-href="http://www.eol.ists.ca/~dunlop/wdb-p95">http://www.eol.ists.ca/~dunlop/wd
-b
--p95
-
-
1.13) Does PostgreSQL have a graphical user interface
-?
-A report generator? A embedded query language interface?
-We have a nice graphical user interface called pgaccess, which is
-shipped as part of the distribtion. Pgaccess also has a report
-generator.
-We also have ecpg, which is an embedded SQL query language interface for
-C. This is also included.
-
1.14) What is a good book to learn SQL?
-Many of our users like The Practical SQL Handbook,
-Bowman et al, Addison Wesley.
-
-
-
Section 2: Installation Questions
-
-
check to see that you have the proper paths set
-
check that the 'postgres' user owns all the right files
-
ensure that there are files in $PGDATA/files, and that they
-are non-empty. If they aren't, then "gmake install" failed for
-some reason
-
-
2.2) when I start up the postmaster, I get
- "FindBackend: could not find a backend to execute..."
- "postmaster: could not find backend to execute..."
-You probably do not have the right path set up. The 'postgres'
-executable needs to be in your path.
-
2.3) The system seems to be confused about commas,
-decimal points, and date formats.
-Check your locale configuration. PostgreSQL uses the locale settings of
-the user that ran the postmaster process. Set those accordingly for
-your operating environment.
-
2.4) How do I install PostgreSQL somewhere other tha
-n
-/usr/local/pgsql?
-You need to edit Makefile.global and change POSTGRESDIR accordingly, or
-create a Makefile.custom and define POSTGRESDIR there.
-
2.5) When I run postmaster, I get a Bad System
-Call core dumped message.
-It could be a variety of problems, but first check to see that you have
-system V extensions installed on your kernel. PostgreSQL requires kernel
-support for shared memory.
-
2.6) When I try to start the postmaster, I get
-IpcMemoryCreate errors.
-You either do not have shared memory configured properly in kernel or
-you need to enlarge the shared memory available in the kernel. The
-exact amount you need depends on your architecture and how many buffers
-you configure postmaster to run with. For most systems, with default
-buffer sizes, you need a minimum of ~760K.
-
2.7) I have changed a source file, but a
-recompile does not see the change?
-The Makefiles do not have the proper dependencies for include files. You
-have to do a 'make clean' and then another 'make'.
-
2.8) How do I prevent other hosts from accessing my
-PostgreSQL backend?
-By default, PostgreSQL only allows connections from the local machine
-using unix domain. You must add the -i flag to the postmaster, and
-enable host-based authentication by modifying the file $PGDATA/pg_hba
-accordingly.
-
2.9) I can't access the database as the 'root'
-user.
-You should not create database users with user id 0(root). They will be
-unable to access the database. This is a security precaution because
-of the ability of any user to dynamically link object modules into the
-database engine.
-
2.10) All my servers crash under concurrent table
-access. Why?
-This problem can be caused by a kernel that is not configured to support
-semaphores.
-
2.11) How do I tune the database engine for
-better performance?
-There are two things that can be done. You can disable fsync() by
-starting the postmaster with a '-o -F' option. This
-will prevent fsync()'s from flushing to disk after every transaction.
-You can also use the postmaster -B option to increase the number of
-shared memory buffers shared among the backend processes. If you make
-this parameter too high, the process will not start or crash
-unexpectedly. Each buffer is 8K and the defualt is 64 buffers.
-You can also use the postgres -S option to increase the maximum
-amount of memory used by each backend process for temporary sorts.
-Each buffer is 1K and the defualt is 512 buffers.
-
2.12) What debugging features are available in
-PostgreSQL?
-PostgreSQL has several features that report status information that can
-be valuable for debugging purposes.
-First, by running configure with the -enable-cassert option,
-many assert()'s monitor the progress of the backend and halt the
-program when something unexpected occurs.
-Both postmaster and postgres have several debug options available.
-First, whenever you start the postmaster, make sure you send the
-standard output and error to a log file, like:
-
+
+ Additional information about PostgreSQL can be found via the
+ PostgreSQL WWW home page at:
+
+ http://postgreSQL.org
+
+ 1.6) Latest release of PostgreSQL
+
+ The latest release of PostgreSQL is version 6.3, which was released on
+ March 1st, 1998.
+
+ We plan to have major releases every three months.
+
+ 1.7) Is there a commercial version of PostgreSQL?
+
+ Illustra Information Technology (a wholly owned subsidiary of Informix
+ Software, Inc.) sells an object-relational DBMS called Illustra that
+ was originally based on postgres. Illustra has cosmetic similarities
+ to PostgreSQL but has more features, is more robust, performs better,
+ and offers real documentation and support. On the flip side, it costs
+
+ 1.8) What documentation is available for PostgreSQL?
+
+ A user manual, manual pages, and some small test examples are included
+ in the distribution. The sql and built-in manual pages are
+ particularly important.
+
+ The www page contains pointers to an implementation guide and five
+ papers written about postgres design concepts and features.
+
+ 1.9) What version of SQL does PostgreSQL use?
+
+ PostgreSQL supports a subset of SQL-92. It has most of the important
+ constructs but lacks some of the functionality. The most visible
+ differences are:
+ * no HAVING clause under a GROUP BY
+
+ On the other hand, you get to create user-defined types, functions,
+ inheritance etc.
+
+ 1.10) Does PostgreSQL work with databases from earlier versions of postgres?
+
+ PostgreSQL v1.09 is compatible with databases created with v1.01.
+
+ Upgrading to 6.3 from earlier releases requires a dump and restore.
+
+ Upgrading to 6.2.1 from pre-6.2 requires a dump and restore.
+
+ Upgrading to 6.2.1 from 6.2 does not require a dump, but see the
+ appropriate /migration file in the distribution.
+
+ Those ugrading from versions earlier than 1.09 must upgrade to 1.09
+ first without a dump/reload, then dump the data from 1.09, and then
+ load it into 6.2.1 or 6.3 beta.
+
+ 1.11) Are there ODBC drivers for PostgreSQL?
+
+ There are two ODBC drivers available, PostODBC and OpenLink ODBC.
+
+ PostODBC is included in the distribution. For all people being
+ interested in PostODBC, there are now two mailing lists devoted to the
+ discussion of PostODBC. The mailing lists are:
+ * postodbc-developers@listse rv.direct.net
+
+ these lists are ordinary majordomo mailing lists. You can subscribe by
+ sending a mail to:
+
+ OpenLink ODBC is very popular. You can get it from
+ http://www.openlinksw.com/postgres.html. It works with our standard
+ ODBC client software so you'll have Postgres ODBC available on every
+ client platform we support (Win, Mac, Unix, VMS).
+
+ We will probably be selling this product to people who need
+ commercial-quality support, but a freeware version will always be
+
+ 1.12) What tools are available for hooking postgres to Web pages?
+
+ A nice introduction to Database-backed Web pages can be seen at:
+ http://www.webtools.com
+
+ For web integration, PHP is an excellent interface. The URL for that
+ is http://www.php.net
+
+ PHP is great for simple stuff, but for more complex stuff, some still
+ use the perl interface and CGI.pm.
+
+ An WWW gatway based on WDB using perl can be downloaded from:
+ * http://www.eol.ists.ca/~dunlop/wdb -p95
+
+ 1.13) Does PostgreSQL have a graphical user interface? A report generator? A
+ embedded query language interface?
+
+ We have a nice graphical user interface called pgaccess, which is
+ shipped as part of the distribtion. Pgaccess also has a report
+ generator.
+
+ We also have ecpg, which is an embedded SQL query language interface
+ for C. This is also included.
+
+ 1.14) What is a good book to learn SQL?
+
+ Many of our users like The Practical SQL Handbook, Bowman et al,
+ Addison Wesley.
+ _________________________________________________________________
+
+Section 2: Installation Questions
+
+ 2.1) initdb doesn't run
+
+ * check to see that you have the proper paths set
+ * check that the 'postgres' user owns all the right files
+ * ensure that there are files in $PGDATA/files, and that they are
+ non-empty. If they aren't, then "gmake install" failed for some
+ reason
+
+ 2.2) when I start up the postmaster, I get "FindBackend: could not find a
+ backend to execute..." "postmaster: could not find backend to execute..."
+
+ You probably do not have the right path set up. The 'postgres'
+ executable needs to be in your path.
+
+ 2.3) The system seems to be confused about commas, decimal points, and date
+ formats.
+
+ Check your locale configuration. PostgreSQL uses the locale settings
+ of the user that ran the postmaster process. Set those accordingly for
+ your operating environment.
+
+ 2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
+
+ You need to edit Makefile.global and change POSTGRESDIR accordingly,
+ or create a Makefile.custom and define POSTGRESDIR there.
+
+ 2.5) When I run postmaster, I get a Bad System Call core dumped message.
+
+ It could be a variety of problems, but first check to see that you
+ have system V extensions installed on your kernel. PostgreSQL requires
+ kernel support for shared memory.
+
+ 2.6) When I try to start the postmaster, I get IpcMemoryCreate errors.
+
+ You either do not have shared memory configured properly in kernel or
+ you need to enlarge the shared memory available in the kernel. The
+ exact amount you need depends on your architecture and how many
+ buffers you configure postmaster to run with. For most systems, with
+ default buffer sizes, you need a minimum of ~760K.
+
+ 2.7) I have changed a source file, but a recompile does not see the change?
+
+ The Makefiles do not have the proper dependencies for include files.
+ You have to do a 'make clean' and then another 'make'.
+
+ 2.8) How do I prevent other hosts from accessing my PostgreSQL backend?
+
+ By default, PostgreSQL only allows connections from the local machine
+ using unix domain. You must add the -i flag to the postmaster, and
+ enable host-based authentication by modifying the file $PGDATA/pg_hba
+ accordingly.
+
+ 2.9) I can't access the database as the 'root' user.
+
+ You should not create database users with user id 0(root). They will
+ be unable to access the database. This is a security precaution
+ because of the ability of any user to dynamically link object modules
+ into the database engine.
+
+ 2.10) All my servers crash under concurrent table access. Why?
+
+ This problem can be caused by a kernel that is not configured to
+ support semaphores.
+
+ 2.11) How do I tune the database engine for better performance?
+
+ There are two things that can be done. You can disable fsync() by
+ starting the postmaster with a '-o -F' option. This will prevent
+ fsync()'s from flushing to disk after every transaction.
+
+ You can also use the postmaster -B option to increase the number of
+ shared memory buffers shared among the backend processes. If you make
+ this parameter too high, the process will not start or crash
+ unexpectedly. Each buffer is 8K and the defualt is 64 buffers.
+
+ You can also use the postgres -S option to increase the maximum amount
+ of memory used by each backend process for temporary sorts. Each
+ buffer is 1K and the defualt is 512 buffers.
+
+ 2.12) What debugging features are available in PostgreSQL?
+
+ PostgreSQL has several features that report status information that
+ can be valuable for debugging purposes.
+
+ First, by running configure with the -enable-cassert option, many
+ assert()'s monitor the progress of the backend and halt the program
+ when something unexpected occurs.
+
+ Both postmaster and postgres have several debug options available.
+ First, whenever you start the postmaster, make sure you send the
+ standard output and error to a log file, like:
+
+
cd /usr/local/pgsql
- ./bin/postmaster >server.log 2>&1 &
-
-
-This will put a server.log file in the top-level PostgreSQL directory.
-This file can contain useful information about problems or errors
-encountered by the server. Postmaster has a -d option that allows even
-more detailed information to be reported. The -d option takes a number
-1-3 that specifies the debug level. Be warned that a debug level
-of 3 generates large log files.
-You can actuall run the postgres backend from the command line, and type
-your SQL statement directly. This is recommended ONLY for debugging
-purposes. Note that a newline terminates the query, not a semicolon. If
-you have compiled with debugging symbols, you can use a debugger
-to see what is happening. Because the backend was not started from the
-postmaster, it is not running in an identical environment and
-locking/backend interaction problems may not be duplicated. Some
-operating system can attach to a running backend directly to diagnose
-problems.
-The postgres program has a -s, -A, -t options that can be very usefull
-for debugging and performance measurements.
-You can also compile with profiling to see what functions are taking
-execution time. The backend profile files will be deposited in the
-pgsql/data/base/dbname directory. The client profile file will be put
-in the current directory.
-The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
-interpreting your query.
-
2.13) How do I enable more than 32 concurrent
-backends?
-Edit include/storage/sinvaladt.h, and change the value of MaxBackendId.
-In the future, we plan to make this a configurable prameter.
-
-
-
Section 3: PostgreSQL Features
-
-
3.1) Does PostgreSQL support nested subqueries?
-Yes.
-
3.2) I've having a lot of problems using rules.
-Currently, the rule system in PostgreSQL has some limitations. It works
-enough to support the view mechanism, but does not handle
-Insert/Update/Delete well.
-
3.3) I can't seem to write into the middle of large
-objects reliably.
-The Inversion large object system now works perfectly. You should no
-longer have problems with large objects.
-
3.4) How can I write client applications to PostgreS
-QL?
-PostgreSQL supports a C-callable library interface called libpq as well
-as many others. See the /src/interfaces directory.
-Others have contributed a perl interface and a WWW gateway to
-PostgreSQL. See the PostgreSQL home pages for more details.
-
3.5) How do I set up a pg_group?
-Currently, there is no easy interface to set up user groups. You have to
-explicitly insert/update the pg_group table. For example:
-
- jolly=> insert into pg_group (groname, grosysid, grolist)
- jolly=> values ('posthackers', '1234', '{5443, 8261}');
+ ./bin/postmaster >server.log 2>&1 &
+
+ This will put a server.log file in the top-level PostgreSQL directory.
+ This file can contain useful information about problems or errors
+ encountered by the server. Postmaster has a -d option that allows even
+ more detailed information to be reported. The -d option takes a number
+ 1-3 that specifies the debug level. Be warned that a debug level of 3
+ generates large log files.
+
+ You can actuall run the postgres backend from the command line, and
+ type your SQL statement directly. This is recommended ONLY for
+ debugging purposes. Note that a newline terminates the query, not a
+ semicolon. If you have compiled with debugging symbols, you can use a
+ debugger to see what is happening. Because the backend was not started
+ from the postmaster, it is not running in an identical environment and
+ locking/backend interaction problems may not be duplicated. Some
+ operating system can attach to a running backend directly to diagnose
+ problems.
+
+ The postgres program has a -s, -A, -t options that can be very usefull
+ for debugging and performance measurements.
+
+ You can also compile with profiling to see what functions are taking
+ execution time. The backend profile files will be deposited in the
+ pgsql/data/base/dbname directory. The client profile file will be put
+ in the current directory.
+
+ The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
+ interpreting your query.
+
+ 2.13) How do I enable more than 32 concurrent backends?
+
+ Edit include/storage/sinvaladt.h, and change the value of
+ MaxBackendId. In the future, we plan to make this a configurable
+ prameter.
+ _________________________________________________________________
+
+Section 3: PostgreSQL Features
+
+ 3.1) Does PostgreSQL support nested subqueries?
+
+ Yes.
+
+ 3.2) I've having a lot of problems using rules.
+
+ Currently, the rule system in PostgreSQL has some limitations. It
+ works enough to support the view mechanism, but does not handle
+ Insert/Update/Delete well.
+
+ 3.3) I can't seem to write into the middle of large objects reliably.
+
+ The Inversion large object system now works perfectly. You should no
+ longer have problems with large objects.
+
+ 3.4) How can I write client applications to PostgreSQL?
+
+ PostgreSQL supports a C-callable library interface called libpq as
+ well as many others. See the /src/interfaces directory.
+
+ Others have contributed a perl interface and a WWW gateway to
+ PostgreSQL. See the PostgreSQL home pages for more details.
+
+ 3.5) How do I set up a pg_group?
+
+ Currently, there is no easy interface to set up user groups. You have
+ to explicitly insert/update the pg_group table. For example:
+
+
+ jolly=> insert into pg_group (groname, grosysid, grolist)
+ jolly=> values ('posthackers', '1234', '{5443, 8261}');
INSERT 548224
- jolly=> grant insert on foo to group posthackers;
+ jolly=> grant insert on foo to group posthackers;
CHANGE
- jolly=>
-
-
- The fields in pg_group are:
-
groname: the group name. This a char16 and should
- be purely alphanumeric. Do not include underscores
- or other punctuation.
-
grosysid: the group id. This is an int4.
- This should be unique for each group.
-
grolist: the list of pg_user id's that belong in the group.
- This is an int4[].
-
-
3.6) What is the exact difference between binary
-cursors and normal cursors?
-See the declare manual page for a description.
-
3.7) What is a R-tree index and what is it used for?
-
-An r-tree index is used for indexing spatial data. A hash index can't
-handle range searches. A B-tree index only handles range searches in a
-single dimension. R-tree's can handle multi-dimensional data. For
-example, if a R-tree index can be built on an attribute of type 'point',
-the system can more efficient answer queries like select all points
-within a bounding rectangle.
-The canonical paper that describes the original R-Tree design is:
-Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial Searching."
-Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data, 45-57.
-You can also find this paper in Stonebraker's "Readings in Database
-Systems"
-Builtin R-Trees can handle polygons and boxes. In theory, R-trees can
-be extended to handle higher number of dimensions. In practice,
-extending R-trees require a bit of work and we don't currently have any
-documentation on how to do it.
-
3.8) What is the maximum size for a tuple?
-Tuples are limited to 8K bytes. Taking into account system attributes
-and other overhead, one should stay well shy of 8,000 bytes to be on the
-safe side. To use attributes larger than 8K, try using the large
-objects interface.
-Tuples do not cross 8k boundaries so a 5k tuple will require 8k of
-storage.
-
3.9) I defined indices but my queries don't seem
-to make use of them. Why?
-PostgreSQL does not automatically maintain statistics. One has to make
-an explicit 'vacuum' call to update the statistics. After statistics
-are updated, the optimizer has a better shot at using indices. Note
-that the optimizer is limited and does not use indices in some
-circumstances (such as OR clauses). For column-specific optimization
-statistics, use 'vacuum analyze'.
-If the system still does not see the index, it is probably because you
-have created an index on a field with the improper *_ops type. For
-example, you have created a CHAR(4) field, but have specified a char_ops
-index type_class.
-See the create_index manual page for information on what type classes
-are available. It must match the field type.
-Postgres does not warn the user when the improper index is created.
-Indexes not used for ORDER BY operations.
-
3.10) How do I do regular expression searches?
-case-insensitive regexp searching?
-See the pgbuiltin manual page. Search for regular expression.
-
3.11) I experienced a server crash during a
-vacuum. How do I remove the lock file?
-See the vacuum manual page.
-
3.12) What is the difference between the various
-character types?
+ jolly=>
+
+ The fields in pg_group are:
+ * groname: the group name. This a char16 and should be purely
+ alphanumeric. Do not include underscores or other punctuation.
+ * grosysid: the group id. This is an int4. This should be unique for
+ each group.
+ * grolist: the list of pg_user id's that belong in the group. This
+ is an int4[].
+
+ 3.6) What is the exact difference between binary cursors and normal cursors?
+
+ See the declare manual page for a description.
+
+ 3.7) What is a R-tree index and what is it used for?
+
+ An r-tree index is used for indexing spatial data. A hash index can't
+ handle range searches. A B-tree index only handles range searches in a
+ single dimension. R-tree's can handle multi-dimensional data. For
+ example, if a R-tree index can be built on an attribute of type
+ 'point', the system can more efficient answer queries like select all
+ points within a bounding rectangle.
+
+ The canonical paper that describes the original R-Tree design is:
+
+ Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial
+ Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
+ 45-57.
+
+ You can also find this paper in Stonebraker's "Readings in Database
+ Systems"
+
+ Builtin R-Trees can handle polygons and boxes. In theory, R-trees can
+ be extended to handle higher number of dimensions. In practice,
+ extending R-trees require a bit of work and we don't currently have
+ any documentation on how to do it.
+
+ 3.8) What is the maximum size for a tuple?
+
+ Tuples are limited to 8K bytes. Taking into account system attributes
+ and other overhead, one should stay well shy of 8,000 bytes to be on
+ the safe side. To use attributes larger than 8K, try using the large
+ objects interface.
+
+ Tuples do not cross 8k boundaries so a 5k tuple will require 8k of
+ storage.
+
+ 3.9) I defined indices but my queries don't seem to make use of them. Why?
+
+ PostgreSQL does not automatically maintain statistics. One has to make
+ an explicit 'vacuum' call to update the statistics. After statistics
+ are updated, the optimizer has a better shot at using indices. Note
+ that the optimizer is limited and does not use indices in some
+ circumstances (such as OR clauses). For column-specific optimization
+ statistics, use 'vacuum analyze'.
+
+ If the system still does not see the index, it is probably because you
+ have created an index on a field with the improper *_ops type. For
+ example, you have created a CHAR(4) field, but have specified a
+ char_ops index type_class.
+
+ See the create_index manual page for information on what type classes
+ are available. It must match the field type.
+
+ Postgres does not warn the user when the improper index is created.
+
+ Indexes not used for ORDER BY operations.
+
+ 3.10) How do I do regular expression searches? case-insensitive regexp
+ searching?
+
+ See the pgbuiltin manual page. Search for regular expression.
+
+ 3.11) I experienced a server crash during a vacuum. How do I remove the lock
+ file?
+
+ See the vacuum manual page.
+
+ 3.12) What is the difference between the various character types?
+
Type Internal Name Notes
--------------------------------------------------
CHAR char 1 character }
VARCHAR(#) varchar size specifies maximum length, no padding
TEXT text length limited only by maximum tuple length
BYTEA bytea variable-length array of bytes
-
-You need to use the internal name when doing internal operations.
-The last four types above are "varlena" types (i.e. the first four bytes
-are the length, followed by the data). CHAR(#) allocates the maximum
-number of bytes no matter how much data is stored in the field.
-TEXT, VARCHAR(#), and BYTEA all have variable length on the disk, and
-because of this, there is a small performance penalty for using them.
-Specifically, the penalty is for access to any columns after the first
-column of this type.
-
3.13) In a query, how do I detect if a field is NULL?
-
-You test the column with IS NULL and IS NOT NULL.
-
3.14) How do I see how the query optimizer is
-evaluating my query?
-See the explain manual page.
-
3.15) How do I create a serial field?
-Postgres does not allow the user to specifiy a user column as type
-SERIAL. Instead, you can use each row's oid field as a unique value.
-However, if you need to dump and reload the database, you need to use
-pgdump's -o option or COPY's WITH OIDS option to preserver the oids.
-We also have a SEQUENCE function that is similar to SERIAL. See the
-create_sequence manual page.
-Another valid way of doing this is to create a function:
-
+
+ You need to use the internal name when doing internal operations.
+
+ The last four types above are "varlena" types (i.e. the first four
+ bytes are the length, followed by the data). CHAR(#) allocates the
+ maximum number of bytes no matter how much data is stored in the
+ field. TEXT, VARCHAR(#), and BYTEA all have variable length on the
+ disk, and because of this, there is a small performance penalty for
+ using them. Specifically, the penalty is for access to any columns
+ after the first column of this type.
+
+ 3.13) In a query, how do I detect if a field is NULL?
+
+ You test the column with IS NULL and IS NOT NULL.
+
+ 3.14) How do I see how the query optimizer is evaluating my query?
+
+ See the explain manual page.
+
+ 3.15) How do I create a serial field?
+
+ Postgres does not allow the user to specifiy a user column as type
+ SERIAL. Instead, you can use each row's oid field as a unique value.
+ However, if you need to dump and reload the database, you need to use
+ pgdump's -o option or COPY's WITH OIDS option to preserver the oids.
+
+ We also have a SEQUENCE function that is similar to SERIAL. See the
+ create_sequence manual page.
+
+ Another valid way of doing this is to create a function:
+
create table my_oids (f1 int4);
insert into my_oids values (1);
create function new_oid () returns int4 as
'update my_oids set f1 = f1 + 1; select f1 from my_oids; '
language 'sql';
-
-
-then:
-
+
+ then:
+
create table my_stuff (my_key int4, value text);
insert into my_stuff values (new_oid(), 'hello');
-
-
-However, keep in mind there is a race condition here where one server
-could do the update, then another one do an update, and they both could
-select the same new id. This statement should be performed within a
-transaction.
-Yet another way is to use general trigger function autoinc()
-from contrib/spi/autoinc.c.
-
3.16) What are the pg_psort.XXX files in my database
-directory?
-They are temporary sort files generated by the query executor.
-For example, if a sort needs to be done to satisfy an ORDER BY,
-some temp files are generated as a result of the sort.
-If you have no transactions or sorts running at the time, it is safe to
-delete the pg_psort.XXX files.
-
3.17) Why can't I connect to my database from
-another machine?
-The default configuration allows only unix domain socket connections
-from the local machine. To enable TCP/IP connections, use the
-postmaster -i option You need to add a host entry to the file
-pgsql/data/pg_hba. See the hba_conf manual page.
-
3.18) How do I find out what indexes or
-operations are defined in the database?
-psql has a variety of backslash commands to show such information. Use
-\? to see them.
-Also try the file pgsql/src/tutorial/syscat.source. It illustrates many of
-the 'select's needed to get information out of the database system
-tables.
-
3.19) What is the time-warp feature and how does
-it relate to vacuum?
-PostgreSQL no longer supports this feature. All support code has been
-removed. This was done to improve performance and reduce disk storage
-overhead.
-
3.20) What is an oid? What is a tid?
-Oids are Postgres's answer to unique row ids or serial columns. Every
-row that is created in Postgres gets a unique oid. All oids generated
-by initdb are less than 16384 (from backend/access/transam.h). All
-post-initdb (user-created) oids are equal or greater that this. All
-these oids are unique not only within a table, or database, but unique
-within the entire postgres installation.
-Postgres uses oids in its internal system tables to link rows in
-separate tables. These oids can be used to identify specific user rows
-and used in joins. It is recommended you use column type oid to store
-oid values. See the sql(l) manual page to see the other internal
-columns. You can create an index on the oid field for faster access.
-Tids are used to indentify specific physical rows with block and offset
-values. Tids change after rows are modified or reloaded. They are used
-by index entries to point to physical rows. They can not be accessed
-through sql.
-
3.21) What is the meaning of some of the terms
-used in Postgres?
-Some of the source code and older documentation use terms that have more
-common usage. Here are some:
-
row, record, tuple
-
attribute, field, column
-
table, class
-
retrieve, select
-
replace, update
-
append, insert
-
oid, serial value
-
portal, cursor
-
range variable, table name, table alias
-
-Please let me know if you think of any more.
-
3.22) What is Genetic Query Optimization?
-The GEQO module in PostgreSQL is intended to solve the query
-optimization problem of joining many tables by means of a Genetic
-Algorithm (GA). It allows the handling of large join queries through
-non-exhaustive search.
-
3.23) How do you remove a column from a table?
-We do not support ALTER TABLE DROP COLUMN, but do this:
-
+
+ However, keep in mind there is a race condition here where one server
+ could do the update, then another one do an update, and they both
+ could select the same new id. This statement should be performed
+ within a transaction.
+
+ Yet another way is to use general trigger function autoinc() from
+ contrib/spi/autoinc.c.
+
+ 3.16) What are the pg_psort.XXX files in my database directory?
+
+ They are temporary sort files generated by the query executor. For
+ example, if a sort needs to be done to satisfy an ORDER BY, some temp
+ files are generated as a result of the sort.
+
+ If you have no transactions or sorts running at the time, it is safe
+ to delete the pg_psort.XXX files.
+
+ 3.17) Why can't I connect to my database from another machine?
+
+ The default configuration allows only unix domain socket connections
+ from the local machine. To enable TCP/IP connections, use the
+ postmaster -i option You need to add a host entry to the file
+ pgsql/data/pg_hba. See the hba_conf manual page.
+
+ 3.18) How do I find out what indexes or operations are defined in the
+ database?
+
+ psql has a variety of backslash commands to show such information. Use
+ \? to see them.
+
+ Also try the file pgsql/src/tutorial/syscat.source. It illustrates
+ many of the 'select's needed to get information out of the database
+ system tables.
+
+ 3.19) What is the time-warp feature and how does it relate to vacuum?
+
+ PostgreSQL no longer supports this feature. All support code has been
+ removed. This was done to improve performance and reduce disk storage
+ overhead.
+
+ 3.20) What is an oid? What is a tid?
+
+ Oids are Postgres's answer to unique row ids or serial columns. Every
+ row that is created in Postgres gets a unique oid. All oids generated
+ by initdb are less than 16384 (from backend/access/transam.h). All
+ post-initdb (user-created) oids are equal or greater that this. All
+ these oids are unique not only within a table, or database, but unique
+ within the entire postgres installation.
+
+ Postgres uses oids in its internal system tables to link rows in
+ separate tables. These oids can be used to identify specific user rows
+ and used in joins. It is recommended you use column type oid to store
+ oid values. See the sql(l) manual page to see the other internal
+ columns. You can create an index on the oid field for faster access.
+
+ Tids are used to indentify specific physical rows with block and
+ offset values. Tids change after rows are modified or reloaded. They
+ are used by index entries to point to physical rows. They can not be
+ accessed through sql.
+
+ 3.21) What is the meaning of some of the terms used in Postgres?
+
+ Some of the source code and older documentation use terms that have
+ more common usage. Here are some:
+ * row, record, tuple
+ * attribute, field, column
+ * table, class
+ * retrieve, select
+ * replace, update
+ * append, insert
+ * oid, serial value
+ * portal, cursor
+ * range variable, table name, table alias
+
+ Please let me know if you think of any more.
+
+ 3.22) What is Genetic Query Optimization?
+
+ The GEQO module in PostgreSQL is intended to solve the query
+ optimization problem of joining many tables by means of a Genetic
+ Algorithm (GA). It allows the handling of large join queries through
+ non-exhaustive search.
+
+ For further information see README.GEQO .
+
+ 3.23) How do you remove a column from a table?
+
+ We do not support ALTER TABLE DROP COLUMN, but do this:
+
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;
-
-
-
3.24) How do SELECT only the first few rows of
-a query?
-See the fetch manual page.
-This only prevents all row results from being transfered to the client.
-The entire query must be evaluated, even if you only want just first few
-rows.
-Consider a query that has and ORDER BY. There is no way to return any
-rows until the entire query is evaluated and sorted.
-
3.25) Why can't I create a column named
-"time"?
-6.2.1 has added some new restricted keywords as we make PostgreSQL more
-ANSI-92 compilant. The next release will have this restriction removed.
-There is a patch on ftp.postgresql.org that will allow this feature now.
-
3.26)How much database disk space is required
-to store data from a typical flat file?
-Consider a file with 300,000 lines with two integers on each line.
-The flat file is 2.4MB. The size of the PostgreSQL database file
-containing this data can be estimated:
+
+ 3.24) How do SELECT only the first few rows of a query?
+
+ See the fetch manual page.
+
+ This only prevents all row results from being transfered to the
+ client. The entire query must be evaluated, even if you only want just
+ first few rows. Consider a query that has and ORDER BY. There is no
+ way to return any rows until the entire query is evaluated and sorted.
+
+ 3.25) Why can't I create a column named "time"?
+
+ 6.2.1 has added some new restricted keywords as we make PostgreSQL
+ more ANSI-92 compilant. The next release will have this restriction
+ removed. There is a patch on ftp.postgresql.org that will allow this
+ feature now.
+
+ 3.26)How much database disk space is required to store data from a typical
+ flat file?
+
+ Consider a file with 300,000 lines with two integers on each line. The
+ flat file is 2.4MB. The size of the PostgreSQL database file
+ containing this data can be estimated:
+
40 bytes + each row header (approximate)
8 bytes + two int fields @ 4 bytes each
4 bytes + pointer on page to tuple
157 rows per page
1911 database pages * 8192 bytes per page = 15,654,912 or 15.5MB
+
Indexes do not contain as much overhead, but do contain the data that
is being indexed, so they can be large also.
-
-
-
Section 4: Extending PostgreSQL
-
-
4.1) I wrote a user-defined function and when I run
-it
-in psql, it dumps core.
-The problem could be a number of things. Try testing your user-defined
-function in a stand alone test program first. Also, make sure you are
-not sending elog NOTICES when the front-end is expecting data, such as
-during a type_in() or type_out() functions
-
4.2) I get messages of the type
- NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!
-You are pfree'ing something that was not palloc'ed. When writing
-user-defined functions, do not include the file "libpq-fe.h". Doing so
-will cause your palloc to be a malloc instead of a free. Then, when the
-backend pfrees the storage, you get the notice message.
-
4.3) I've written some nifty new types and functions
- for
-PostgreSQL.
-Please share them with other PostgreSQL users. Send your extensions to
-mailing list, and they will eventually end up in the contrib/
-subdirectory.
-
4.4) How do I write a C function to return a tuple?<
-/H3>
-This requires extreme wizardry, so extreme that the authors have not
-ever tried it, though in principle it can be done. The short answer is
-... you can't. This capability is forthcoming in the future.
-
-
-
Section 5: Bugs
-
-
5.1) How do I make a bug report?
-Check the current FAQ at
-href="http://postgreSQL.org">http://postgreSQL.org
-Also check out our ftp site
-href="ftp://ftp.postgreSQL.org/pub">ftp://ftp.postgreSQL.org/pub to
-see if there is a more recent PostgreSQL version.
-You can also fill out the "bug-template" file and send it to:
-
-This is the address of the developers mailing list.
-
-
projects / postgresql.git / commitdiff