SQL spec references in ri_triggers code to match SQL:2008.
Now that what we’re implementing isn’t SQL92, we probably shouldn’t
cite chapter and verse in that spec anymore. Also fix some comments
that talked about MATCH FULL but in fact were in code that’s also
used for MATCH SIMPLE. No code changes in this commit, just

– Remove derived fields from RI_QueryKey, and do a bit of other
cleanup. We really only need the foreign key constraint’s OID and
the query type code to uniquely identify each plan we are caching
for FK checks. The other stuff that was in the struct had no
business being used as part of a hash key, and was all just being
copied from struct RI_ConstraintInfo anyway. Get rid of the
unnecessary fields, and readjust various function APIs to make them
use RI_ConstraintInfo not RI_QueryKey as info source. I’d be
surprised if this makes any measurable performance difference, but
it certainly feels cleaner.

– Allow ON UPDATE/DELETE SET DEFAULT plans to be cached. Once upon a
time, somebody was worried that cached RI plans wouldn’t get remade
with new default values after ALTER TABLE … SET DEFAULT, so they
didn’t allow caching of plans for ON UPDATE/DELETE SET DEFAULT
actions. That time is long gone, though (and even at the time I
doubt this was the greatest hazard posed by ALTER TABLE…). So
allow these triggers to cache their plans just like the others. The
cache_plan argument to ri_PlanCheck is now vestigial, since there
are no callers that don’t pass “true”; but I left it alone in case
there is any future need for it.

– Improve comments about why SET DEFAULT triggers must recheck for
matches. I was confused about this, so try to make it clearer for
the next person. (This seems like a fairly inefficient way of
dealing with a corner case, but I don’t have a better idea offhand.
Maybe if there were a way to turn off the RI_FKey_keyequal_upd_fk
event filter temporarily?)

– Share RI trigger code between NO ACTION and RESTRICT cases. These
triggers are identical except for whether ri_Check_Pk_Match is to be
called, so factor out the common code to save a couple hundred
lines. Also, eliminate null-column checks in ri_Check_Pk_Match,
since they’re duplicate with the calling functions and require
unnecessary complication in its API statement. Simplify the way
code is shared between RI_FKey_check_ins and RI_FKey_check_upd, too.

– Add pgbench option to add foreign key constraints to the standard
scenario. The option –foreign-keys, used at initialization time,
will create foreign key constraints for the columns that represent
references to other tables’ primary keys. This can help in
benchmarking FK performance. Jeff Janes

– Improve tests for whether we can skip queueing RI enforcement
triggers. During an update of a PK row, we can skip firing the RI
trigger if any old key value is NULL, because then the row could not
have had any matching rows in the FK table. Conversely, during an
update of an FK row, the outcome is determined if any new key value
is NULL. In either case it becomes unnecessary to compare
individual key values. This patch was inspired by discussion of Vik
Reykja’s patch to use IS NOT DISTINCT semantics for the key
comparisons. In the event there is no need for that and so this
patch looks nothing like his, but he should still get credit for
having re-opened consideration of the trigger skip logic.

– Cache the results of ri_FetchConstraintInfo in a backend-local
cache. Extracting data from pg_constraint turned out to take as
much as 10% of the runtime in a bulk-update case where the foreign
key column wasn’t changing, because we did it over again for each
tuple. Fix that by maintaining a backend-local cache of the
results. This is really a pretty small patch, but converting the
trigger functions to work with pointers rather than local struct
variables requires a lot of mechanical changes.

– Increase MAX_SYSCACHE_CALLBACKS from 20 to 32. By my count there
are 18 callers of CacheRegisterSyscacheCallback in the core code in
HEAD, so we are potentially leaving as few as 2 slots for any add-on
code to use (though possibly not all these callers would actually
activate in any particular session). That doesn’t seem like a lot
of headroom, so let’s pump it up a little.

– Remove incomplete/incorrect support for zero-column foreign keys.
The original coding in ri_triggers.c had partial support for the
concept of zero-column foreign key constraints. But this is not
defined in the SQL standard, nor was it ever allowed by any other
part of Postgres, nor was it very fully implemented even here (eg
there was no support for preventing PK-table deletions that would
violate the constraint). Doesn’t seem very useful to carry 100-plus
lines of code for a corner case that no one is interested in making
work. Instead, just add a check that the column list read from
pg_constraint is non-empty.

– Fix memory leak in ARRAY(SELECT …) subqueries. Repeated execution
of an uncorrelated ARRAY_SUBLINK sub-select (which I think can only
happen if the sub-select is embedded in a larger, correlated
subquery) would leak memory for the duration of the query, due to
not reclaiming the array generated in the previous execution. Per
bug #6698 from Armando Miraglia. Diagnosis and fix idea by Heikki,
patch itself by me. This has been like this all along, so
back-patch to all supported versions.

Peter Eisentraut pushed:

– Remove confusing half sentence from legal notice. Pointed out by
Stefan Kaltenbrunner

– Make placeholders in SQL command help more consistent and precise.
To avoid divergent names on related pages, avoid ambiguities, and
reduce translation work a little.

– Replace int2/int4 in C code with int16/int32. The latter was
already the dominant use, and it’s preferable because in C the
convention is that intXX means XX bits. Therefore, allowing mixed
use of int2, int4, int8, int16, int32 is obviously confusing.
Remove the typedefs for int2 and int4 for now. They don’t seem to
be widely used outside of the PostgreSQL source tree, and the few
uses can probably be cleaned up by the time this ships.
– Update copyright year in forgotten places. Found by Stefan

– pg_dump: Add missing newlines at end of messages

Alvaro Herrera pushed:

– pg_dump: Fix verbosity level in LO progress messages. In passing,
reword another instance of the same message that was gratuitously
different. Author: Josh Kupershmidt after a bug report by Bosco

– Repair comment mangled by a pgindent run long ago.

Heikki Linnakangas pushed:

– Add a small cache of locks owned by a resource owner in
ResourceOwner. This speeds up reassigning locks to the parent
owner, when the transaction holds a lot of locks, but only a few of
them belong to the current resource owner. This is particularly
helps pg_dump when dumping a large number of objects. The cache can
hold up to 15 locks in each resource owner. After that, the cache is
marked as overflowed, and we fall back to the old method of scanning
the whole local lock table. The tradeoff here is that the cache has
to be scanned whenever a lock is released, so if the cache is too
large, lock release becomes more expensive. 15 seems enough to cover
pg_dump, and doesn’t have much impact on lock release. Jeff Janes,
reviewed by Amit Kapila and Heikki Linnakangas.

– Move WAL continuation record information to WAL page header. The
continuation record only contained one field, xl_rem_len, so it
makes things simpler to just include it in the WAL page header. This
wastes four bytes on pages that don’t begin with a continuation from
previos page, plus four bytes on every page, because of padding.
The motivation of this is to make it easier to calculate how much
space a WAL record needs. Before this patch, it depended on how many
page boundaries the record crosses. The motivation of that, in turn,
is to separate the allocation of space in the WAL from the copying
of the record data to the allocated space. Keeping the calculation
of space required simple helps to keep the critical section of
allocating the space from WAL short. But that’s not included in this
patch yet. Bump WAL version number again, as this is an
incompatible change.

– Don’t waste the last segment of each 4GB logical log file. The
comments claimed that wasting the last segment made it easier to do
calculations with XLogRecPtrs, because you don’t have problems
representing last-byte-position-plus-1 that way. In my experience,
however, it only made things more complicated, because the there was
two ways to represent the boundary at the beginning of a logical log
file: logid = n+1 and xrecoff = 0, or as xlogid = n and xrecoff =
4GB – XLOG_SEG_SIZE. Some functions were picky about which
representation was used. Also, use a 64-bit segment number instead
of the log/seg combination, to point to a certain WAL segment. We
assume that all platforms have a working 64-bit integer type
nowadays. This is an incompatible change in WAL format, so bumping
WAL version number.

– Replace XLogRecPtr struct with a 64-bit integer. This simplifies
code that needs to do arithmetic on XLogRecPtrs. To avoid changing
on-disk format of data pages, the LSN on data pages is still stored
in the old format. That should keep pg_upgrade happy. However, we
have XLogRecPtrs embedded in the control file, and in the structs
that are sent over the replication protocol, so this changes breaks
compatibility of pg_basebackup and server. I didn’t do anything
about this in this patch, per discussion on -hackers, the right
thing to do would to be to change the replication protocol to be
architecture-independent, so that you could use a newer version of
pg_receivexlog, for example, against an older server version.

– Allow WAL record header to be split across pages. This saves a few
bytes of WAL space, but the real motivation is to make it
predictable how much WAL space a record requires, as it no longer
depends on whether we need to waste the last few bytes at end of WAL
page because the header doesn’t fit. The total length field of WAL
record, xl_tot_len, is moved to the beginning of the WAL record
header, so that it is still always found on the first page where a
WAL record begins. Bump WAL version number again as this is an
incompatible change.

– Use LL suffix for 64-bit constants. Per warning from buildfarm
member ‘locust’. At least I think this what’s making it upset.

– Oops. Remove stray paren. I didn’t notice this on my laptop as I

– Use UINT64CONST for 64-bit integer constants. Peter Eisentraut
advised me that UINT64CONST is the proper way to do that, not LL

– I missed some references to xlogid/xrecoff in Win32-only code. Fix.

Robert Haas pushed:

– Document that && can be used to search arrays. Also, add some
cross-links to the indexing documentation, so it’s easier to notice
that && and other array operators have index support. Ryan Kelly,
edited by me.

– Make pgbench -i emit only one-tenth as many status messages. These
days, even a wimpy system can insert 10000 tuples in the blink of an
eye, so there’s no real need for this much verbosity. Per complaint
from Tatsuo Ishii.

Patch rifiutate (per adesso)

Nessuno è stato scontetao questa settimana. :-)

Patch in attesa

Joachim Wieland sent in another revision of the patch to implement
parallel pg_dump.

Robert Haas sent in another revision of the patch to avoid heavyweight
locking on hash metapages.

Kyotaro HORIGUCHI sent in another revision of the patch to skip
checkpoint on promoting from streaming replication.

Jeff Davis sent in two more revisions of the patch to fix fsync
behavior on initdb.

Alexander Korotkov sent in a patch to reduce gistchoose index bloat.

Zoltan Boszormenyi sent in four more revisions of the patch to
implement lock_timeout and SIGALARM framework.

Peter Eisentraut sent in another revision of the patch to prevent
sub-main functions in the backend from returning anything.

Honza Horak sent in another revision of the patch to allow PostgreSQL
to answer on more than one Unix socket.

Alvaro Herrera and Kyotaro HORIGUCHI traded patches to fix PL/Perl in
SQL_ASCII-encoded databases.

Greg Smith sent in a patch to try to avoid running with a full fsync
request queue.

Robert Haas sent in a patch to make pgbench less yappy.

Peter Eisentraut sent in a patch to revive AC_PROG_INSTALL.

John Lumby sent in a patch intended to enable using INSERT …
RETURNING in a RULE on a partitioned table.

Alexander Korotkov sent in a patch to fix an issue with GiST indexing
on points.

Alexander Korotkov sent in another revision of the patch for SP-GiST
for ranges, this time using a k-d tree instead of quad-tree.

Kevin Grittner and Alvaro Herrera traded patches to implement foreign
key locks.

Tom Lane sent in another revision of the patch to fix pg_dump
dependencies in the –section case.

Dimitri Fontaine sent in another revision of the event trigger patch.

David Kerr sent in a patch to allow using an empty backup_label in
streaming replication.

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