Core by no means makes excessive use of these functions, but quite a large
number of those usages do require the caller to call strlen() on the
returned string. This is quite wasteful since these functions do already
have a good idea of the length of the string, so we might as well just
have them return that.
Reviewed-by: Andrew Gierth
Discussion: https://postgr.es/m/CAApHDvrm2A5x2uHYxsqriO2cUaGcFvND%2BksC9e7Tjep0t2RK_A%40mail.gmail.com
Choose names that fit into the conventions for wait event names
(particularly, that multi-word names are in the style MultiWordName)
and hopefully convey more information to non-hacker users than the
previous names did.
Also rename SerializablePredicateLockListLock to
SerializablePredicateListLock; the old name was long enough to cause
table formatting problems, plus the double occurrence of "Lock" seems
confusing/error-prone.
Also change a couple of particularly opaque LWLock field names.
Discussion: https://postgr.es/m/28683.1589405363@sss.pgh.pa.us
Includes some manual cleanup of places that pgindent messed up,
most of which weren't per project style anyway.
Notably, it seems some people didn't absorb the style rules of
commit c9d297751, because there were a bunch of new occurrences
of function calls with a newline just after the left paren, all
with faulty expectations about how the rest of the call would get
indented.
PostgreSQL provides set of template index access methods, where opclasses have
much freedom in the semantics of indexing. These index AMs are GiST, GIN,
SP-GiST and BRIN. There opclasses define representation of keys, operations on
them and supported search strategies. So, it's natural that opclasses may be
faced some tradeoffs, which require user-side decision. This commit implements
opclass parameters allowing users to set some values, which tell opclass how to
index the particular dataset.
This commit doesn't introduce new storage in system catalog. Instead it uses
pg_attribute.attoptions, which is used for table column storage options but
unused for index attributes.
In order to evade changing signature of each opclass support function, we
implement unified way to pass options to opclass support functions. Options
are set to fn_expr as the constant bytea expression. It's possible due to the
fact that opclass support functions are executed outside of expressions, so
fn_expr is unused for them.
This commit comes with some examples of opclass options usage. We parametrize
signature length in GiST. That applies to multiple opclasses: tsvector_ops,
gist__intbig_ops, gist_ltree_ops, gist__ltree_ops, gist_trgm_ops and
gist_hstore_ops. Also we parametrize maximum number of integer ranges for
gist__int_ops. However, the main future usage of this feature is expected
to be json, where users would be able to specify which way to index particular
json parts.
Catversion is bumped.
Discussion: https://postgr.es/m/d22c3a18-31c7-1879-fc11-4c1ce2f5e5af%40postgrespro.ru
Author: Nikita Glukhov, revised by me
Reviwed-by: Nikolay Shaplov, Robert Haas, Tom Lane, Tomas Vondra, Alvaro Herrera
Traditionally autovacuum has only ever invoked a worker based on the
estimated number of dead tuples in a table and for anti-wraparound
purposes. For the latter, with certain classes of tables such as
insert-only tables, anti-wraparound vacuums could be the first vacuum that
the table ever receives. This could often lead to autovacuum workers being
busy for extended periods of time due to having to potentially freeze
every page in the table. This could be particularly bad for very large
tables. New clusters, or recently pg_restored clusters could suffer even
more as many large tables may have the same relfrozenxid, which could
result in large numbers of tables requiring an anti-wraparound vacuum all
at once.
Here we aim to reduce the work required by anti-wraparound and aggressive
vacuums in general, by triggering autovacuum when the table has received
enough INSERTs. This is controlled by adding two new GUCs and reloptions;
autovacuum_vacuum_insert_threshold and
autovacuum_vacuum_insert_scale_factor. These work exactly the same as the
existing scale factor and threshold controls, only base themselves off the
number of inserts since the last vacuum, rather than the number of dead
tuples. New controls were added rather than reusing the existing
controls, to allow these new vacuums to be tuned independently and perhaps
even completely disabled altogether, which can be done by setting
autovacuum_vacuum_insert_threshold to -1.
We make no attempt to skip index cleanup operations on these vacuums as
they may trigger for an insert-mostly table which continually doesn't have
enough dead tuples to trigger an autovacuum for the purpose of removing
those dead tuples. If we were to skip cleaning the indexes in this case,
then it is possible for the index(es) to become bloated over time.
There are additional benefits to triggering autovacuums based on inserts,
as tables which never contain enough dead tuples to trigger an autovacuum
are now more likely to receive a vacuum, which can mark more of the table
as "allvisible" and encourage the query planner to make use of Index Only
Scans.
Currently, we still obey vacuum_freeze_min_age when triggering these new
autovacuums based on INSERTs. For large insert-only tables, it may be
beneficial to lower the table's autovacuum_freeze_min_age so that tuples
are eligible to be frozen sooner. Here we've opted not to zero that for
these types of vacuums, since the table may just be insert-mostly and we
may otherwise freeze tuples that are still destined to be updated or
removed in the near future.
There was some debate to what exactly the new scale factor and threshold
should default to. For now, these are set to 0.2 and 1000, respectively.
There may be some motivation to adjust these before the release.
Author: Laurenz Albe, Darafei Praliaskouski
Reviewed-by: Alvaro Herrera, Masahiko Sawada, Chris Travers, Andres Freund, Justin Pryzby
Discussion: https://postgr.es/m/CAC8Q8t%2Bj36G_bLF%3D%2B0iMo6jGNWnLnWb1tujXuJr-%2Bx8ZCCTqoQ%40mail.gmail.com
swap_relation_files() calls toast_get_valid_index() to find and lock
this index, just before swapping with the rebuilt TOAST index. The
latter function releases the lock before returning. Potential for
mischief is low; a concurrent session can issue ALTER INDEX ... SET
(fillfactor = ...), which is not alarming. Nonetheless, changing
pg_class.relfilenode without a lock is unconventional. Back-patch to
9.5 (all supported versions), because another fix needs this.
Discussion: https://postgr.es/m/20191226001521.GA1772687@rfd.leadboat.com
Introduce a GUC and a tablespace option to control I/O prefetching, much
like effective_io_concurrency, but for work that is done on behalf of
many client sessions.
Use the new setting in heapam.c instead of the hard-coded formula
effective_io_concurrency + 10 introduced by commit 558a9165e0. Go with
a default value of 10 for now, because it's a round number pretty close
to the value used for that existing case.
Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com
Our usual practice for "poor man's enum" catalog columns is to define
macros for the possible values and use those, not literal constants,
in C code. But for some reason lost in the mists of time, this was
never done for typalign/attalign or typstorage/attstorage. It's never
too late to make it better though, so let's do that.
The reason I got interested in this right now is the need to duplicate
some uses of the TYPSTORAGE constants in an upcoming ALTER TYPE patch.
But in general, this sort of change aids greppability and readability,
so it's a good idea even without any specific motivation.
I may have missed a few places that could be converted, and it's even
more likely that pending patches will re-introduce some hard-coded
references. But that's not fatal --- there's no expectation that
we'd actually change any of these values. We can clean up stragglers
over time.
Discussion: https://postgr.es/m/16457.1583189537@sss.pgh.pa.us
This also involves renaming src/include/utils/hashutils.h, which
becomes src/include/common/hashfn.h. Perhaps an argument can be
made for keeping the hashutils.h name, but it seemed more
consistent to make it match the name of the file, and also more
descriptive of what is actually going on here.
Patch by me, reviewed by Suraj Kharage and Mark Dilger. Off-list
advice on how not to break the Windows build from Davinder Singh
and Amit Kapila.
Discussion: http://postgr.es/m/CA+TgmoaRiG4TXND8QuM6JXFRkM_1wL2ZNhzaUKsuec9-4yrkgw@mail.gmail.com
Deduplication reduces the storage overhead of duplicates in indexes that
use the standard nbtree index access method. The deduplication process
is applied lazily, after the point where opportunistic deletion of
LP_DEAD-marked index tuples occurs. Deduplication is only applied at
the point where a leaf page split would otherwise be required. New
posting list tuples are formed by merging together existing duplicate
tuples. The physical representation of the items on an nbtree leaf page
is made more space efficient by deduplication, but the logical contents
of the page are not changed. Even unique indexes make use of
deduplication as a way of controlling bloat from duplicates whose TIDs
point to different versions of the same logical table row.
The lazy approach taken by nbtree has significant advantages over a GIN
style eager approach. Most individual inserts of index tuples have
exactly the same overhead as before. The extra overhead of
deduplication is amortized across insertions, just like the overhead of
page splits. The key space of indexes works in the same way as it has
since commit dd299df8 (the commit that made heap TID a tiebreaker
column).
Testing has shown that nbtree deduplication can generally make indexes
with about 10 or 15 tuples for each distinct key value about 2.5X - 4X
smaller, even with single column integer indexes (e.g., an index on a
referencing column that accompanies a foreign key). The final size of
single column nbtree indexes comes close to the final size of a similar
contrib/btree_gin index, at least in cases where GIN's posting list
compression isn't very effective. This can significantly improve
transaction throughput, and significantly reduce the cost of vacuuming
indexes.
A new index storage parameter (deduplicate_items) controls the use of
deduplication. The default setting is 'on', so all new B-Tree indexes
automatically use deduplication where possible. This decision will be
reviewed at the end of the Postgres 13 beta period.
There is a regression of approximately 2% of transaction throughput with
synthetic workloads that consist of append-only inserts into a table
with several non-unique indexes, where all indexes have few or no
repeated values. The underlying issue is that cycles are wasted on
unsuccessful attempts at deduplicating items in non-unique indexes.
There doesn't seem to be a way around it short of disabling
deduplication entirely. Note that deduplication of items in unique
indexes is fairly well targeted in general, which avoids the problem
there (we can use a special heuristic to trigger deduplication passes in
unique indexes, since we're specifically targeting "version bloat").
Bump XLOG_PAGE_MAGIC because xl_btree_vacuum changed.
No bump in BTREE_VERSION, since the representation of posting list
tuples works in a way that's backwards compatible with version 4 indexes
(i.e. indexes built on PostgreSQL 12). However, users must still
REINDEX a pg_upgrade'd index to use deduplication, regardless of the
Postgres version they've upgraded from. This is the only way to set the
new nbtree metapage flag indicating that deduplication is generally
safe.
Author: Anastasia Lubennikova, Peter Geoghegan
Reviewed-By: Peter Geoghegan, Heikki Linnakangas
Discussion:
https://postgr.es/m/55E4051B.7020209@postgrespro.ruhttps://postgr.es/m/4ab6e2db-bcee-f4cf-0916-3a06e6ccbb55@postgrespro.ru
Tuple conversion incorrectly concluded that no conversion was needed
as long as all the attributes lined up. But if the source tuple has a
missing attribute (from addition of a column with default), then the
destination tupdesc might not reflect the same default. The typical
symptom was that the affected columns would be unexpectedly NULL.
Repair by always forcing conversion if the source has missing
attributes, which will be filled in by the deform operation. (In
theory we could optimize for when the destination has the same
default, but that seemed overkill.)
Backpatch to 11 where missing attributes were added.
Per bug #16242.
Vik Fearing (discovery, code, testing) and me (analysis, testcase).
Discussion: https://postgr.es/m/16242-d1c9fca28445966b@postgresql.org
Using a lookup table of digit pairs reduces the number of divisions
needed, and calculating the length upfront saves some work; these
ideas are taken from the code previously committed for floats.
David Fetter, reviewed by Kyotaro Horiguchi, Tels, and me.
Discussion: https://postgr.es/m/20190924052620.GP31596%40fetter.org
Instead of always calling heap_fetch_toast_slice during detoasting,
invoke a table AM callback which, when the toast table is a heap
table, will be heap_fetch_toast_slice.
This makes it possible for a table AM other than heap to be used
as a TOAST table. It also completes the series of commits intended
to improve the interaction of tableam with TOAST that began with
commit 8b94dab06617ef80a0901ab103ebd8754427ef5a; detoast.c is
now, hopefully, fully AM-independent.
Patch by me, reviewed by Andres Freund and Peter Eisentraut.
Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com
This follows multiple complains from Peter Geoghegan, Andres Freund and
Alvaro Herrera that this issue ought to be dug more before actually
happening, if it happens.
Discussion: https://postgr.es/m/20191226144606.GA5659@alvherre.pgsql
The following renaming is done so as source files related to index
access methods are more consistent with table access methods (the
original names used for index AMs ware too generic, and could be
confused as including features related to table AMs):
- amapi.h -> indexam.h.
- amapi.c -> indexamapi.c. Here we have an equivalent with
backend/access/table/tableamapi.c.
- amvalidate.c -> indexamvalidate.c.
- amvalidate.h -> indexamvalidate.h.
- genam.c -> indexgenam.c.
- genam.h -> indexgenam.h.
This has been discussed during the development of v12 when table AM was
worked on, but the renaming never happened.
Author: Michael Paquier
Reviewed-by: Fabien Coelho, Julien Rouhaud
Discussion: https://postgr.es/m/20191223053434.GF34339@paquier.xyz
The new function, heap_fetch_toast_slice, is shared between
toast_fetch_datum_slice and toast_fetch_datum, and does all the
work of scanning the TOAST table, fetching chunks, and storing
them into the space allocated for the result varlena.
As an incidental side effect, this allows toast_fetch_datum_slice
to perform the scan with only a single scankey if all chunks are
being fetched, which might have some tiny performance benefit.
Discussion: http://postgr.es/m/CA+TgmobBzxwFojJ0zV0Own3dr09y43hp+OzU2VW+nos4PMXWEg@mail.gmail.com
Tuple conversion support in tupconvert.c is able to convert rowtypes
between two relations, inner and outer, which are logically equivalent
but have a different ordering or even dropped columns (used mainly for
inheritance tree and partitions). This makes use of attribute mappings,
which are simple arrays made of AttrNumber elements with a length
matching the number of attributes of the outer relation. The length of
the attribute mapping has been treated as completely independent of the
mapping itself until now, making it easy to pass down an incorrect
mapping length.
This commit refactors the code related to attribute mappings and moves
it into an independent facility called attmap.c, extracted from
tupconvert.c. This merges the attribute mapping with its length,
avoiding to try to guess what is the length of a mapping to use as this
is computed once, when the map is built.
This will avoid mistakes like what has been fixed in dc816e58, which has
used an incorrect mapping length by matching it with the number of
attributes of an inner relation (a child partition) instead of an outer
relation (a partitioned table).
Author: Michael Paquier
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/20191121042556.GD153437@paquier.xyz
Commit d5406dea25 used a slightly
novel, and wrong, approach to compute the length of the last
toast chunk. It worked fine unless the last chunk happened to
have the largest possible size.
Rework some of the checks for bad TOAST chunks to be a bit simpler
and easier to understand. These checks verify that (1) we get all
and only the chunk numbers we expect to see and (2) each chunk has
the expected size. However, the existing code was a bit hard to
understand, at least for me; try to make it clearer.
As part of that, have toast_fetch_datum_slice check the relationship
between endchunk and totalchunks only with an Assert() rather than
checking every chunk number against both values. There's no need to
check that relationship in production builds because it's not a
function of whether on-disk corruption is present; it's just a
question of whether the code does the right math.
Also, have toast_fetch_datum_slice() use ereport(ERROR) rather than
elog(ERROR). Commit fd6ec93bf8 made
the two functions inconsistent with each other.
Rename assorted variables for better clarity and consistency, and
move assorted variables from function scope to the function's main
loop. Remove a few variables that are used only once entirely.
Patch by me, reviewed by Peter Eisentraut.
Discussion: http://postgr.es/m/CA+TgmobBzxwFojJ0zV0Own3dr09y43hp+OzU2VW+nos4PMXWEg@mail.gmail.com
This reworks the reloption parsing and build of a couple of index AMs by
creating new structures for each index AM's options. This split was
already done for BRIN, GIN and GiST (which actually has a fillfactor
parameter), but not for hash, B-tree and SPGiST which relied on
StdRdOptions due to an overlap with the default option set.
This saves a couple of bytes for rd_options in each relcache entry with
indexes making use of relation options, and brings more consistency
between all index AMs. While on it, add a couple of AssertMacro() calls
to make sure that utility macros to grab values of reloptions are used
with the expected index AM.
Author: Nikolay Shaplov
Reviewed-by: Amit Langote, Michael Paquier, Álvaro Herrera, Dent John
Discussion: https://postgr.es/m/4127670.gFlpRb6XCm@x200m
In detoast_attr_slice, VARSIZE_ANY was used to compute compressed length
of on-disk TOAST values. That's incorrect, because the varlena value may
be just a TOAST pointer, producing either bogus value or crashing.
This is likely why the code was crashing on big-endian machines before
540f316809 replaced the VARSIZE with VARSIZE_ANY, which however only
masked the issue.
Reported-by: Rushabh Lathia
Discussion: https://postgr.es/m/CAL-OGkthU9Gs7TZchf5OWaL-Gsi=hXqufTxKv9qpNG73d5na_g@mail.gmail.com
reloptions.h includes since ba748f7 a set of macros to handle reloption
types in a way similar to how parseRelOptions() works. They have never
been used in the core code, and we have more simple methods now to parse
and fill in rd_options for a given relation depending on its relkind, so
remove this interface to simplify things.
Per discussion between Amit Langote, Álvaro Herrera and me.
Discussion: https://postgr.es/m/CA+HiwqE6zbNO92az6pp5GiTw4tr-9rfCE0t84whQSP+YwSKjMQ@mail.gmail.com
Partitioned tables do not have relation options yet, but, similarly to
what's done for views which have their own parsing table, it could make
sense to introduce new parameters for some of the existing default ones
like fillfactor, autovacuum, etc. Splitting things has the advantage to
make the information stored in rd_options include only the necessary
information, reducing the amount of memory used for a relcache entry
with partitioned tables if new reloptions are introduced at this level.
Author: Nikolay Shaplov
Reviewed-by: Amit Langote, Michael Paquier
Discussion: https://postgr.es/m/1627387.Qykg9O6zpu@x200m
When maintaining or merging patches, one of the most common sources
for conflicts are the list of objects in makefiles. Especially when
the split across lines has been changed on both sides, which is
somewhat common due to attempting to stay below 80 columns, those
conflicts are unnecessarily laborious to resolve.
By splitting, and alphabetically sorting, OBJS style lines into one
object per line, conflicts should be less frequent, and easier to
resolve when they still occur.
Author: Andres Freund
Discussion: https://postgr.es/m/20191029200901.vww4idgcxv74cwes@alap3.anarazel.de
Historically, the code to build relation options has been shaped the
same way in multiple code paths by using a set of datums in input with
the options parsed with a static table which is then filled with the
option values. This introduces a new common routine in reloptions.c to
do most of the legwork for the in-core code paths.
Author: Amit Langote
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CA+HiwqGsoSn_uTPPYT19WrtR7oYpYtv4CdS0xuedTKiHHWuk_g@mail.gmail.com
The old names for the attribute-detoasting functions names included
the word "heap," which seems outdated now that the heap is only one of
potentially many table access methods.
On the other hand, toast_insert_or_update and toast_delete are
heap-specific, so rename them by adding "heap_" as a prefix.
Not all of the work of making the TOAST system fully accessible to AMs
other than the heap is done yet, but there seems to be little harm in
getting this renaming out of the way now. Commit
8b94dab066 already divided up the
functions among various files partially according to whether it was
intended that they should be heap-specific or AM-agnostic, so this is
just clarifying the division contemplated by that commit.
Patch by me, reviewed and tested by Prabhat Sabu, Thomas Munro,
Andres Freund, and Álvaro Herrera.
Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com
Commit 4d0e994eed added support for partial TOAST decompression, so the
decompression is interrupted after producing the requested prefix. For
prefix and slices near the beginning of the entry, this may saves a lot
of decompression work.
That however only deals with decompression - the whole compressed entry
was still fetched and re-assembled, even though the compression used
only a small fraction of it. This commit improves that by computing how
much compressed data may be needed to decompress the requested prefix,
and then fetches only the necessary part.
We always need to fetch a bit more compressed data than the requested
(uncompressed) prefix, because the prefix may not be compressible at all
and pglz itself adds a bit of overhead. That means this optimization is
most effective when the requested prefix is much smaller than the whole
compressed entry.
Author: Binguo Bao
Reviewed-by: Andrey Borodin, Tomas Vondra, Paul Ramsey
Discussion: https://www.postgresql.org/message-id/flat/CAL-OGkthU9Gs7TZchf5OWaL-Gsi=hXqufTxKv9qpNG73d5na_g@mail.gmail.com
The code was enforcing AccessExclusiveLock for all custom relation
options, which is incorrect as the APIs allow a custom lock level to be
set.
While on it, fix a couple of inconsistencies in the tests and the README
of dummy_index_am.
Oversights in commit 773df88.
Discussion: https://postgr.es/m/20190925234152.GA2115@paquier.xyz
All our current in core relation options of type string (not many,
admittedly) behave in reality like enums. But after seeing an
implementation for enum reloptions, it's clear that strings are messier,
so introduce the new reloption type. Switch all string options to be
enums instead.
Fortunately we have a recently introduced test module for reloptions, so
we don't lose coverage of string reloptions, which may still be used by
third-party modules.
Authors: Nikolay Shaplov, Álvaro Herrera
Reviewed-by: Nikita Glukhov, Aleksandr Parfenov
Discussion: https://postgr.es/m/43332102.S2V5pIjXRx@x200m
Relation options can define a lock mode other than AccessExclusiveMode
since 47167b7, but modules defining custom relation options did not
really have a way to enforce that. Correct that by extending the
current API set so as modules can define a custom lock mode.
Author: Michael Paquier
Reviewed-by: Kuntal Ghosh
Discussion: https://postgr.es/m/20190920013831.GD1844@paquier.xyz
In-core relation options can use a custom lock mode since 47167b7, that
has lowered the lock available for some autovacuum parameters. However
it forgot to consider custom relation options. This causes failures
with ALTER TABLE SET when changing a custom relation option, as its lock
is not defined. The existing APIs to define a custom reloption does not
allow to define a custom lock mode, so enforce its initialization to
AccessExclusiveMode which should be safe enough in all cases. An
upcoming patch will extend the existing APIs to allow a custom lock mode
to be defined.
The problem can be reproduced with bloom indexes, so add a test there.
Reported-by: Nikolay Sharplov
Analyzed-by: Thomas Munro, Michael Paquier
Author: Michael Paquier
Reviewed-by: Kuntal Ghosh
Discussion: https://postgr.es/m/20190920013831.GD1844@paquier.xyz
Backpatch-through: 9.6
detoast.c/h contain functions required to detoast a datum, partially
or completely, plus a few other utility functions for examining the
size of toasted datums.
toast_internals.c/h contain functions that are used internally to the
TOAST subsystem but which (mostly) do not need to be accessed from
outside.
heaptoast.c/h contains code that is intrinsically specific to the
heap AM, either because it operates on HeapTuples or is based on the
layout of a heap page.
detoast.c and toast_internals.c are placed in
src/backend/access/common rather than src/backend/access/heap. At
present, both files still have dependencies on the heap, but that will
be improved in a future commit.
Patch by me, reviewed and tested by Prabhat Sabu, Thomas Munro,
Andres Freund, and Álvaro Herrera.
Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com
The message was included as a parameter when this function was added in
dcb2bda9b7, but I don't think it has ever served any useful purpose.
Let's stop spreading it pointlessly.
Reviewed by Amit Langote and Peter Eisentraut.
Discussion: https://postgr.es/m/20190806224728.GA17233@alvherre.pgsql
Originally, Postgres Lists were a more or less exact reimplementation of
Lisp lists, which consist of chains of separately-allocated cons cells,
each having a value and a next-cell link. We'd hacked that once before
(commit d0b4399d8) to add a separate List header, but the data was still
in cons cells. That makes some operations -- notably list_nth() -- O(N),
and it's bulky because of the next-cell pointers and per-cell palloc
overhead, and it's very cache-unfriendly if the cons cells end up
scattered around rather than being adjacent.
In this rewrite, we still have List headers, but the data is in a
resizable array of values, with no next-cell links. Now we need at
most two palloc's per List, and often only one, since we can allocate
some values in the same palloc call as the List header. (Of course,
extending an existing List may require repalloc's to enlarge the array.
But this involves just O(log N) allocations not O(N).)
Of course this is not without downsides. The key difficulty is that
addition or deletion of a list entry may now cause other entries to
move, which it did not before.
For example, that breaks foreach() and sister macros, which historically
used a pointer to the current cons-cell as loop state. We can repair
those macros transparently by making their actual loop state be an
integer list index; the exposed "ListCell *" pointer is no longer state
carried across loop iterations, but is just a derived value. (In
practice, modern compilers can optimize things back to having just one
loop state value, at least for simple cases with inline loop bodies.)
In principle, this is a semantics change for cases where the loop body
inserts or deletes list entries ahead of the current loop index; but
I found no such cases in the Postgres code.
The change is not at all transparent for code that doesn't use foreach()
but chases lists "by hand" using lnext(). The largest share of such
code in the backend is in loops that were maintaining "prev" and "next"
variables in addition to the current-cell pointer, in order to delete
list cells efficiently using list_delete_cell(). However, we no longer
need a previous-cell pointer to delete a list cell efficiently. Keeping
a next-cell pointer doesn't work, as explained above, but we can improve
matters by changing such code to use a regular foreach() loop and then
using the new macro foreach_delete_current() to delete the current cell.
(This macro knows how to update the associated foreach loop's state so
that no cells will be missed in the traversal.)
There remains a nontrivial risk of code assuming that a ListCell *
pointer will remain good over an operation that could now move the list
contents. To help catch such errors, list.c can be compiled with a new
define symbol DEBUG_LIST_MEMORY_USAGE that forcibly moves list contents
whenever that could possibly happen. This makes list operations
significantly more expensive so it's not normally turned on (though it
is on by default if USE_VALGRIND is on).
There are two notable API differences from the previous code:
* lnext() now requires the List's header pointer in addition to the
current cell's address.
* list_delete_cell() no longer requires a previous-cell argument.
These changes are somewhat unfortunate, but on the other hand code using
either function needs inspection to see if it is assuming anything
it shouldn't, so it's not all bad.
Programmers should be aware of these significant performance changes:
* list_nth() and related functions are now O(1); so there's no
major access-speed difference between a list and an array.
* Inserting or deleting a list element now takes time proportional to
the distance to the end of the list, due to moving the array elements.
(However, it typically *doesn't* require palloc or pfree, so except in
long lists it's probably still faster than before.) Notably, lcons()
used to be about the same cost as lappend(), but that's no longer true
if the list is long. Code that uses lcons() and list_delete_first()
to maintain a stack might usefully be rewritten to push and pop at the
end of the list rather than the beginning.
* There are now list_insert_nth...() and list_delete_nth...() functions
that add or remove a list cell identified by index. These have the
data-movement penalty explained above, but there's no search penalty.
* list_concat() and variants now copy the second list's data into
storage belonging to the first list, so there is no longer any
sharing of cells between the input lists. The second argument is
now declared "const List *" to reflect that it isn't changed.
This patch just does the minimum needed to get the new implementation
in place and fix bugs exposed by the regression tests. As suggested
by the foregoing, there's a fair amount of followup work remaining to
do.
Also, the ENABLE_LIST_COMPAT macros are finally removed in this
commit. Code using those should have been gone a dozen years ago.
Patch by me; thanks to David Rowley, Jesper Pedersen, and others
for review.
Discussion: https://postgr.es/m/11587.1550975080@sss.pgh.pa.us
a96c41f has introduced the option for heap, but it still lacked the
variant to control the behavior for toast relations.
While on it, refactor the tests so as they stress more scenarios with
the various values that vacuum_index_cleanup can use. It would be
useful to couple those tests with pageinspect to check that pages are
actually cleaned up, but this is left for later.
Author: Masahiko Sawada, Michael Paquier
Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAD21AoCqs8iN04RX=i1KtLSaX5RrTEM04b7NHYps4+rqtpWNEg@mail.gmail.com
There were a number of issues in the recent commits which include typos,
code and comments mismatch, leftover function declarations. Fix them.
Reported-by: Alexander Lakhin
Author: Alexander Lakhin, Amit Kapila and Amit Langote
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/ef0c0232-0c1d-3a35-63d4-0ebd06e31387@gmail.com
This is still using the 2.0 version of pg_bsd_indent.
I thought it would be good to commit this separately,
so as to document the differences between 2.0 and 2.1 behavior.
Discussion: https://postgr.es/m/16296.1558103386@sss.pgh.pa.us
Tom Lane