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Backpatch FAQ entry for null concatenation.
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Bruce Momjian
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Sat Dec 2 07:15:34 EST 2006</P>
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<P>Last updated: Mon Dec 11 17:45:54 EST 2006</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:bruce@momjian.us">bruce@momjian.us</A>)
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development team?<BR>
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<A href="#item1.13">1.13</A>) How does PostgreSQL compare to other
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<SMALL>DBMS</SMALL>s?<BR>
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<H2 align="center">User Client Questions</H2>
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<A href="#item2.1">2.1</A>) What interfaces are available for
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PostgreSQL with Web pages?<BR>
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<A href="#item2.3">2.3</A>) Does PostgreSQL have a graphical user
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interface?<BR>
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<H2 align="center">Administrative Questions</H2>
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<A href="#item3.1">3.1</A>) How do I install PostgreSQL somewhere other
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searches and case-insensitive regular expression searches? How do I
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use an index for case-insensitive searches?<BR>
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<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
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is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
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NULL</SMALL> or not?<BR>
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is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
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How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
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<A href="#item4.10">4.10</A>) What is the difference between the
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various character types?<BR>
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<A href="#item4.11.1">4.11.1</A>) How do I create a
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</ul>
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</li>
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</ul>
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<H3 id="item1.9">1.9) How do I find out about known bugs or
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missing features?</H3>
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<li>The new feature is added to the
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<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> list</li>
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</ul>
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<P>PostgreSQL does not use a bug tracking system because we find
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it more efficient to respond directly to email and keep the
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<A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
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<a href="http://www.postgresql.org/developer/sourcecode/">CVS</a>
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log messages. Even the release notes do not list every change
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made to the software.</P>
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<H3 id="item1.10">1.10) What documentation is available?</H3>
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<P>PostgreSQL includes extensive documentation, including a large
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"http://mysite.verizon.net/Graeme_Birchall/id1.html">http://mysite.verizon.net/Graeme_Birchall/id1.html</A>
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</LI>
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</UL>
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<H3 id="item1.12">1.12) How do I submit a patch or join the development
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team?</H3>
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<P>There are three major areas for potential performance
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improvement:</P>
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<DL>
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<DT><B>Query Changes</B></DT>
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PostgreSQL minor releases are designed to fix only common bugs
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with the least risk. The community considers <i>not</i> upgrading
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more risky that upgrading.</P>
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<P>Major releases (e.g. from 7.3 to 7.4) often change the internal
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format of system tables and data files. These changes are often complex,
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so we don't maintain backward compatibility for data files. A dump/reload
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<P>There are also system tables beginning with <I>pg_</I> that describe
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these too.</P>
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<P>Use <I>psql -l</I> will list all databases.</P>
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<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
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</PRE>
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<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
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disk space used by the expired rows.</P>
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<H3 id="item4.4">4.4) What is the maximum size for a row, a
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table, and a database?</H3>
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@ -735,7 +735,7 @@ table?</TD><TD>unlimited</TD></TR>
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<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
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use very little space.</P>
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<H3 id="item4.6">4.6) Why are my queries slow? Why don't they
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use my indexes?</H3>
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@ -760,7 +760,7 @@ table?</TD><TD>unlimited</TD></TR>
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However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
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often will use an index because only a small portion of the table
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is returned.</P>
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<P>If you believe the optimizer is incorrect in choosing a
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sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
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run query again to see if an index scan is indeed faster.</P>
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@ -823,10 +823,14 @@ table?</TD><TD>unlimited</TD></TR>
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identical values that differ only in case. To force a particular
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case to be stored in the column, use a <SMALL>CHECK</SMALL>
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constraint or a trigger.</P>
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<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
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is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
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How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
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<H3 id="item4.9">4.9) In a query, how do I detect if a field
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is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
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NULL</SMALL> or not?</H3>
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is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
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How can I sort on whether a field is <SMALL> NULL</SMALL> or not?</H3>
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<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
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NOT NULL</SMALL>, like this:</P>
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@ -837,6 +841,13 @@ table?</TD><TD>unlimited</TD></TR>
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WHERE col IS NULL;
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</PRE>
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<P>To concatentate with possible <SMALL>NULL</SMALL>s, use <I>COALESCE()</I>,
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like this:</P>
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<PRE>
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SELECT COALESCE(col1, '') || COALESCE(col2, '')
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FROM tab
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</PRE>
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<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
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and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
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Things that are <I>true</I> will sort higher than things that are <I>false</I>,
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@ -937,7 +948,7 @@ length</TD></TR>
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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</PRE>
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<H3 id="item4.11.3">4.11.3) Doesn't <I>currval()</I>
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lead to a race condition with other users?</H3>
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double-quotes around table or column names during table creation.
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When double-quotes are used, table and column names (called
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identifiers) are stored <a
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href="http://www.postgresql.org/docs/8.2/static/sql-syntax.html#SQL-SYNTAX-IDENTIFIERS">
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href="http://www.postgresql.org/docs/current/static/sql-syntax.html#SQL-SYNTAX-IDENTIFIERS">
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case-sensitive</a>, meaning you must use double-quotes when
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referencing the names in a query. Some interfaces, like pgAdmin,
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automatically double-quote identifiers during table creation.
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