Some Documentation Changes - Mailing list pgsql-patches

1.  In keeping with the recent discussion that there should be more
said about views, stored procedures, and triggers, in the tutorial, I
have added a bit of verbiage to that end.

2.  Some formatting changes to the datetime discussion, as well as
addition of a citation of a relevant book on calendars.

Index: advanced.sgml
===================================================================
RCS file: /projects/cvsroot/pgsql-server/doc/src/sgml/advanced.sgml,v
retrieving revision 1.38
diff -c -u -r1.38 advanced.sgml
--- advanced.sgml    29 Nov 2003 19:51:36 -0000    1.38
+++ advanced.sgml    30 Dec 2003 01:58:24 -0000
@@ -65,10 +65,24 @@

    <para>
     Views can be used in almost any place a real table can be used.
-    Building views upon other views is not uncommon.
+    Building views upon other views is not uncommon.  You may cut down
+    on the difficulty of building complex queries by constructing them
+    in smaller, easier-to-verify pieces, using views.  Views may be
+    used to reveal specific table columns to users that legitimately
+    need access to some of the data, but who shouldn't be able to look
+    at the whole table.
    </para>
-  </sect1>

+   <para>
+    Views differ from <quote> real tables </quote> in that they are
+    not, by default, updatable.  If they join together several tables,
+    it may be troublesome to update certain columns since the
+    <emphasis>real</emphasis> update that must take place requires
+    identifying the relevant rows in the source tables.  This is
+    discussed further in <xref linkend="rules-views-update">.
+    </para>
+
+  </sect1>

   <sect1 id="tutorial-fk">
    <title>Foreign Keys</title>
@@ -387,6 +401,169 @@
    </para>
   </sect1>

+  <sect1 id="tutorial-storedprocs">
+   <title> Stored Procedures </title>
+
+   <indexterm zone="tutorial-storedprocs">
+     <primary>stored procedures</primary>
+   </indexterm>
+
+   <para> Stored procedures are code that runs inside the database
+   system.  Numerous languages may be used to implement functions and
+   procedures; most built-in code is implemented in C.  The
+   <quote>basic</quote> loadable procedural language for
+   <productname>PostgreSQL</productname> is <xref linkid="plpgsql">.
+   Numerous other languages may also be used, including <xref
+   linkid="plperl">, <xref linkid="pltcl">, and <xref
+   linkid="plpython">.
+   </para>
+
+   <para> There are several ways that stored procedures are really
+   helpful:
+
+   <itemizedlist>
+
+   <listitem><para> To centralize data validation code into the
+   database </para>
+
+   <para> Your system may use client software written in several
+   languages, perhaps with a <quote>web application</quote>
+   implemented in PHP, a <quote>server application</quote> implemented
+   in Java, and a <quote> report writer</quote> implemented in Perl.
+   In the absence of stored procedures, you will likely find that data
+   validation code must be implemented multiple times, in multiple
+   languages, once for each application.</para>
+
+   <para> By implementing data validation in stored procedures,
+   running in the database, it can behave uniformly for all these
+   systems, and you do not need to worry about synchronizing
+   validation procedures across the languages.</para>
+
+   </listitem>
+
+   <listitem><para> Reducing round trips between client and server
+   </para>
+
+   <para>A stored procedure may submit multiple queries, looking up
+   information and adding in links to additional tables.  This takes
+   place without requiring that the client submit multiple queries,
+   and without requiring any added network traffic.
+   </para>
+
+   <para> As a matter of course, the queries share a single
+   transaction context, and there may also be savings in the
+   evaluation of query plans, that will be similar between invocations
+   of a given stored procedure.  </para></listitem>
+
+   <listitem><para> To simplify queries. </para>
+
+   <para> For instance, if you are commonly checking the TLD on domain
+   names, you might create a stored procedure for this purpose, and so
+   be able to use queries such as <command> select domain, tld(domain)
+   from domains; </command> instead of having to put verbose code
+   using <function>substr()</function> into each query.
+   </para>
+
+   <para> It is particularly convenient to use scripting languages
+   like Perl, Tcl, and Python to <quote>grovel through strings</quote>
+   since they are designed for <quote>text processing.</quote></para>
+
+   <para> The binding to the R statistical language allows
+   implementing complex statistical queries inside the database,
+   instead of having to draw the data out.
+   </listitem>
+
+   <listitem><para> Increasing the level of abstraction</para>
+
+   <para> If data is accessed exclusively through stored procedures,
+   then the structures of tables may be changed without there needing
+   to be any visible change in the API used by programmers.  In some
+   systems, users are <emphasis>only</emphasis> allowed access to
+   stored procedures to update data, and cannot do direct updates to
+   tables.
+   </para>
+
+   </listitem>
+
+   </itemizedlist>
+   </para>
+
+   <para> These benefits build on one another: careful use of stored
+   procedures can simultaneously improve reliability and performance,
+   whilst simplifying database access code and improving portability
+   across client platforms and languages.  For instance, consider that
+   a stored procedure can cheaply query tables in the database to
+   validate the correctness of data provided as input. </para>
+
+   <para> Instead of requiring a whole series of queries to create an
+   object, and to look up parent/subsidiary objects to link it to, a
+   stored procedure can do all of this efficiently in the database
+   server, improving performance, and eliminating whole classes of
+   errors. </para>
+
+  </sect1>
+
+  <sect1 id="tutorial-triggers">
+   <title> Triggers </title>
+
+   <indexterm zone="tutorial-triggers">
+     <primary>triggers</primary>
+   </indexterm>
+
+   <para> Triggers allow running a function either before or after
+   update (<command>INSERT</command>, <command>DELETE</command>,
+   <command>UPDATE</command>) operations, which can allow you to do
+   some very clever things. </para>
+
+   <itemizedlist>
+
+   <listitem><para> Data Validation  </para>
+
+   <para> Instead of explicitly coding validation checks as part of a
+   stored procedure, they may be introduced as <command>BEFORE</command>
+   triggers.  The trigger function checks the input values, raising an
+   exception if it finds invalid input.</para>
+
+   <para> Note that this is how foreign key checks are implemented in
+   <productname>PostgreSQL</productname>; when you define a foreign
+   key, you will see a message similar to the following:
+<screen>
+NOTICE:  CREATE TABLE will create implicit trigger(s) for FOREIGN KEY check(s)
+</screen></para>
+
+   <para> In some cases, it may be appropriate for a trigger function
+   to insert data in order to <emphasis>make</emphasis> the input valid.  For
+   instance, if a newly created object needs a status code in a status
+   table, the trigger might automatically do that.</para>
+   </listitem>
+
+   <listitem><para> Audit logs </para>
+
+   <para> One may use <command>AFTER</command> triggers to monitor updates to
+   vital tables, and <command>INSERT</command> entries into log tables to
+   provide a more permanent record of those updates.  </para>
+   </listitem>
+
+   <listitem><para> Replication </para>
+
+   <para> The <application>RServ</application> replication system uses
+   <command>AFTER</command> triggers to track which rows have changed on the
+   <quote>master</quote> system and therefore need to be copied over to
+   <quote>slave</quote> systems.</para>
+
+   <para> <command>
+     CREATE TRIGGER "_rserv_trigger_t_" AFTER INSERT OR DELETE OR UPDATE ON "my_table"
+        FOR EACH ROW EXECUTE PROCEDURE "_rserv_log_" ('10');
+   </command></para>
+   </listitem>
+
+   </itemizedlist>
+
+   <para> Notice that there are strong parallels between what can be
+   accomplished using triggers and stored procedures, particularly in
+   regards to data validation.  </para>
+
+  </sect1>

   <sect1 id="tutorial-conclusion">
    <title>Conclusion</title>
Index: datetime.sgml
===================================================================
RCS file: /projects/cvsroot/pgsql-server/doc/src/sgml/datetime.sgml,v
retrieving revision 2.39
diff -c -u -r2.39 datetime.sgml
--- datetime.sgml    1 Dec 2003 20:34:53 -0000    2.39
+++ datetime.sgml    30 Dec 2003 01:58:25 -0000
@@ -11,8 +11,8 @@
    strings, and are broken up into distinct fields with a preliminary
    determination of what kind of information may be in the
    field. Each field is interpreted and either assigned a numeric
-   value, ignored, or rejected.
-   The parser contains internal lookup tables for all textual fields,
+   value, ignored, or rejected.</para>
+   <para> The parser contains internal lookup tables for all textual fields,
    including months, days of the week, and time
    zones.
   </para>
@@ -1056,21 +1056,21 @@
    years.
   </para>

-  <para>
-   The papal bull of February 1582 decreed that 10 days should be dropped
-   from October 1582 so that 15 October should follow immediately after
-   4 October.
-   This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
-   countries followed shortly after, but Protestant countries were
-   reluctant to change, and the Greek orthodox countries didn't change
-   until the start of the 20th century.
-
-   The reform was observed by Great Britain and Dominions (including what is
-   now the USA) in 1752.
-   Thus 2 September 1752 was followed by 14 September 1752.
+  <para> The papal bull of February 1582 decreed that 10 days should
+  be dropped from October 1582 so that 15 October should follow
+  immediately after 4 October.</para>
+
+  <para> This was observed in Italy, Poland, Portugal, and Spain.
+  Other Catholic countries followed shortly after, but Protestant
+  countries were reluctant to change, and the Greek orthodox countries
+  didn't change until the start of the 20th century.</para>
+
+  <para> The reform was observed by Great Britain and Dominions
+  (including what is now the USA) in 1752.  Thus 2 September 1752 was
+  followed by 14 September 1752.</para>

-   This is why Unix systems have the <command>cal</command> program
-   produce the following:
+   <para> This is why Unix systems have the <command>cal</command>
+   program produce the following:

 <screen>
 $ <userinput>cal 9 1752</userinput>
@@ -1094,19 +1094,24 @@
     </para>
    </note>

-  <para>
-   Different calendars have been developed in various parts of the
-   world, many predating the Gregorian system.
+  <para> Different calendars have been developed in various parts of
+  the world, many predating the Gregorian system.</para>

-   For example,
-   the beginnings of the Chinese calendar can be traced back to the 14th
-   century BC. Legend has it that the Emperor Huangdi invented the
-   calendar in 2637 BC.
+  <para> For example, the beginnings of the Chinese calendar can be
+  traced back to the 14th century BC. Legend has it that the Emperor
+  Huangdi invented the calendar in 2637 BC.</para>

-   The People's Republic of China uses the Gregorian calendar
-   for civil purposes. The Chinese calendar is used for determining
-   festivals.
+  <para> The People's Republic of China uses the Gregorian calendar
+  for civil purposes. The Chinese calendar is used for determining
+  festivals.
   </para>
+
+  <para> If you are interested in this sort of thing, <citation>
+  Calendrical Calculations: The Millennium Edition </citation> by by
+  Edward M. Reingold and Nachum Dershowitz is an excellent reference,
+  describing some 25 calendars, and providing software for displaying
+  them and converting between them.</para>
+
  </sect1>
 </appendix>
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