From d94fa4183f9bf24078b59df2e6b255e511e601e9 Mon Sep 17 00:00:00 2001 From: Bruce Momjian Date: Mon, 11 Dec 2006 22:48:08 +0000 Subject: Backpatch FAQ entry for null concatenation. --- doc/src/FAQ/FAQ.html | 51 +++++++++++++++++++++++++----------------- doc/src/sgml/ref/psql-ref.sgml | 4 ++-- 2 files changed, 33 insertions(+), 22 deletions(-) (limited to 'doc/src') diff --git a/doc/src/FAQ/FAQ.html b/doc/src/FAQ/FAQ.html index 301903a5114..b04db6b4748 100644 --- a/doc/src/FAQ/FAQ.html +++ b/doc/src/FAQ/FAQ.html @@ -10,7 +10,7 @@ alink="#0000ff">

Frequently Asked Questions (FAQ) for PostgreSQL

-

Last updated: Sat Dec 2 07:15:34 EST 2006

+

Last updated: Mon Dec 11 17:45:54 EST 2006

Current maintainer: Bruce Momjian (bruce@momjian.us) @@ -42,7 +42,7 @@ development team?
1.13) How does PostgreSQL compare to other DBMSs?
- +

User Client Questions

2.1) What interfaces are available for @@ -51,7 +51,7 @@ PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface?
- +

Administrative Questions

3.1) How do I install PostgreSQL somewhere other @@ -86,8 +86,8 @@ searches and case-insensitive regular expression searches? How do I use an index for case-insensitive searches?
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?
+ is NULL? How do I concatenate possible NULLs? + How can I sort on whether a field is NULL or not?
4.10) What is the difference between the various character types?
4.11.1) How do I create a @@ -281,7 +281,7 @@ - +

1.9) How do I find out about known bugs or missing features?

@@ -307,7 +307,7 @@
  • The new feature is added to the TODO list
    • - +

    PostgreSQL does not use a bug tracking system because we find it more efficient to respond directly to email and keep the TODO @@ -318,7 +318,7 @@ CVS log messages. Even the release notes do not list every change made to the software.

    - +

    1.10) What documentation is available?

    PostgreSQL includes extensive documentation, including a large @@ -370,7 +370,7 @@ "http://mysite.verizon.net/Graeme_Birchall/id1.html">http://mysite.verizon.net/Graeme_Birchall/id1.html - +

    1.12) How do I submit a patch or join the development team?

    @@ -503,7 +503,7 @@

    There are three major areas for potential performance improvement:

    - +
    Query Changes
    @@ -583,7 +583,7 @@ PostgreSQL minor releases are designed to fix only common bugs with the least risk. The community considers not upgrading more risky that upgrading.

    - +

    Major releases (e.g. from 7.3 to 7.4) often change the internal format of system tables and data files. These changes are often complex, so we don't maintain backward compatibility for data files. A dump/reload @@ -638,7 +638,7 @@

    There are also system tables beginning with pg_ that describe these too.

    - +

    Use psql -l will list all databases.

    Also try the file pgsql/src/tutorial/syscat.source. It @@ -660,7 +660,7 @@

    You might then want to do VACUUM FULL tab to reclaim the disk space used by the expired rows.

    - +

    4.4) What is the maximum size for a row, a table, and a database?

    @@ -735,7 +735,7 @@ table?unlimited

    NULLs are stored as bitmaps, so they use very little space.

    - +

    4.6) Why are my queries slow? Why don't they use my indexes?

    @@ -760,7 +760,7 @@ table?unlimited However, LIMIT combined with ORDER BY often will use an index because only a small portion of the table is returned.

    - +

    If you believe the optimizer is incorrect in choosing a sequential scan, use SET enable_seqscan TO 'off' and run query again to see if an index scan is indeed faster.

    @@ -823,10 +823,14 @@ table?unlimited identical values that differ only in case. To force a particular case to be stored in the column, use a CHECK constraint or a trigger.

    - + + 4.9) In a query, how do I detect if a field + is NULL? How do I concatenate possible NULLs? + How can I sort on whether a field is NULL or not?
    +

    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?

    + is NULL? How do I concatenate possible NULLs? + How can I sort on whether a field is NULL or not?

    You test the column with IS NULL and IS NOT NULL, like this:

    @@ -837,6 +841,13 @@ table?unlimited WHERE col IS NULL; +

    To concatentate with possible NULLs, use COALESCE(), + like this:

    +
    +   SELECT COALESCE(col1, '') || COALESCE(col2, '')
+   FROM tab
+
    +

    To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers in your ORDER BY clause. Things that are true will sort higher than things that are false, @@ -937,7 +948,7 @@ length execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); new_id = execute("SELECT currval('person_id_seq')"); - +

    4.11.3) Doesn't currval() lead to a race condition with other users?

    @@ -1091,7 +1102,7 @@ length double-quotes around table or column names during table creation. When double-quotes are used, table and column names (called identifiers) are stored + href="http://www.postgresql.org/docs/current/static/sql-syntax.html#SQL-SYNTAX-IDENTIFIERS"> case-sensitive, meaning you must use double-quotes when referencing the names in a query. Some interfaces, like pgAdmin, automatically double-quote identifiers during table creation. diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml index 0cba5772dcf..100a3d00a4b 100644 --- a/doc/src/sgml/ref/psql-ref.sgml +++ b/doc/src/sgml/ref/psql-ref.sgml @@ -1,5 +1,5 @@ @@ -89,7 +89,7 @@ PostgreSQL documentation SQL and psql meta-commands with this option. To achieve that, you could pipe the string into psql, like - this: echo "\x \\ SELECT * FROM foo;" | psql. + this: echo '\x \\ SELECT * FROM foo;' | psql. (\\ is the separator meta-command.) -- cgit v1.2.3