-"http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess
-
-We also include ecpg, which is an embedded SQL query language interface for
-C.
-
-
2.4) What languages are available to
-communicate with PostgreSQL?
-
-We have:
-
C(libpq)
-
C++(libpq++)
-
Embedded C(ecpg)
-
Java(jdbc)
-
Perl(perl5)
-
ODBC(odbc)
-
Python(PyGreSQL)
-
TCL(libpgtcl)
-
C Easy API(libpgeasy)
-
Embedded HTML(PHP from http://www.php.net)-
-
-
-
Administrative Questions
-
-
-
3.1) Why does initdb fail?
-
-
check that you don't have any of the previous version's binaries in
-your path
-
check to see that you have the proper paths set
-
check that the postgres user owns the proper files
-
-
-
3.2) How do I install PostgreSQL somewhere
-other than /usr/local/pgsql?
-
-The simplest way is to specify the --prefix option when running configure.
-If you forgot to do that, you can edit Makefile.global and change POSTGRESDIR
-accordingly, or create a Makefile.custom and define POSTGRESDIR there.
-
-
-
3.3) When I start the postmaster, I get a Bad
-System Call or core dumped message. Why?
-
-It could be a variety of problems, but first check to see that you
-have system V extensions installed in your kernel. PostgreSQL requires
-kernel support for shared memory and semaphores.
-
-
-
3.4) When I try to start the postmaster, I
-get
IpcMemoryCreate errors. Why?
-
-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
-and backend processes you configure postmaster to run with.
-For most systems, with default numbers of buffers and processes, you
-
-
3.5) When I try to start the postmaster, I
-get
IpcSemaphoreCreate errors. Why?
-
-If the error message is IpcSemaphoreCreate: semget failed (No space
-left on device) then your kernel is not configured with enough
-semaphores. Postgres needs one semaphore per potential backend process.
-A temporary solution is to start the postmaster with a smaller limit on
-the number of backend processes. Use -N with a parameter less
-than the default of 32. A more permanent solution is to increase your
-kernel's
SEMMNS and
SEMMNI parameters.
-
-If the error message is something else, you might not have semaphore
-support configured in your kernel at all.
-
-
-
3.6) How do I prevent other hosts from
-accessing my PostgreSQL database?
-
-By default, PostgreSQL only allows connections from the local machine
-using Unix domain sockets. Other machines will not be able to connect
-unless you add the -i flag to the postmaster,
-and enable host-based authentication by modifying the file
-$PGDATA/pg_hba.conf accordingly. This will allow TCP/IP connections.
-
-
3.7) Why can't I connect to my database from
-
-The default configuration allows only unix domain socket connections
-from the local machine. To enable TCP/IP connections, make sure the
-postmaster has been started with the -i option, and add an
-appropriate host entry to the file
-
pgsql/data/pg_hba.conf. See the
pg_hba.conf manual page.
-
-
-
3.8) Why can't I access the database as the root
-
-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
-
-
-
3.9) All my servers crash under concurrent
-
-This problem can be caused by a kernel that is not configured to support
-
-
-
3.10) How do I tune the database engine for
-
-Certainly, indices can speed up queries. The EXPLAIN command
-allows you to see how PostgreSQL is interpreting your query, and which
-
-If you are doing a lot of INSERTs, consider doing them in a large
-batch using the COPY command. This is much faster than single
-individual INSERTS. Second, statements not in a BEGIN
-WORK/COMMIT transaction block are considered to be in their
-own transaction. Consider performing several statements in a single
-transaction block. This reduces the transaction overhead. Also
-consider dropping and recreating indices when making large data
-
-There are several tuning 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 used by the backend processes. If you make this
-parameter too high, the postmaster may not start up because you've exceeded
-your kernel's limit on shared memory space.
-Each buffer is 8K and the default is 64 buffers.
-
-You can also use the backend -S option to increase the maximum amount
-of memory used by the backend process for temporary sorts. The -S value
-is measured in kilobytes, and the default is 512 (ie, 512K).
-
-You can also use the CLUSTER command to group data in base tables to
-match an index. See the cluster(l) manual page for more details.
-
-
-
3.11) What debugging features are available in
-
-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 contains 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
-that specifies the debug level. Be warned that high debug level values
-generate large log files.
-
-If the postmaster is not running, you can actually 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.
-
-If the postmaster is running, start psql in one window,
-then find the PID of the postgres process used by
-psql. Use a debugger to attach to the postgres
-PID. You can set breakpoints in the debugger and issue
-queries from psql. If you are debugging postgres startup,
-you can set PGOPTIONS="-W n", then start psql. This will cause
-startup to delay for n seconds so you can attach with the
-debugger and trace through the startup sequence.
-
-The postgres program has -s, -A, and -t options that can be very useful
-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 client's current directory.
-
-
-
3.12) I get 'Sorry, too many clients' when trying
-
-You need to increase the postmaster's limit on how many concurrent backend
-
-In Postgres 6.5 and up, the default limit is 32 processes. You can
-increase it by restarting the postmaster with a suitable -N
-value. With the default configuration you can set -N as large as
-1024; if you need more, increase MAXBACKENDS in
-include/config.h and rebuild. You can set the default value of
--N at configuration time, if you like, using configure's
-
--with-maxbackends switch.
-
-Note that if you make -N larger than 32, you must also increase
--B beyond its default of 64; -B must be at least twice -N, and
-probably should be more than that for best performance. For large
-numbers of backend processes, you are also likely to find that you need
-to increase various Unix kernel configuration parameters. Things to
-check include the maximum size of shared memory blocks,
-SHMMAX, the maximum number of semaphores,
-SEMMNS and SEMMNI, the maximum number of
-processes, NPROC, the maximum number of processes per
-user, MAXUPRC, and the maximum number of open files,
-NFILE and NINODE. The reason that Postgres
-has a limit on the number of allowed backend processes is so that you
-can ensure that your system won't run out of resources.
-
-In Postgres versions prior to 6.5, the maximum number of backends was
-64, and changing it required a rebuild after altering the MaxBackendId
-constant in
include/storage/sinvaladt.h.-
-
3.13) What are the pg_tempNNN.NN files in my
-
-They are temporary files generated by the query executor. For
-example, if a sort needs to be done to satisfy an ORDER BY, and
-the sort requires more space than the backend's -S parameter allows,
-then temp files are created to hold the extra data.
-
-The temp files should go away automatically, but might not if a backend
-crashes during a sort. If you have no transactions running at the time,
-it is safe to delete the pg_tempNNN.NN files.
-
-
-
-
-
-
4.1) 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. There are postgres and psql
-SET commands to control the date format. Set those accordingly for
-your operating environment.
-
-
-
4.2) What is the exact difference between
-binary cursors and normal cursors?
-
-See the
DECLARE manual page for a description.
-
-
4.3) How do I SELECT only the first few
-
-See the
FETCH manual page, or use SELECT ... LIMIT....
-
-The entire query may have to be evaluated, even if you only want the
-first few rows. Consider a query that has an ORDER BY.
-If there is an index that matches the ORDER BY,
-PostgreSQL may be able to evaluate only the first few records requested,
-or the entire query may have to be evaluated until the desired rows have
-
-
4.4) How do I get a list of tables, or other
-information I see in
psql?-
-You can read the source code for psql, file
-pgsql/src/bin/psql/psql.c. It contains SQL commands that generate the
-output for psql's backslash commands. You can also start psql
-with the -E option so that it will print out the queries it uses
-to execute the commands you give.
-
-
-
4.5) How do you remove a column from a
-
-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;
-
-
-
-
-
4.6) What is the maximum size for a
-
-These are the limits:
-
-Maximum size for a database? unlimited (60GB databases exist)
-Maximum size for a table? unlimited on all operating systems
-Maximum size for a row? 8k, configurable to 32k
-Maximum number of rows in a table? unlimited
-Maximum number of columns table? unlimited
-Maximum number of indexes on a table? unlimited
-
-
-Of course, these are not actually unlimited, but limited to available
-
-To change the maximum row size, edit include/config.h and change
-BLCKSZ. To use attributes larger than 8K, you can also
-use the large object interface.
-
-Row length limit will be removed in 7.1.
-
-
-
4.7)How much database disk space is required to
-store data from a typical flat file?
-
-A Postgres database can require about six and a half times the disk space
-required to store the data in a flat file.
-
-Consider a file of 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 at 14MB:
-
- 36 bytes: each row header (approximate)
- + 8 bytes: two int fields @ 4 bytes each
- + 4 bytes: pointer on page to tuple
- ----------------------------------------
- 48 bytes per row
-
- The data page size in PostgreSQL is 8192 bytes (8 KB), so:
-
- 8192 bytes per page
- ------------------- = 171 rows per database page (rounded up)
- 48 bytes per row
-
- 300000 data rows
- -------------------- = 1755 database pages
- 171 rows per page
-
-1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB)
-
-
-Indexes do not contain as much overhead, but do contain the data that is
-being indexed, so they can be large also.
-
-
4.8) How do I find out what indices or
-operations are defined in the database?
-
-psql has a variety of backslash commands to show such information. Use
-
-Also try the file pgsql/src/tutorial/syscat.source. It
-illustrates many of the SELECTs needed to get information from
-the database system tables.
-
-
-
4.9) My queries are slow or don't make
-use of the indexes. 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 knows how many rows in the table,
-and can better decide if it should use indices. Note that the optimizer
-does not use indices in cases when the table is small because a
-sequential scan would be faster.
-
-For column-specific optimization statistics, use VACUUM
-ANALYZE. VACUUM ANALYZE is important for complex
-multi-join queries, so the optimizer can estimate the number of rows
-returned from each table, and choose the proper join order. The backend
-does not keep track of column statistics on its own, so VACUUM
-ANALYZE must be run to collect them periodically.
-
-Indexes are usually not used for ORDER BY operations: a
-sequential scan followed by an explicit sort is faster than an indexscan
-of all tuples of a large table, because it takes fewer disk accesses.
-
-When using wild-card operators such as LIKE or ~, indices can
-only be used if the beginning of the search is anchored to the start of
-the string. So, to use indices, LIKE searches should not
-begin with %, and ~(regular expression searches) should
-start with ^.
-
-
4.10) How do I see how the query optimizer is
-
-See the
EXPLAIN manual page.
-
-
4.11) What is an R-tree index?
-
-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 an 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
-
-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.
-
-
-
4.12) What is Genetic Query
-
-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
-
-For further information see the documentation.
-
-
-
-
4.13) How do I do regular expression searches and
-case-insensitive regexp searching?
-
-The ~ operator does regular-expression matching, and ~*
-does case-insensitive regular-expression matching. There is no
-case-insensitive variant of the LIKE operator, but you can get the
-effect of case-insensitive LIKE with this:
- WHERE lower(textfield) LIKE lower(pattern)
-
-
-
4.14) In a query, how do I detect if a field
-
-You test the column with IS NULL and IS NOT NULL.
-
-
-
4.15) What is the difference between the
-various character types?
-
-Type Internal Name Notes
---------------------------------------------------
-"char" char 1 character
-CHAR(#) bpchar blank padded to the specified fixed length
-VARCHAR(#) varchar size specifies maximum length, no padding
-TEXT text length limited only by maximum row length
-BYTEA bytea variable-length array of bytes
-
-You will see the internal name when examining system catalogs
-and in some error messages.
-
-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 all columns after the
-first column of this type.
-
-
-
4.16.1) How do I create a
-serial/auto-incrementing field?
-
-PostgreSQL supports SERIAL data type. It auto-creates a
-sequence and index on the column. For example, this...
- CREATE TABLE person (
- id SERIAL,
- name TEXT
- );
-
-...is automatically translated into this...
- CREATE SEQUENCE person_id_seq;
- CREATE TABLE person (
- id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
- name TEXT
- );
- CREATE UNIQUE INDEX person_id_key ON person ( id );
-
-See the create_sequence manual page for more information about sequences.
-
-You can also use each row's oid field as a unique value. However, if
-you need to dump and reload the database, you need to use pg_dump's -o
-option or
COPY WITH OIDS option to preserve the oids.
-
-For more details, see Bruce Momjian's chapter on
-
-
4.16.2) How do I get the back the generated SERIAL value after an insert?
-Probably the simplest approach is to to retrieve the next SERIAL value from the sequence object with the
nextval() function
before inserting and then insert it explicitly. Using the example table in
4.16.1, that might look like this:- $newSerialID = nextval('person_id_seq');
- INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
-
-You would then also have the new value stored in $newSerialID for use in other queries (e.g., as a foreign key to the person table). Note that the name of the automatically-created SEQUENCE object will be named <table>_<serialcolumn>_seq, where table and serialcolumn are the names of your table and your SERIAL column, respectively.
-Similarly, you could retrieve the just-assigned SERIAL value with the currval() function after it was inserted by default, e.g.,
- INSERT INTO person (name) VALUES ('Blaise Pascal');
- $newID = currval('person_id_seq');
-
-Finally, you could use the
oid returned from the-INSERT statement to lookup the default value, though this is probably
-the least portable approach. In perl, using DBI with Edmund Mergl's
-DBD::Pg module, the oid value is made available via
-$sth->{pg_oid_status} after $sth->execute().
-
-
4.16.3) Don't currval() and nextval() lead to a race condition with other
-concurrent backend processes?
-
-No. That has been handled by the backends.
-
-
-
4.17) What is an oid? What is a tid?
-
-Oids are PostgreSQL's answer to unique row ids. Every row that is
-created in PostgreSQL gets a unique oid. All oids generated during
-initdb are less than 16384 (from backend/access/transam.h). All
-user-created oids are equal or greater that this. By default, all these
-oids are unique not only within a table, or database, but unique within
-the entire PostgreSQL installation.
-
-PostgreSQL uses oids in its internal system tables to link rows between
-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.
-
-Oids are assigned to all new rows from a central area that is used by
-all databases. If you want to change the oid to something else, or if
-you want to make a copy of the table, with the original oid's, there is
-no reason you can't do it:
-
- CREATE TABLE new_table(old_oid oid, mycol int);
- SELECT INTO new SELECT old_oid, mycol FROM old;
- COPY new TO '/tmp/pgtable';
- DELETE FROM new;
- COPY new WITH OIDS FROM '/tmp/pgtable';
-
-
-Tids are used to identify 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.
-
-
-
4.18) What is the meaning of some of the terms
-
-Some of the source code and older documentation use terms that have more
-common usage. Here are some:
-
-
table, relation, class
-
row, record, tuple
-
column, field, attribute
-
retrieve, select
-
replace, update
-
append, insert
-
oid, serial value
-
portal, cursor
-
range variable, table name, table alias
-
-
4.19) Why do I get the error "FATAL: palloc
-failure: memory exhausted?"
-
-It is possible you have run out of virtual memory on your system, or
-your kernel has a low limit for certain resources. Try this before
-starting the postmaster:
-
- ulimit -d 65536
- limit datasize 64m
-
-
-Depending on your shell, only one of these may succeed, but it will set
-your process data segment limit much higher and perhaps allow the query
-to complete. This command applies to the current process, and all
-subprocesses created after the command is run. If you are having a problem
-with the SQL client because the backend is returning too much data, try
-it before starting the client.
-
-
4.20) How do I tell what PostgreSQL version I
-
-From
psql, type
select version();-
-
4.21) My large-object operations get invalid
-large obj descriptor. Why?
-
-You need to put BEGIN WORK and COMMIT
- around any use of a large object handle, that is,
-surrounding
lo_open ...
lo_close.-
-Current PostgreSQL enforces the rule by closing large object handles at
-transaction commit, which will be instantly upon completion of the
-lo_open command if you are not inside a transaction. So the
-first attempt to do anything with the handle will draw invalid large
-obj descriptor. So code that used to work (at least most of the
-time) will now generate that error message if you fail to use a
-
-If you are using a client interface like ODBC you may need to set
-
-
4.22) How do I create a column that will default to the
-Use now():
- CREATE TABLE test (x int, modtime timestamp default now() );
-
-
4.23) Why are my subqueries using IN so
-Currently, we join subqueries to outer queries by sequential scanning
-the result of the subquery for each row of the outer query. A workaround
-is to replace IN with EXISTS. For example,
-change:
- SELECT *
- FROM tab
- WHERE col1 IN (SELECT col2 FROM TAB2)
-
-to:
- SELECT *
- FROM tab
- WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
-
-We hope to fix this limitation in a future release.
-
-
4.24) How do I do an outer join?
-PostgreSQL does not support outer joins in the current release. They can
-be simulated using UNION and NOT IN. For
-example, when joining tab1 and tab2, the following query
-does an outer join of the two tables:
- SELECT tab1.col1, tab2.col2
- FROM tab1, tab2
- WHERE tab1.col1 = tab2.col1
- UNION ALL
- SELECT tab1.col1, NULL
- FROM tab1
- WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
- ORDER BY tab1.col1
-
-
-
-
-
-
-
5.1) I wrote a user-defined function. When
-I run it in
psql, why does it dump core?
-
-The problem could be a number of things. Try testing your user-defined
-function in a stand alone test program first.
-
-
5.2) What does the message:
-
NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set! mean?
-
-You are pfree'ing something that was not palloc'ed.
-Beware of mixing malloc/free and palloc/pfree.
-
-
-
5.3) How can I contribute some nifty new types and
-functions for PostgreSQL?
-
-
-Send your extensions to the pgsql-hackers mailing list, and they will
-eventually end up in the
contrib/ subdirectory.
-
-
-
5.4) How do I write a C function to return a
-
-This requires wizardry so extreme that the authors have never
-tried it, though in principle it can be done.
-
-
5.5) I have changed a source file. Why does the
-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.
- You
-have to do a
make clean and then another
make.-
-
-
-
projects / postgresql.git / commitdiff