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Improvements to Ian Barwick patch.
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Bruce Momjian
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doc/FAQ
36
doc/FAQ
@ -1,7 +1,7 @@
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sun Oct 13 22:49:56 EDT 2002
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Last updated: Sun Oct 13 23:15:09 EDT 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@ -52,7 +52,7 @@
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3.6) How do I tune the database engine for better performance?
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3.7) What debugging features are available?
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3.8) Why do I get "Sorry, too many clients" when trying to connect?
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3.9) What are the pg_sorttempNNN.NN files in my database directory?
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3.9) What is in the pgsql_tmp directory?
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3.10) Why do I need to do a dump and restore to upgrade PostgreSQL
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releases?
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@ -197,8 +197,7 @@
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distribution or the MS Windows FAQ at
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http://www.PostgreSQL.org/docs/faq-mswin.html.
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A native port to some Microsoft platforms is currently being worked
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upon.
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A native port to MS Win NT/2000/XP is currently being worked on.
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1.5) Where can I get PostgreSQL?
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@ -368,7 +367,7 @@
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1.15) How can I financially assist PostgreSQL?
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PostgreSQL has had a first-class infrastructure since we started in
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1994. This is all thanks to Marc Fournier, who has created and managed
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1996. This is all thanks to Marc Fournier, who has created and managed
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this infrastructure over the years.
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Quality infrastructure is very important to an open-source project. It
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@ -419,9 +418,9 @@
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2.3) Does PostgreSQL have a graphical user interface? A report generator?
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An embedded query language interface?
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We have a nice graphical user interface called PgAccess, which is
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shipped as part of the distribution. PgAccess also has a report
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generator. The Web page is http://www.pgaccess.org/.
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We have a nice graphical user interface called PgAccess which can also
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be used as a report generator. The Web page is
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http://www.pgaccess.org/.
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We also include ecpg, which is an embedded SQL query language
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interface for C.
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@ -433,7 +432,7 @@
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* C++ (libpq++)
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* Embedded C (ecpg)
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* Java (jdbc)
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* Perl (DBD::Pg)
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* Perl (DBD::Pg and perl5)
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* ODBC (odbc)
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* Python (PyGreSQL)
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* TCL (libpgtcl)
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@ -441,7 +440,8 @@
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* PHP ('pg_' functions, Pear::DB)
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Additional interfaces are available at
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http://www.PostgreSQL.org/interfaces.html.
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http://www.PostgreSQL.org/interfaces.html and
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http://gborg.PostgreSQL.org.
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_________________________________________________________________
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Administrative Questions
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@ -596,16 +596,16 @@
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was 64, and changing it required a rebuild after altering the
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MaxBackendId constant in include/storage/sinvaladt.h.
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3.9) What are the pg_tempNNN.NN files in my database directory?
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3.9) What are the pgsql_tmp directory?
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They are temporary files generated by the query executor. For example,
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if a sort needs to be done to satisfy an ORDER BY, and the sort
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requires more space than the backend's -S parameter allows, then
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temporary files are created to hold the extra data.
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The temporary files should be deleted automatically, but might not if
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a backend crashes during a sort. If you have no backends running at
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the time, it is safe to delete the pg_tempNNN.NN files.
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The temporary files are usually deleted automatically, but might
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remain if a backend crashes during a sort. A stop and restart of the
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postmaster will remove files from those directories.
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3.10) Why do I need to do a dump and restore to upgrade between major
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PostgreSQL releases?
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@ -649,8 +649,8 @@
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4.4) How do you remove a column from a table?
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Prior to version 7.3, ALTER TABLE DROP COLUMN is not supported. You
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can do this instead:
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This functionality was added in release 7.3 with ALTER TABLE DROP
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COLUMN. In earlier versions, you can do this:
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BEGIN;
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LOCK TABLE old_table;
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SELECT ... -- select all columns but the one you want to remove
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@ -815,11 +815,11 @@
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Case-insensitive equality comparisons are normally expressed as:
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SELECT *
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FROM tab
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WHERE LOWER(col) = 'abc';
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WHERE lower(col) = 'abc';
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This will not use an standard index. However, if you create a
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functional index, it will be used:
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CREATE INDEX tabindex ON tab (LOWER(col));
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CREATE INDEX tabindex ON tab (lower(col));
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4.13) In a query, how do I detect if a field is NULL?
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@ -14,7 +14,7 @@
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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: Sun Oct 13 22:49:56 EDT 2002</P>
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<P>Last updated: Sun Oct 13 23:15:09 EDT 2002</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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@ -78,8 +78,8 @@
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<A href="#3.7">3.7</A>) What debugging features are available?<BR>
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<A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
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clients"</I> when trying to connect?<BR>
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<A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
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files in my database directory?<BR>
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<A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I>
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directory?<BR>
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<A href="#3.10">3.10</A>) Why do I need to do a dump and restore
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to upgrade PostgreSQL releases?<BR>
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@ -250,10 +250,11 @@
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<P>The database server can run on Windows NT and Win2k using
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Cygwin, the Cygnus Unix/NT porting library. See
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<I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
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at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
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at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">
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http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
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<p>A native port to some Microsoft platforms is currently being worked
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upon.</p>
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<p>A native port to MS Win NT/2000/XP is currently being worked
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on.</p>
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<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
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@ -484,7 +485,7 @@
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PostgreSQL?</H4>
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<P>PostgreSQL has had a first-class infrastructure since we started
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in 1994. This is all thanks to Marc Fournier, who has created
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in 1996. This is all thanks to Marc Fournier, who has created
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and managed this infrastructure over the years.</P>
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<P>Quality infrastructure is very important to an open-source
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@ -545,9 +546,9 @@
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interface? A report generator? An embedded query language
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interface?</H4>
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<P>We have a nice graphical user interface called PgAccess, which is
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shipped as part of the distribution. PgAccess also has a report
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generator. The Web page is <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
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<P>We have a nice graphical user interface called PgAccess which can
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also be used as a report generator. The Web page is
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<A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
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<P>We also include <I>ecpg</I>, which is an embedded SQL query
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language interface for C.</P>
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@ -565,7 +566,7 @@
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<LI>Java (jdbc)</LI>
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<LI>Perl (DBD::Pg)</LI>
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<LI>Perl (DBD::Pg and perl5)</LI>
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<LI>ODBC (odbc)</LI>
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@ -578,7 +579,9 @@
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<LI>PHP ('pg_' functions, Pear::DB)</LI>
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</UL>
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<P>Additional interfaces are available at
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<a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>.
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<a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>
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and
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<a href="http://gborg.PostgreSQL.org">http://gborg.PostgreSQL.org</A>.
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</P>
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<HR>
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@ -765,8 +768,8 @@
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the MaxBackendId constant in
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<I>include/storage/sinvaladt.h</I>.</P>
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<H4><A name="3.9">3.9</A>) What are the <I>pg_tempNNN.NN</I>
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files in my database directory?</H4>
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<H4><A name="3.9">3.9</A>) What are the <I>pgsql_tmp</I>
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directory?</H4>
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<P>They are temporary files generated by the query executor. For
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example, if a sort needs to be done to satisfy an <SMALL>ORDER
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@ -774,10 +777,9 @@
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<I>-S</I> parameter allows, then temporary files are created to
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hold the extra data.</P>
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<P>The temporary files should be deleted automatically, but might
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not if a backend crashes during a sort. If you have no backends
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running at the time, it is safe to delete the pg_tempNNN.NN
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files.</P>
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<P>The temporary files are usually deleted automatically, but might
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remain if a backend crashes during a sort. A stop and restart of the
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<I>postmaster</I> will remove files from those directories.</P>
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<H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore
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to upgrade between major PostgreSQL releases?</H4>
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@ -790,10 +792,10 @@
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data in a generic format that can then be loaded in using the new internal
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format.</P>
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<p>In releases where the on-disk format does not change, the
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<i>pg_upgrade</i> script can be used to upgrade without a dump/restore.
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The release notes mention whether <i>pg_upgrade</i> is available for the
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release.</p>
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<P>In releases where the on-disk format does not change, the
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<I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
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The release notes mention whether <I>pg_upgrade</I> is available for the
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release.</P>
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<HR>
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@ -831,8 +833,9 @@
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<H4><A name="4.4">4.4</A>) How do you remove a column from a
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table?</H4>
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<P>Prior to version 7.3, <SMALL>ALTER TABLE DROP COLUMN</SMALL> is not supported.
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You can do this instead:</P>
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<P>This functionality was added in release 7.3 with
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<SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
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you can do this:</P>
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<PRE>
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BEGIN;
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LOCK TABLE old_table;
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@ -1023,13 +1026,13 @@
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<PRE>
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SELECT *
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FROM tab
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WHERE LOWER(col) = 'abc';
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WHERE lower(col) = 'abc';
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</PRE>
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This will not use an standard index. However, if you create a
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functional index, it will be used:
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<PRE>
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CREATE INDEX tabindex ON tab (LOWER(col));
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CREATE INDEX tabindex ON tab (lower(col));
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</PRE>
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<H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
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@ -1118,7 +1121,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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and your <SMALL>SERIAL</SMALL> column, respectively.
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<P>Alternatively, you could retrieve the assigned
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<SMALL>SERIAL</SMALL> value with the <I>currval</I>() function
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<SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
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<I>after</I> it was inserted by default, e.g.,</P>
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<PRE>
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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@ -1135,7 +1138,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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<H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
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<I>nextval()</I> lead to a race condition with other users?</H4>
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<P>No. <i>currval</i>() returns the current value assigned by your
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<P>No. <I>currval()</I> returns the current value assigned by your
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backend, not by all users.</P>
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<H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
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