# PostgreSQL TODO List

Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
Last updated: Thu Jul 7 22:44:34 EDT 2005

The most recent version of this document can be viewed at
http://www.postgresql.org/docs/faqs.TODO.html.

A hyphen, "-", marks changes that will appear in the upcoming 8.1 release.

Bracketed items, "[]", have more detail.

This list contains all known PostgreSQL bugs and feature requests. If
you would like to work on an item, please read the Developer's FAQ
first.

# Administration

- Remove behavior of postmaster -o after making postmaster/postgres flags unique
- Allow limits on per-db/role connections
- Allow pooled connections to list all prepared queries

  This would allow an application inheriting a pooled connection to know the queries prepared in the current session.

- Allow major upgrades without dump/reload, perhaps using pg_upgrade [pg_upgrade]
- Check for unreferenced table files created by transactions that were in-progress when the server terminated abruptly
- Allow administrators to safely terminate individual sessions either via an SQL function or SIGTERM

  Currently SIGTERM of a backend can lead to lock table corruption.

- -Prevent dropping user that still owns objects, or auto-drop the objects
- Set proper permissions on non-system schemas during db creation

  Currently all schemas are owned by the super-user because they are copied from the template1 database.

- -Add the client IP address and port to pg_stat_activity
- Support table partitioning that allows a single table to be stored in subtables that are partitioned based on the primary key or a WHERE clause
- Improve replication solutions
  - Load balancing

    You can use any of the master/slave replication servers to use a standby server for data warehousing. To allow read/write queries to multiple servers, you need multi-master replication like pgcluster.

  - Allow replication over unreliable or non-persistent links
- Configuration files
  - Add "include file" functionality in postgresql.conf
  - Allow postgresql.conf values to be set so they can not be changed by the user
  - Allow commenting of variables in postgresql.conf to restore them to defaults
  - Allow pg_hba.conf settings to be controlled via SQL

    This would require a new global table that is dumped to flat file for use by the postmaster. We do a similar thing for pg_shadow currently.

- Tablespaces
  - Allow a database in tablespace t1 with tables created in tablespace t2 to be used as a template for a new database created with default tablespace t2

    All objects in the default database tablespace must have default tablespace specifications. This is because new databases are created by copying directories. If you mix default tablespace tables and tablespace-specified tables in the same directory, creating a new database from such a mixed directory would create a new database with tables that had incorrect explicit tablespaces. To fix this would require modifying pg_class in the newly copied database, which we don't currently do.

  - Allow reporting of which objects are in which tablespaces

    This item is difficult because a tablespace can contain objects from multiple databases. There is a server-side function that returns the databases which use a specific tablespace, so this requires a tool that will call that function and connect to each database to find the objects in each database for that tablespace.

    - Add a GUC variable to control the tablespace for temporary objects and sort files

      It could start with a random tablespace from a supplied list and cycle through the list.

    - Allow WAL replay of CREATE TABLESPACE to work when the directory structure on the recovery computer is different from the original
    - Allow per-tablespace quotas
- Point-In-Time Recovery (PITR)
  - Allow point-in-time recovery to archive partially filled write-ahead logs [pitr]

    Currently only full WAL files are archived. This means that the most recent transactions aren't available for recovery in case of a disk failure. This could be triggered by a user command or a timer.

  - Automatically force archiving of partially-filled WAL files when pg_stop_backup() is called or the server is stopped

    Doing this will allow administrators to know more easily when the archive contins all the files needed for point-in-time recovery.

  - Create dump tool for write-ahead logs for use in determining transaction id for point-in-time recovery
  - Allow a warm standby system to also allow read-only queries [pitr]

    This is useful for checking PITR recovery.

  - Allow the PITR process to be debugged and data examined

# Monitoring

- Allow server log information to be output as INSERT statements

  This would allow server log information to be easily loaded into a database for analysis.

- Add ability to monitor the use of temporary sort files
- -Add session start time and last statement time to pg_stat_activity
- Allow server logs to be remotely read using SQL commands
- -Add a function that returns the start time of the postmaster

# Data Types

- Remove Money type, add money formatting for decimal type
- Change NUMERIC to enforce the maximum precision, and increase it
- Add NUMERIC division operator that doesn't round?

  Currently NUMERIC _rounds_ the result to the specified precision. This means division can return a result that multiplied by the divisor is greater than the dividend, e.g. this returns a value > 10:

SELECT (10::numeric(2,0) / 6::numeric(2,0))::numeric(2,0) * 6;

The positive modulus result returned by NUMERICs might be considered
inaccurate, in one sense.

- Have sequence dependency track use of DEFAULT sequences, seqname.nextval?
- Disallow changing default expression of a SERIAL column?
- Fix data types where equality comparison isn't intuitive, e.g. box
- Prevent INET cast to CIDR if the unmasked bits are not zero, or zero the bits
- Prevent INET cast to CIDR from droping netmask, SELECT '1.1.1.1'::inet::cidr
- Allow INET + INT4 to increment the host part of the address, or throw an error on overflow
- Add 'tid != tid ' operator for use in corruption recovery
- Dates and Times
  - Allow infinite dates just like infinite timestamps
  - Add a GUC variable to allow output of interval values in ISO8601 format
  - Merge hardwired timezone names with the TZ database; allow either kind everywhere a TZ name is currently taken
  - Allow customization of the known set of TZ names (generalize the present australian_timezones hack)
  - Allow TIMESTAMP WITH TIME ZONE to store the original timezone information, either zone name or offset from UTC [timezone]

    If the TIMESTAMP value is stored with a time zone name, interval computations should adjust based on the time zone rules, e.g. adding 24 hours to a timestamp would yield a different result from adding one day.

  - Add ISO INTERVAL handling
    - Add support for day-time syntax, INTERVAL '1 2:03:04' DAY TO SECOND
    - Add support for year-month syntax, INTERVAL '50-6' YEAR TO MONTH
    - For syntax that isn't uniquely ISO or PG syntax, like '1:30' or '1', treat as ISO if there is a range specification clause, and as PG if there no clause is present, e.g. interpret

      '1:30' MINUTE TO SECOND as '1 minute 30 seconds', and interpret '1:30' as '1 hour, 30 minutes'

    - Interpret INTERVAL '1 year' MONTH as CAST (INTERVAL '1 year' AS INTERVAL MONTH), and this should return '12 months'
    - Round or truncate values to the requested precision, e.g. INTERVAL '11 months' AS YEAR should return one or zero
    - Support precision, CREATE TABLE foo (a INTERVAL MONTH(3))
- Arrays
  - Allow NULLs in arrays
  - Allow MIN()/MAX() on arrays
  - Delay resolution of array expression's data type so assignment coercion can be performed on empty array expressions
  - Modify array literal representation to handle array index lower bound of other than one
- Binary Data
  - Improve vacuum of large objects, like /contrib/vacuumlo?
  - Add security checking for large objects

    Currently large objects entries do not have owners. Permissions can only be set at the pg_largeobject table level.

  - Auto-delete large objects when referencing row is deleted
  - Allow read/write into TOAST values like large objects

    This requires the TOAST column to be stored EXTERNAL.

# Functions

- -Add function to return compressed length of TOAST data values
- Allow INET subnet tests using non-constants to be indexed
- Add transaction_timestamp(), statement_timestamp(), clock_timestamp() functionality

  Current CURRENT_TIMESTAMP returns the start time of the current transaction, and gettimeofday() returns the wallclock time. This will make time reporting more consistent and will allow reporting of the statement start time.

- Add pg_get_acldef(), pg_get_typedefault(), and pg_get_attrdef()
- Allow to_char() to print localized month names
- Allow functions to have a schema search path specified at creation time
- Allow substring/replace() to get/set bit values
- Allow to_char() on interval values to accumulate the highest unit requested

  Some special format flag would be required to request such accumulation. Such functionality could also be added to EXTRACT. Prevent accumulation that crosses the month/day boundary because of the uneven number of days in a month.

  - to_char(INTERVAL '1 hour 5 minutes', 'MI') => 65
  - to_char(INTERVAL '43 hours 20 minutes', 'MI' ) => 2600
  - to_char(INTERVAL '43 hours 20 minutes', 'WK:DD:HR:MI') => 0:1:19:20
  - to_char(INTERVAL '3 years 5 months','MM') => 41
- Prevent to_char() on interval from returning meaningless values

  For example, to_char('1 month', 'mon') is meaningless. Basically, most date-related parameters to to_char() are meaningless for intervals because interval is not anchored to a date.

# Multi-Language Support

- Add NCHAR (as distinguished from ordinary varchar),
- Allow locale to be set at database creation

  Currently locale can only be set during initdb. No global tables have locale-aware columns. However, the database template used during database creation might have locale-aware indexes. The indexes would need to be reindexed to match the new locale.

- Allow encoding on a per-column basis

  Right now only one encoding is allowed per database.

- Support multiple simultaneous character sets, per SQL92
- Improve UTF8 combined character handling?
- Add octet_length_server() and octet_length_client()
- Make octet_length_client() the same as octet_length()?

# Views / Rules

- Automatically create rules on views so they are updateable, per SQL99

  We can only auto-create rules for simple views. For more complex cases users will still have to write rules.

- Add the functionality for WITH CHECK OPTION clause of CREATE VIEW
- Allow NOTIFY in rules involving conditionals
- Have views on temporary tables exist in the temporary namespace
- Allow temporary views on non-temporary tables
- Allow RULE recompilation

# SQL Commands

- -Add BETWEEN SYMMETRIC/ASYMMETRIC
- Change LIMIT/OFFSET and FETCH/MOVE to use int8
- -Add E'' escape string marker so eventually ordinary strings can treat backslashes literally, for portability
- -Allow additional tables to be specified in DELETE for joins

  UPDATE already allows this (UPDATE...FROM) but we need similar functionality in DELETE. It's been agreed that the keyword should be USING, to avoid anything as confusing as DELETE FROM a FROM b.

- Add CORRESPONDING BY to UNION/INTERSECT/EXCEPT
- -Allow REINDEX to rebuild all database indexes
- Add ROLLUP, CUBE, GROUPING SETS options to GROUP BY
- Allow SET CONSTRAINTS to be qualified by schema/table name
- Allow TRUNCATE ... CASCADE/RESTRICT
- Allow PREPARE of cursors
- Allow PREPARE to automatically determine parameter types based on the SQL statement
- Allow finer control over the caching of prepared query plans

  Currently, queries prepared via the libpq API are planned on first execute using the supplied parameters --- allow SQL PREPARE to do the same. Also, allow control over replanning prepared queries either manually or automatically when statistics for execute parameters differ dramatically from those used during planning.

- Allow LISTEN/NOTIFY to store info in memory rather than tables?

  Currently LISTEN/NOTIFY information is stored in pg_listener. Storing such information in memory would improve performance.

- Add optional textual message to NOTIFY

  This would allow an informational message to be added to the notify message, perhaps indicating the row modified or other custom information.

- Add a GUC variable to warn about non-standard SQL usage in queries
- Add MERGE command that does UPDATE/DELETE, or on failure, INSERT (rules, triggers?)
- Add NOVICE output level for helpful messages like automatic sequence/index creation
- Add COMMENT ON for all cluster global objects (roles, databases and tablespaces)
- -Add an option to automatically use savepoints for each statement in a multi-statement transaction.

  When enabled, this would allow errors in multi-statement transactions to be automatically ignored.

- Make row-wise comparisons work per SQL spec
- Add RESET CONNECTION command to reset all session state

  This would include resetting of all variables (RESET ALL), dropping of temporary tables, removing any NOTIFYs, cursors, open transactions, prepared queries, currval()s, etc. This could be used for connection pooling. We could also change RESET ALL to have this functionality. The difficult of this features is allowing RESET ALL to not affect changes made by the interface driver for its internal use. One idea is for this to be a protocol-only feature. Another approach is to notify the protocol when a RESET CONNECTION command is used.

- Add GUC to issue notice about queries that use unjoined tables
- CREATE
  - Allow CREATE TABLE AS to determine column lengths for complex expressions like SELECT col1 || col2
  - Use more reliable method for CREATE DATABASE to get a consistent copy of db?
  - Currently the system uses the operating system COPY command to create a new database. Add ON COMMIT capability to CREATE TABLE AS SELECT
- UPDATE
  - Allow UPDATE to handle complex aggregates [update]?
  - Allow an alias to be provided for the target table in UPDATE/DELETE

    This is not SQL-spec but many DBMSs allow it.

  - Allow UPDATE tab SET ROW (col, ...) = (...) for updating multiple columns
  - Allow FOR UPDATE queries to do NOWAIT locks
- ALTER
  - Have ALTER TABLE RENAME rename SERIAL sequence names
  - Add ALTER DOMAIN TYPE
  - Allow ALTER TABLE ... ALTER CONSTRAINT ... RENAME
  - Allow ALTER TABLE to change constraint deferrability and actions
  - Disallow dropping of an inherited constraint
  - Allow objects to be moved to different schemas
  - Allow ALTER TABLESPACE to move to different directories
  - Allow databases to be moved to different tablespaces
  - Allow moving system tables to other tablespaces, where possible

    Currently non-global system tables must be in the default database tablespace. Global system tables can never be moved.

  - Prevent child tables from altering constraints like CHECK that were inherited from the parent table
- CLUSTER
  - Automatically maintain clustering on a table

    This might require some background daemon to maintain clustering during periods of low usage. It might also require tables to be only paritally filled for easier reorganization. Another idea would be to create a merged heap/index data file so an index lookup would automatically access the heap data too. A third idea would be to store heap rows in hashed groups, perhaps using a user-supplied hash function.

  - Add default clustering to system tables

    To do this, determine the ideal cluster index for each system table and set the cluster setting during initdb.

- COPY
  - Allow COPY to report error lines and continue

    This requires the use of a savepoint before each COPY line is processed, with ROLLBACK on COPY failure.

  - -Allow COPY to understand \x as a hex byte
  - Have COPY return the number of rows loaded/unloaded?
  - -Allow COPY to optionally include column headings in the first line
  - -Allow COPY FROM ... CSV to interpret newlines and carriage returns in data
- GRANT/REVOKE
  - Allow column-level privileges
    - Allow GRANT/REVOKE permissions to be applied to all schema objects with one command

      The proposed syntax is:

      GRANT SELECT ON ALL TABLES IN public TO phpuser; GRANT SELECT ON NEW TABLES IN public TO phpuser;

    - Allow GRANT/REVOKE permissions to be inherited by objects based on schema permissions
- CURSOR
  - Allow UPDATE/DELETE WHERE CURRENT OF cursor

    This requires using the row ctid to map cursor rows back to the original heap row. This become more complicated if WITH HOLD cursors are to be supported because WITH HOLD cursors have a copy of the row and no FOR UPDATE lock.

  - Prevent DROP TABLE from dropping a row referenced by its own open cursor?
  - Allow pooled connections to list all open WITH HOLD cursors

    Because WITH HOLD cursors exist outside transactions, this allows them to be listed so they can be closed.

- INSERT
  - Allow INSERT/UPDATE of the system-generated oid value for a row
  - Allow INSERT INTO tab (col1, ..) VALUES (val1, ..), (val2, ..)
  - Allow INSERT/UPDATE ... RETURNING new.col or old.col

    This is useful for returning the auto-generated key for an INSERT. One complication is how to handle rules that run as part of the insert.

- SHOW/SET
  - -Have SHOW ALL show descriptions for server-side variables
  - Add SET PERFORMANCE_TIPS option to suggest INDEX, VACUUM, VACUUM ANALYZE, and CLUSTER
  - Add SET PATH for schemas?

    This is basically the same as SET search_path.

- Server-Side Languages
  - -Allow PL/PgSQL's RAISE function to take expressions

    Currently only constants are supported.

  - -Change PL/PgSQL to use palloc() instead of malloc()
  - Handle references to temporary tables that are created, destroyed, then recreated during a session, and EXECUTE is not used

    This requires the cached PL/PgSQL byte code to be invalidated when an object referenced in the function is changed.

  - Fix PL/pgSQL RENAME to work on variables other than OLD/NEW
  - Allow function parameters to be passed by name, get_employee_salary(emp_id => 12345, tax_year => 2001)
  - Add Oracle-style packages
  - Add table function support to pltcl, plperl, plpython?
  - Allow PL/pgSQL to name columns by ordinal position, e.g. rec.(3)
  - -Allow PL/pgSQL EXECUTE query_var INTO record_var;
  - Add capability to create and call PROCEDURES
  - Allow PL/pgSQL to handle %TYPE arrays, e.g. tab.col%TYPE[]
  - Add MOVE to PL/pgSQL
  - Pass arrays natively instead of as text between plperl and postgres
  - Add support for polymorphic arguments and return types to plperl

# Clients

- Add a libpq function to support Parse/DescribeStatement capability
- Prevent libpq's PQfnumber() from lowercasing the column name?
- Allow libpq to access SQLSTATE so pg_ctl can test for connection failure

  This would be used for checking if the server is up.

- Have initdb set DateStyle based on locale?
- Have pg_ctl look at PGHOST in case it is a socket directory?
- Add a schema option to createlang
- Allow pg_ctl to work properly with configuration files located outside the PGDATA directory

  pg_ctl can not read the pid file because it isn't located in the config directory but in the PGDATA directory. The solution is to allow pg_ctl to read and understand postgresql.conf to find the data_directory value.

- psql
  - Have psql show current values for a sequence
  - Move psql backslash database information into the backend, use mnemonic commands? [psql]

    This would allow non-psql clients to pull the same information out of the database as psql.

  - Fix psql's display of schema information (Neil)
  - Allow psql \pset boolean variables to set to fixed values, rather than toggle
  - Consistently display privilege information for all objects in psql
  - Improve psql's handling of multi-line queries
- pg_dump
  - Have pg_dump use multi-statement transactions for INSERT dumps
  - Allow pg_dump to use multiple -t and -n switches [pg_dump]
  - Add dumping of comments on composite type columns
  - Add dumping of comments on index columns
  - Replace crude DELETE FROM method of pg_dumpall --clean for cleaning of roles with separate DROP commands
  - -Add dumping and restoring of LOB comments
  - Stop dumping CASCADE on DROP TYPE commands in clean mode
  - Add full object name to the tag field. eg. for operators we need '=(integer, integer)', instead of just '='.
  - Add pg_dumpall custom format dumps.

    This is probably best done by combining pg_dump and pg_dumpall into a single binary.

  - Add CSV output format
  - Update pg_dump and psql to use the new COPY libpq API (Christopher)
- ecpg
  - Docs

    Document differences between ecpg and the SQL standard and information about the Informix-compatibility module.

  - Solve cardinality > 1 for input descriptors / variables?
  - Add a semantic check level, e.g. check if a table really exists
  - fix handling of DB attributes that are arrays
  - Use backend PREPARE/EXECUTE facility for ecpg where possible
  - Implement SQLDA
  - Fix nested C comments
  - sqlwarn[6] should be 'W' if the PRECISION or SCALE value specified
  - Make SET CONNECTION thread-aware, non-standard?
  - Allow multidimensional arrays
  - Add internationalized message strings

# Referential Integrity

- Add MATCH PARTIAL referential integrity
- Add deferred trigger queue file

  Right now all deferred trigger information is stored in backend memory. This could exhaust memory for very large trigger queues. This item involves dumping large queues into files.

- -Implement shared row locks and use them in RI triggers
- Change foreign key constraint for array -> element to mean element in array?
- Allow DEFERRABLE UNIQUE constraints?
- Allow triggers to be disabled [trigger]

  Currently the only way to disable triggers is to modify the system tables.

- With disabled triggers, allow pg_dump to use ALTER TABLE ADD FOREIGN KEY

  If the dump is known to be valid, allow foreign keys to be added without revalidating the data.

- Allow statement-level triggers to access modified rows
- Support triggers on columns (Greg Sabino Mullane)
- Remove CREATE CONSTRAINT TRIGGER

  This was used in older releases to dump referential integrity constraints.

- Enforce referential integrity for system tables
- Allow AFTER triggers on system tables

  System tables are modified in many places in the backend without going through the executor and therefore not causing triggers to fire. To complete this item, the functions that modify system tables will have to fire triggers.

# Dependency Checking

- Flush cached query plans when the dependent objects change
- Track dependencies in function bodies and recompile/invalidate

# Exotic Features

- Add SQL99 WITH clause to SELECT
- Add SQL99 WITH RECURSIVE to SELECT
- Add pre-parsing phase that converts non-ISO syntax to supported syntax

  This could allow SQL written for other databases to run without modification.

- Allow plug-in modules to emulate features from other databases
- SQL*Net listener that makes PostgreSQL appear as an Oracle database to clients
- Allow queries across databases or servers with transaction semantics

  This can be done using dblink and two-phase commit.

- -Add two-phase commit
- Add the features of packages
  - Make private objects accessable only to objects in the same schema
  - Allow current_schema.objname to access current schema objects
  - Add session variables
  - Allow nested schemas

# Indexes

- Allow inherited tables to inherit index, UNIQUE constraint, and primary key, foreign key
- UNIQUE INDEX on base column not honored on INSERTs/UPDATEs from inherited table: INSERT INTO inherit_table (unique_index_col) VALUES (dup) should fail

  The main difficulty with this item is the problem of creating an index that can span more than one table.

- Add UNIQUE capability to non-btree indexes
- -Use indexes for MIN() and MAX()

  MIN/MAX queries can already be rewritten as SELECT col FROM tab ORDER BY col {DESC} LIMIT 1. Completing this item involves doing this transformation automatically.

- -Use index to restrict rows returned by multi-key index when used with non-consecutive keys to reduce heap accesses

  For an index on col1,col2,col3, and a WHERE clause of col1 = 5 and col3 = 9, spin though the index checking for col1 and col3 matches, rather than just col1; also called skip-scanning.

- Prevent index uniqueness checks when UPDATE does not modify the column

  Uniqueness (index) checks are done when updating a column even if the column is not modified by the UPDATE.

- Fetch heap pages matching index entries in sequential order

  Rather than randomly accessing heap pages based on index entries, mark heap pages needing access in a bitmap and do the lookups in sequential order. Another method would be to sort heap ctids matching the index before accessing the heap rows.

- -Allow non-bitmap indexes to be combined by creating bitmaps in memory

  This feature allows separate indexes to be ANDed or ORed together. This is particularly useful for data warehousing applications that need to query the database in an many permutations. This feature scans an index and creates an in-memory bitmap, and allows that bitmap to be combined with other bitmap created in a similar way. The bitmap can either index all TIDs, or be lossy, meaning it records just page numbers and each page tuple has to be checked for validity in a separate pass.

- Allow the creation of on-disk bitmap indexes which can be quickly combined with other bitmap indexes

  Such indexes could be more compact if there are only a few distinct values. Such indexes can also be compressed. Keeping such indexes updated can be costly.

- Allow use of indexes to search for NULLs

  One solution is to create a partial index on an IS NULL expression.

- Allow accurate statistics to be collected on indexes with more than one column or expression indexes, perhaps using per-index statistics
- Add fillfactor to control reserved free space during index creation
- Allow the creation of indexes with mixed ascending/descending specifiers
- -Fix incorrect rtree results due to wrong assumptions about "over" operator semantics
- GIST
  - Add more GIST index support for geometric data types
  - -Add concurrency to GIST
  - Allow GIST indexes to create certain complex index types, like digital trees (see Aoki)
- Hash
  - Pack hash index buckets onto disk pages more efficiently

    Currently no only one hash bucket can be stored on a page. Ideally several hash buckets could be stored on a single page and greater granularity used for the hash algorithm.

  - Consider sorting hash buckets so entries can be found using a binary search, rather than a linear scan
  - In hash indexes, consider storing the hash value with or instead of the key itself

# Fsync

- Improve commit_delay handling to reduce fsync()
- Determine optimal fdatasync/fsync, O_SYNC/O_DSYNC options
- Allow multiple blocks to be written to WAL with one write()
- Add an option to sync() before fsync()'ing checkpoint files
- Add program to test if fsync has a delay compared to non-fsync

# Cache Usage

- Allow free-behind capability for large sequential scans, perhaps using posix_fadvise()

  Posix_fadvise() can control both sequential/random file caching and free-behind behavior, but it is unclear how the setting affects other backends that also have the file open, and the feature is not supported on all operating systems.

- Consider use of open/fcntl(O_DIRECT) to minimize OS caching, especially for WAL writes

  O_DIRECT doesn't have the same media write guarantees as fsync, so it is in addition to the fsync method, not in place of it.

- -Cache last known per-tuple offsets to speed long tuple access
- Speed up COUNT(*)

  We could use a fixed row count and a +/- count to follow MVCC visibility rules, or a single cached value could be used and invalidated if anyone modifies the table. Another idea is to get a count directly from a unique index, but for this to be faster than a sequential scan it must avoid access to the heap to obtain tuple visibility information.

- Allow data to be pulled directly from indexes

  Currently indexes do not have enough tuple visibility information to allow data to be pulled from the index without also accessing the heap. One way to allow this is to set a bit to index tuples to indicate if a tuple is currently visible to all transactions when the first valid heap lookup happens. This bit would have to be cleared when a heap tuple is expired.

- Consider automatic caching of queries at various levels:
  - Parsed query tree
  - Query execute plan
  - Query results
- -Allow the size of the buffer cache used by temporary objects to be specified as a GUC variable

  Larger local buffer cache sizes requires more efficient handling of local cache lookups.

- Improve the background writer

  Allow the background writer to more efficiently write dirty buffers from the end of the LRU cache and use a clock sweep algorithm to write other dirty buffers to reduced checkpoint I/O

- Allow sequential scans to take advantage of other concurrent sequentiqal scans, also called "Synchronised Scanning"

  One possible implementation is to start sequential scans from the lowest numbered buffer in the shared cache, and when reaching the end wrap around to the beginning, rather than always starting sequential scans at the start of the table.

# Vacuum

- Improve speed with indexes

  For large table adjustements during vacuum, it is faster to reindex rather than update the index.

- Reduce lock time by moving tuples with read lock, then write lock and truncate table

  Moved tuples are invisible to other backends so they don't require a write lock. However, the read lock promotion to write lock could lead to deadlock situations.

- -Add a warning when the free space map is too small
- Maintain a map of recently-expired rows

  This allows vacuum to target specific pages for possible free space without requiring a sequential scan.

- Auto-fill the free space map by scanning the buffer cache or by checking pages written by the background writer
- Create a bitmap of pages that need vacuuming

  Instead of sequentially scanning the entire table, have the background writer or some other process record pages that have expired rows, then VACUUM can look at just those pages rather than the entire table. In the event of a system crash, the bitmap would probably be invalidated.

- Auto-vacuum
  - Move into the backend code
  - Use free-space map information to guide refilling
  - Do VACUUM FULL if table is nearly empty?
  - Improve xid wraparound detection by recording per-table rather than per-database

# Locking

- -Make locking of shared data structures more fine-grained

  This requires that more locks be acquired but this would reduce lock contention, improving concurrency.

- Add code to detect an SMP machine and handle spinlocks accordingly from distributted.net, http://www1.distributed.net/source, in client/common/cpucheck.cpp

  On SMP machines, it is possible that locks might be released shortly, while on non-SMP machines, the backend should sleep so the process holding the lock can complete and release it.

- -Improve SMP performance on i386 machines

  i386-based SMP machines can generate excessive context switching caused by lock failure in high concurrency situations. This may be caused by CPU cache line invalidation inefficiencies.

- Research use of sched_yield() for spinlock acquisition failure
- Fix priority ordering of read and write light-weight locks (Neil)

# Startup Time Improvements

- Experiment with multi-threaded backend [thread]

  This would prevent the overhead associated with process creation. Most operating systems have trivial process creation time compared to database startup overhead, but a few operating systems (WIn32, Solaris) might benefit from threading. Also explore the idea of a single session using multiple threads to execute a query faster.

- Add connection pooling

  It is unclear if this should be done inside the backend code or done by something external like pgpool. The passing of file descriptors to existing backends is one of the difficulties with a backend approach.

# Write-Ahead Log

- Eliminate need to write full pages to WAL before page modification [wal]

  Currently, to protect against partial disk page writes, we write full page images to WAL before they are modified so we can correct any partial page writes during recovery. These pages can also be eliminated from point-in-time archive files.

  - -Add ability to turn off full page writes
  - When off, write CRC to WAL and check file system blocks on recovery
  - Write full pages during file system write and not when the page is modified in the buffer cache

    This allows most full page writes to happen in the background writer. It might cause problems for applying WAL on recovery into a partially-written page, but later the full page will be replaced from WAL.

- Reduce WAL traffic so only modified values are written rather than entire rows?
- Add WAL index reliability improvement to non-btree indexes
- Allow the pg_xlog directory location to be specified during initdb with a symlink back to the /data location
- Allow WAL information to recover corrupted pg_controldata
- Find a way to reduce rotational delay when repeatedly writing last WAL page

  Currently fsync of WAL requires the disk platter to perform a full rotation to fsync again. One idea is to write the WAL to different offsets that might reduce the rotational delay.

- Allow buffered WAL writes and fsync

  Instead of guaranteeing recovery of all committed transactions, this would provide improved performance by delaying WAL writes and fsync so an abrupt operating system restart might lose a few seconds of committed transactions but still be consistent. We could perhaps remove the 'fsync' parameter (which results in an an inconsistent database) in favor of this capability.

- -Eliminate WAL logging for CREATE TABLE AS when not doing WAL archiving
- -Change WAL to use 32-bit CRC, for performance reasons

# Optimizer / Executor

- Add missing optimizer selectivities for date, r-tree, etc
- Allow ORDER BY ... LIMIT # to select high/low value without sort or index using a sequential scan for highest/lowest values

  Right now, if no index exists, ORDER BY ... LIMIT # requires we sort all values to return the high/low value. Instead The idea is to do a sequential scan to find the high/low value, thus avoiding the sort. MIN/MAX already does this, but not for LIMIT > 1.

- Precompile SQL functions to avoid overhead
- Create utility to compute accurate random_page_cost value
- Improve ability to display optimizer analysis using OPTIMIZER_DEBUG
- Have EXPLAIN ANALYZE highlight poor optimizer estimates
- -Use CHECK constraints to influence optimizer decisions

  CHECK constraints contain information about the distribution of values within the table. This is also useful for implementing subtables where a tables content is distributed across several subtables.

- Consider using hash buckets to do DISTINCT, rather than sorting

  This would be beneficial when there are few distinct values.

- ANALYZE should record a pg_statistic entry for an all-NULL column
- Log queries where the optimizer row estimates were dramatically different from the number of rows actually found?

# Miscellaneous Performance

- Do async I/O for faster random read-ahead of data

  Async I/O allows multiple I/O requests to be sent to the disk with results coming back asynchronously.

- Use mmap() rather than SYSV shared memory or to write WAL files?

  This would remove the requirement for SYSV SHM but would introduce portability issues. Anonymous mmap (or mmap to /dev/zero) is required to prevent I/O overhead.

- Consider mmap()'ing files into a backend?

  Doing I/O to large tables would consume a lot of address space or require frequent mapping/unmapping. Extending the file also causes mapping problems that might require mapping only individual pages, leading to thousands of mappings. Another problem is that there is no way to _prevent_ I/O to disk from the dirty shared buffers so changes could hit disk before WAL is written.

- Add a script to ask system configuration questions and tune postgresql.conf
- Use a phantom command counter for nested subtransactions to reduce per-tuple overhead

# Source Code

- Add use of 'const' for variables in source tree
- Rename some /contrib modules from pg* to pg_*
- Move some things from /contrib into main tree
- Move some /contrib modules out to their own project sites
- Remove warnings created by -Wcast-align
- Move platform-specific ps status display info from ps_status.c to ports
- Add optional CRC checksum to heap and index pages
- Improve documentation to build only interfaces (Marc)
- Remove or relicense modules that are not under the BSD license, if possible
- Remove memory/file descriptor freeing before ereport(ERROR)
- Acquire lock on a relation before building a relcache entry for it
- Promote debug_query_string into a server-side function current_query()
- Allow the identifier length to be increased via a configure option
- Remove Win32 rename/unlink looping if unnecessary
- -Remove kerberos4 from source tree
- Allow cross-compiling by generating the zic database on the target system
- Improve NLS maintenace of libpgport messages linked onto applications
- Allow ecpg to work with MSVC and BCC
- -Make src/port/snprintf.c thread-safe
- Add xpath_array() to /contrib/xml2 to return results as an array
- Allow building in directories containing spaces

  This is probably not possible because 'gmake' and other compiler tools do not fully support quoting of paths with spaces.

- Allow installing to directories containing spaces

  This is possible if proper quoting is added to the makefiles for the install targets. Because PostgreSQL supports relocatable installs, it is already possible to install into a directory that doesn't contain spaces and then copy the install to a directory with spaces.

- Fix cross-compiling of time zone database via 'zic'
- Fix sgmltools so PDFs can be generated with bookmarks
- Add C code on Unix to copy directories for use in creating new databases
- Win32
  - Remove configure.in check for link failure when cause is found
  - Remove readdir() errno patch when runtime/mingwex/dirent.c rev 1.4 is released
  - Remove psql newline patch when we find out why mingw outputs an extra newline
  - Allow psql to use readline once non-US code pages work with backslashes
  - Re-enable timezone output on log_line_prefix '%t' when a shorter timezone string is available
  - Improve dlerror() reporting string
  - Fix problem with shared memory on the Win32 Terminal Server
  - Add support for Unicode

    To fix this, the data needs to be converted to/from UTF16/UTF8 so the Win32 wcscoll() can be used, and perhaps other functions like towupper(). However, UTF8 already works with normal locales but provides no ordering or character set classes.

- Wire Protocol Changes
  - Allow dynamic character set handling
  - Add decoded type, length, precision
  - Use compression?
  - Update clients to use data types, typmod, schema.table.column/ names of result sets using new query protocol

## Developers who have claimed items are:

- Alvaro is Alvaro Herrera <alvherre@dcc.uchile.cl>
- Andrew is Andrew Dunstan <andrew@dunslane.net>
- Bruce is Bruce Momjian <pgman@candle.pha.pa.us> of Software Research Assoc.
- Christopher is Christopher Kings-Lynne <chriskl@familyhealth.com.au> of Family Health Network
- Claudio is Claudio Natoli <claudio.natoli@memetrics.com>
- D'Arcy is D'Arcy J.M. Cain <darcy@druid.net> of The Cain Gang Ltd.
- Fabien is Fabien Coelho <coelho@cri.ensmp.fr>
- Gavin is Gavin Sherry <swm@linuxworld.com.au> of Alcove Systems Engineering
- Greg is Greg Sabino Mullane <greg@turnstep.com>
- Hiroshi is Hiroshi Inoue <Inoue@tpf.co.jp>
- Jan is Jan Wieck <JanWieck@Yahoo.com> of Afilias, Inc.
- Joe is Joe Conway <mail@joeconway.com>
- Karel is Karel Zak <zakkr@zf.jcu.cz>
- Magnus is Magnus Hagander <mha@sollentuna.net>
- Marc is Marc Fournier <scrappy@hub.org> of PostgreSQL, Inc.
- Matthew T. O'Connor <matthew@zeut.net>
- Michael is Michael Meskes <meskes@postgresql.org> of Credativ
- Neil is Neil Conway <neilc@samurai.com>
- Oleg is Oleg Bartunov <oleg@sai.msu.su>
- Peter is Peter Eisentraut <peter_e@gmx.net>
- Philip is Philip Warner <pjw@rhyme.com.au> of Albatross Consulting Pty. Ltd.
- Rod is Rod Taylor <pg@rbt.ca>
- Simon is Simon Riggs <simon@2ndquadrant.com>
- Stephan is Stephan Szabo <sszabo@megazone23.bigpanda.com>
- Tatsuo is Tatsuo Ishii <t-ishii@sra.co.jp> of Software Research Assoc.
- Tom is Tom Lane <tgl@sss.pgh.pa.us> of Red Hat