POSTGRESQL: Summary of Changes since last release (7.4.1)
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26 Jan 2004
This is a summary of most changes since code versions marked 7_4_1,
rather than a weekly news bulletin, a summary of desired future items,
or the definitive list of what's in any particular release. The
intention is to help everybody understand what's coming and what might
be affected, though most importantly, where you might add value to the
community as a designer, developer, tester, technical author or
advocate.
So far in this release dev cycle, major functionality will effect
- PERFORMANCE
- OPTIMIZER/EXECUTOR
- ROBUSTNESS
- SECURITY
Other code changes are summarised and their major impacts noted.
These notes cover major changes and are not guaranteed complete, or even
fully tested. Many additional patches to the latest full release have
been submitted and these are appreciated just as much, even though they
have *mostly* more isolated effects. Documentation changes continue,
though aren't described here, neither are client side
utilities/interfaces.
Nothing mentioned here is DEFINITELY in 7.5 or any future release;
testing of everything mentioned here is encouraged and appreciated, for
regression, performance and robustness. There is not yet a CVS branch
specifically for any later release than 7_4_1; these changes are not yet
even guaranteed to build into a consistent release when taken together.
Description of changes is designed to highlight benefit and impact, as
well as identifying specific areas of code change and potential knock-on
effects.
MAJOR FUNCTIONALITY
PERFORMANCE
- Major new memory buffer cache algorithm has now been implemented using
the Adaptive Replacement Cache algorithm. The implementation should have
positive benefit for everybody's workload, since ARC will adapt to a
variety of situations and has been designed to allow Vacuum to avoid
interfering with user applications. (Jan) src/backend/buffer
- New performance profiling of Intel CPU has allowed new spinlock code
to achieve performance/throughput gains of up to 10% using DBT-2 (OLTP)
workloads. Further gains to follow? (Manfred Spraul/Tom)
src/backend/storage/lmgr/s_lock.c
- TIP 9 now needs changing! Cross-data-type comparisons are now
indexable by btrees. All the existing cross-type comparison operators
(int2/int4/int8 and float4/float8) have appropriate support. (Tom)
Implications for user defined types and indices also? [HACKERS] 8-Nov-03
- All operations on TEMP relations are no longer logged in WAL, nor are
they involved in checkpoints, thus improving performance. (Tom)
- Index performance improved when scanning highly non-unique indices;
will greatly improve performance of cursor/fetch logic. B-tree's
initial-positioning-strategy code has been improved so that we start
scan at first entry, rather than reading in all entries that share that
index value before we begin to scan. (Tom, after Dimitry Tkach)
- Heap access code is now faster when using compressed columns in-line;
previous assumption was that all compressed columns were also toasted
(Tom)
- Optimized calling performance for dynamically loaded C functions. Hash
table added to cache lookups of 'C'-language functions. Some limited
testing suggests that this puts the lookup speed for external functions
just about on par with built-in functions. (Tom)
OPTIMIZER/EXECUTOR IMPROVEMENTS
- Genetic Optimizer usage has been re-analyzed; geqo defaults have now
been set to more effective values which are expected to significantly
improve plan selection for complex multi-way joins (> 10-way).
geqo_effort setting now offers an easy 1..10 setting (like IBM DB2),
that allows this to be controlled realistically by user/DBA. New
heuristic added to significantly reduce number of join plans attempted
before geqo begins. (Tom)
- Avoid redundant unique-ification step on subqueries where the result
is already known to be unique (i.e. it is a SELECT DISTINCT ...
subquery, IN subqueries that use UNION/INTERSECT/EXCEPT (without ALL)).
Also set join_in_selectivity correctly. (Tom)
- Avoid redundant projection step when scanning a table that we need all
the columns from. In case of SELECT INTO, we have to check that the
hasoids flag matches the desired output type, too. (Tom)
- Repair mis-estimation of indexscan CPU costs. When an indexqual
contains a run-time key (that is, a nonconstant expression compared to
the index variable), the key is evaluated just once per scan, but we
were charging costs as though it were evaluated once per visited index
entry. (Tom)
- Avoid planner failure for cases involving Cartesian products inside IN
(sub-SELECT) constructs. (Tom)
ROBUSTNESS
- Local buffer manager is no longer used for newly-created non-TEMP
relations; a new non-TEMP relation goes through the shared bufmgr and
thus will participate normally in checkpoints. TEMP relations use the
local buffer manager throughout their lifespan. (Tom)
- Massive overhaul of pg_dump: make use of dependency information from
pg_depend to determine a safe dump order. Defaults and check
constraints
can be emitted either as part of a table or domain definition, or
separately if that's needed to break a dependency loop. Lots of old
half-baked code for controlling dump order removed. Performance work has
also occurred to address some regressions in performance this caused.
(Tom)
- Changes to ALTER .. SET PATH allows ordered dumps to restore without
error - pg_restore options to select restore order now removed - not
needed (Tom)
- In backend/access/transam/ add warning to AtEOXact_SPI() to catch
cases where the current txn has been committed without SPI_finish()
being called
first. Allows detection of resource leaks... (Joe)
- psql memory allocation is being cleaned up, using safer calls
(Bruce/Neil)
- Transaction logic now enhanced to stay intact even at final stage of
resource failure conditions, such as running out of disk space etc (Tom)
SECURITY
- New permission-checking code. Rather than relying on the query context
of a rangetable entry to identify what permissions it wants checked,
store a full AclMode mask in each RTE, and check exactly those bits.
This allows an RTE specifying, say, INSERT privilege on a view to be
copied into a derived UPDATE query without changing meaning. (Tom)
OTHER NEW FUNCTIONALITY
- Add "WITH / WITHOUT OIDS" clause to CREATE TABLE AS. This allows the
user to explicitly specify whether OIDs should be included in the
newly-created relation; useful because it provides a way for application
authors to ensure their applications are compatible with future versions
of (in which the relation created by CREATE TABLE AS won't include OIDs
by default). (Neil)
- Add more kinds of exprs that can be accepted after a CREATE SCHEMA
(Neil)
- Info Schema enhanced further to support named function parameters
(Dennis)
- Change factorial to return a numeric (Gavin)
- Comments can now be set on individual Cast, Conversion, Op Class,
Large Object and Language (s) (Chris)
- Have psql \dn show only visible temp schemas using current_schemas()
- Have psql '\i ~/<tab><tab>' actually load files it displays from home
dir
- Allow psql \du to show groups, and add \dg for groups
- Allow pg_dump to dump CREATE CONVERSION (Chris)
SUMMARY OF OTHER CODE CHANGES
- Remove the explicit casting of NULL literals to a pointer in a wide
variety of code locations (Neil)
- Streamline local buffer manager code: Since it's no longer necessary
to fsync relations as they move out of the local buffers into shared
buffers, quite a lot of smgr.c/md.c/fd.c code is no longer needed and
has been removed: there's no concept of a dirty relation anymore in
md.c/fd.c, and we never fsync anything but WAL. (Tom)
- Add operator strategy and comparison-value datatype fields to ScanKey.
Remove the 'strategy map' code - Passing the strategy number to the
index AM directly should now be simpler and faster. Changes to
ScanKeyEntryInitialize() API touches quite a lot of files. (Tom)
- nbtree function _bt_first is now substantially changed/simplified
(Tom)
- Change PG_DELAY from msec to usec and use it consistently rather than
select(). Add Win32 Sleep() for delay. (Bruce)
- Supporting relaxing of ALTER...SET PATH requires changing the API for
GUC assign_hook functions, which touches a lot of places (Tom)
- In src/backend/access/ we have major changes in heap, nbtree, transam
and
common code
- Almost all files src/backend/commands have changes; mostly robustness
- JDBC interface has been moved out into its own project to improve the
focus on this popular and important area of client code.
MAJOR IMPACTS NOTED (Upgrades, performance, change of defaults etc)
- initdb forced due to pg_proc change to support Named function
parameters
- initdb forced due to change of stored rule representation.
- initdb recommended to allow picking up Info Schema changes
- geqo defaults have now been set to significantly more effective
values; we should expect increased optimization elapsed times for very
large queries, though hopefully outweighed by a significant improvement
in plan selection
MAJOR WORK IN PROGRESS (Much less accurate than the above...)
- Win32 - Changes to many areas, especially the postmaster, ipc, libpq
etc, to streamline and allow a single source Win32 port - the Win32 port
is coming closer! (Claudio)
- Replication - involving remote copying of WAL logs, then cut-over and
automatic catch-up on secondary node (Jan)
- Background writer work progresses, which is likely to improve overall
scalability/performance by smoothing dirty blocks writes; forms the
basis for increased server availability also (Jan)
- Initial stages of Named Function parameter support have been
committed. pg_proc has a column to store names, and CREATE FUNCTION can
insert data into it, but that's all as yet. (Dennis)
- Buffer manager locking changes: lock contention data have shown that
the BufMgrLock is the major source of contention under heavy load,
effecting multi-CPU SMP scalability. Patch to rework the bufmgr's
locking scheme to be more granular and further perf testing may yield
further perf gains. (Neil)
- Backend internal data structure changes: list rewrite: the linked list
implementation used throughout the backend is being redesigned for
constant-time length and append operations. This was done because
lappend() is called quite frequently, and allows some ugly code
(FastList) to be removed. (Neil)
- Hash index changes: complete wrap up unique hash indexes, as well as
some improvements to hash index concurrent performance. (Neil)
- Customizable ANALYZE function for user definable functions is generic
functionality being added, though as base for PostGIS enhancements
(Mark)
- Re-evaluation of Genetic Query Optimizer parameters and usage will
likely continue for some and any real usage scenarios/observations are
welcome
WHAT **ISN'T** IN THIS RELEASE (YET!)
- Many TODO items still to be claimed...
- SQL Commands, standardisation and compatibility features
- Referential Integrity & Inheritance
- Administration (client interfaces may have changed)
- National/Multi-language support extensions
DEVELOPER FEATURES (logging, debugging etc)
- bootstrap can now be cancelled using CTRL-C (Tom)
- use debug_shared_buffers = <seconds> to show ARC mem buffer contents
(Jan)
All corrections and changes welcome...if this is well received, then I
will monitor pgsql-commiters to keep track of things. Please let me know
about even minor technical mistakes, though please lets not revisit the
designs of everything again! -- Simon
