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Postgres FD Implementation
Commit f7e514d0 authored by Bruce Momjian's avatar Bruce Momjian
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Update FAQ by eliminating non-frequent items like large objects and 

extending questions.  Update wording of various entries.
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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Apr 23 14:59:01 EDT 2005
Last updated: Sat Apr 23 16:49:43 EDT 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
......@@ -67,29 +67,17 @@
4.11.3) Doesn't currval() lead to a race condition with other users?
4.11.4) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a TID?
4.12) What is an OID? What is a CTID?
4.13) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
4.14) How do I tell what PostgreSQL version I am running?
4.15) Why does my large-object operations get "invalid large obj
descriptor"?
4.16) How do I create a column that will default to the current time?
4.17) How do I perform an outer join?
4.18) How do I perform queries using multiple databases?
4.19) How do I return multiple rows or columns from a function?
4.20) Why do I get "relation with OID ##### does not exist" errors
4.15) How do I create a column that will default to the current time?
4.16) How do I perform an outer join?
4.17) How do I perform queries using multiple databases?
4.18) How do I return multiple rows or columns from a function?
4.19) Why do I get "relation with OID ##### does not exist" errors
when accessing temporary tables in PL/PgSQL functions?
4.21) What encryption options are available?
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
it dump core?
5.2) How can I contribute some nifty new types and functions to
PostgreSQL?
5.3) How do I write a C function to return a tuple?
5.4) I have changed a source file. Why does the recompile not see the
change?
4.20) What encryption options are available?
_________________________________________________________________
General Questions
......@@ -529,8 +517,8 @@
LIMIT 1;
If you believe the optimizer is incorrect in choosing a sequential
scan, use SET enable_seqscan TO 'off' and run tests to see if an index
scan is indeed faster.
scan, use SET enable_seqscan TO 'off' and run query again to see if an
index scan is indeed faster.
When using wild-card operators such as LIKE or ~, indexes can only be
used in certain circumstances:
......@@ -576,13 +564,13 @@
4.9) In a query, how do I detect if a field is NULL? How can I sort on
whether a field is NULL or not?
You test the column with IS NULL and IS NOT NULL.
You test the column with IS NULL and IS NOT NULL, like this:
SELECT *
FROM tab
WHERE col IS NULL;
To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
in your WHERE clause. Things that are true will sort higher than
in your ORDER BY clause. Things that are true will sort higher than
things that are false, so the following will put NULL entries at the
top of the resulting list:
SELECT *
......@@ -603,9 +591,9 @@
The first four types above are "varlena" types (i.e., the first four
bytes on disk are the length, followed by the data). Thus the actual
space used is slightly greater than the declared size. However, these
data types are also subject to compression or being stored out-of-line
by TOAST, so the space on disk might also be less than expected.
space used is slightly greater than the declared size. However, long
values are also subject to compression, so the space on disk might
also be less than expected.
VARCHAR(n) is best when storing variable-length strings and it limits
how long a string can be. TEXT is for strings of unlimited length,
with a maximum of one gigabyte.
......@@ -647,8 +635,8 @@
You would then also have the new value stored in new_id 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.
<table>_< serialcolumn>_seq, where table and serialcolumn are the
names of your table and your SERIAL column, respectively.
Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
......@@ -667,7 +655,7 @@
transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted transactions.
4.12) What is an OID? What is a TID?
4.12) What is an OID? What is a CTID?
Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
......@@ -680,9 +668,9 @@
single table. and are therefore less likely to overflow. SERIAL8 is
available for storing eight-byte sequence values.
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.
CTIDs are used to identify specific physical rows with block and
offset values. CTIDs change after rows are modified or reloaded. They
are used by index entries to point to physical rows.
4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
......@@ -703,26 +691,12 @@
From psql, type SELECT version();
4.15) Why does my large-object operations get "invalid large obj descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close.
Currently PostgreSQL enforces the rule by closing large object handles
at transaction commit. 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 transaction.
If you are using a client interface like ODBC you may need to set
auto-commit off.
4.16) How do I create a column that will default to the current time?
4.15) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
4.17) How do I perform an outer join?
4.16) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples:
......@@ -740,7 +714,7 @@
is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
INNER joins.
4.18) How do I perform queries using multiple databases?
4.17) How do I perform queries using multiple databases?
There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
......@@ -750,13 +724,13 @@
course, a client can also make simultaneous connections to different
databases and merge the results on the client side.
4.19) How do I return multiple rows or columns from a function?
4.18) How do I return multiple rows or columns from a function?
It is easy using set-returning functions,
http://techdocs.postgresql.org/guides/SetReturningFunctions
.
4.20) Why do I get "relation with OID ##### does not exist" errors when
4.19) Why do I get "relation with OID ##### does not exist" errors when
accessing temporary tables in PL/PgSQL functions?
PL/PgSQL caches function scripts, and an unfortunate side effect is
......@@ -767,7 +741,7 @@
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
4.21) What encryption options are available?
4.20) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.
......@@ -779,33 +753,4 @@
or ssh, rather than PostgreSQL's native SSL connections.)
* Database user passwords are automatically encrypted when stored in
the system tables.
* The server can run using an encrypted file system.
_________________________________________________________________
Extending PostgreSQL
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) How can I contribute some nifty new types and functions to PostgreSQL?
Send your extensions to the pgsql-hackers mailing list, and they will
eventually end up in the contrib/ subdirectory.
5.3) How do I write a C function to return a tuple?
In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the
Programmer's Guide for more information. An example of a
table-returning function defined in C can be found in
contrib/tablefunc.
5.4) I have changed a source file. Why does the recompile 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. If you are using
GCC you can use the --enable-depend option of configure to have the
compiler compute the dependencies automatically.
* The server can also run using an encrypted file system.
doc/src/FAQ/FAQ.html
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