advanced.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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 <chapter id="tutorial-advanced">
  <title>Advanced Features</title>

  <sect1 id="tutorial-advanced-intro">
   <title>Introduction</title>

   <para>
    In the previous chapter we have covered the basics of using
    <acronym>SQL</acronym> to store and access your data in
    <productname>PostgreSQL</productname>.  We will now discuss some
    more advanced features of <acronym>SQL</acronym> that simplify
    management and prevent loss or corruption of your data.  Finally,
    we will look at some <productname>PostgreSQL</productname>
    extensions.
   </para>

   <para>
    This chapter will on occasion refer to examples found in <xref
    linkend="tutorial-sql"> to change or improve them, so it will be
    of advantage if you have read that chapter.  Some examples from
    this chapter can also be found in
    <filename>advanced.sql</filename> in the tutorial directory.  This
    file also contains some example data to load, which is not
    repeated here.  (Refer to <xref linkend="tutorial-sql-intro"> for
    how to use the file.)
   </para>
  </sect1>


  <sect1 id="tutorial-views">
   <title>Views</title>

   <indexterm zone="tutorial-views">
    <primary>view</primary>
   </indexterm>

   <para>
    Refer back to the queries in <xref linkend="tutorial-join">.
    Suppose the combined listing of weather records and city location
    is of particular interest to your application, but you do not want
    to type the query each time you need it.  You can create a
    <firstterm>view</firstterm> over the query, which gives a name to
    the query that you can refer to like an ordinary table.

<programlisting>
CREATE VIEW myview AS
    SELECT city, temp_lo, temp_hi, prcp, date, location
        FROM weather, cities
        WHERE city = name;

SELECT * FROM myview;
</programlisting>
   </para>

   <para>
    Making liberal use of views is a key aspect of good SQL database
    design.  Views allow you to encapsulate the details of the
    structure of your tables, which may change as your application
    evolves, behind consistent interfaces.
   </para>

   <para>
    Views can be used in almost any place a real table can be used.
    Building views upon other views is not uncommon.
   </para>
  </sect1>


  <sect1 id="tutorial-fk">
   <title>Foreign Keys</title>

   <indexterm zone="tutorial-fk">
    <primary>foreign key</primary>
   </indexterm>

   <indexterm zone="tutorial-fk">
    <primary>referential integrity</primary>
   </indexterm>

   <para>
    Recall the <classname>weather</classname> and
    <classname>cities</classname> tables from <xref
    linkend="tutorial-sql">.  Consider the following problem:  You
    want to make sure that no one can insert rows in the
    <classname>weather</classname> table that do not have a matching
    entry in the <classname>cities</classname> table.  This is called
    maintaining the <firstterm>referential integrity</firstterm> of
    your data.  In simplistic database systems this would be
    implemented (if at all) by first looking at the
    <classname>cities</classname> table to check if a matching record
    exists, and then inserting or rejecting the new
    <classname>weather</classname> records.  This approach has a
    number of problems and is very inconvenient, so
    <productname>PostgreSQL</productname> can do this for you.
   </para>

   <para>
    The new declaration of the tables would look like this:

<programlisting>
CREATE TABLE cities (
        city     varchar(80) primary key,
        location point
);

CREATE TABLE weather (
        city      varchar(80) references cities(city),
        temp_lo   int,
        temp_hi   int,
        prcp      real,
        date      date
);
</programlisting>

    Now try inserting an invalid record:

<programlisting>
INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');
</programlisting>

<screen>
ERROR:  insert or update on table "weather" violates foreign key constraint "weather_city_fkey"
DETAIL:  Key (city)=(Berkeley) is not present in table "cities".
</screen>
   </para>

   <para>
    The behavior of foreign keys can be finely tuned to your
    application.  We will not go beyond this simple example in this
    tutorial, but just refer you to <xref linkend="ddl">
    for more information.  Making correct use of
    foreign keys will definitely improve the quality of your database
    applications, so you are strongly encouraged to learn about them.
   </para>
  </sect1>


  <sect1 id="tutorial-transactions">
   <title>Transactions</title>

   <indexterm zone="tutorial-transactions">
    <primary>transaction</primary>
   </indexterm>

   <para>
    <firstterm>Transactions</> are a fundamental concept of all database
    systems.  The essential point of a transaction is that it bundles
    multiple steps into a single, all-or-nothing operation.  The intermediate
    states between the steps are not visible to other concurrent transactions,
    and if some failure occurs that prevents the transaction from completing,
    then none of the steps affect the database at all.
   </para>

   <para>
    For example, consider a bank database that contains balances for various
    customer accounts, as well as total deposit balances for branches.
    Suppose that we want to record a payment of $100.00 from Alice's account
    to Bob's account.  Simplifying outrageously, the SQL commands for this
    might look like

<programlisting>
UPDATE accounts SET balance = balance - 100.00
    WHERE name = 'Alice';
UPDATE branches SET balance = balance - 100.00
    WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Alice');
UPDATE accounts SET balance = balance + 100.00
    WHERE name = 'Bob';
UPDATE branches SET balance = balance + 100.00
    WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Bob');
</programlisting>
   </para>

   <para>
    The details of these commands are not important here; the important
    point is that there are several separate updates involved to accomplish
    this rather simple operation.  Our bank's officers will want to be
    assured that either all these updates happen, or none of them happen.
    It would certainly not do for a system failure to result in Bob
    receiving $100.00 that was not debited from Alice.  Nor would Alice long
    remain a happy customer if she was debited without Bob being credited.
    We need a guarantee that if something goes wrong partway through the
    operation, none of the steps executed so far will take effect.  Grouping
    the updates into a <firstterm>transaction</> gives us this guarantee.
    A transaction is said to be <firstterm>atomic</>: from the point of
    view of other transactions, it either happens completely or not at all.
   </para>

   <para>
    We also want a
    guarantee that once a transaction is completed and acknowledged by
    the database system, it has indeed been permanently recorded
    and won't be lost even if a crash ensues shortly thereafter.
    For example, if we are recording a cash withdrawal by Bob,
    we do not want any chance that the debit to his account will
    disappear in a crash just after he walks out the bank door.
    A transactional database guarantees that all the updates made by
    a transaction are logged in permanent storage (i.e., on disk) before
    the transaction is reported complete.
   </para>

   <para>
    Another important property of transactional databases is closely
    related to the notion of atomic updates: when multiple transactions
    are running concurrently, each one should not be able to see the
    incomplete changes made by others.  For example, if one transaction
    is busy totalling all the branch balances, it would not do for it
    to include the debit from Alice's branch but not the credit to
    Bob's branch, nor vice versa.  So transactions must be all-or-nothing
    not only in terms of their permanent effect on the database, but
    also in terms of their visibility as they happen.  The updates made
    so far by an open transaction are invisible to other transactions
    until the transaction completes, whereupon all the updates become
    visible simultaneously.
   </para>

   <para>
    In <productname>PostgreSQL</>, a transaction is set up by surrounding
    the SQL commands of the transaction with
    <command>BEGIN</> and <command>COMMIT</> commands.  So our banking
    transaction would actually look like

<programlisting>
BEGIN;
UPDATE accounts SET balance = balance - 100.00
    WHERE name = 'Alice';
-- etc etc
COMMIT;
</programlisting>
   </para>

   <para>
    If, partway through the transaction, we decide we do not want to
    commit (perhaps we just noticed that Alice's balance went negative),
    we can issue the command <command>ROLLBACK</> instead of
    <command>COMMIT</>, and all our updates so far will be canceled.
   </para>

   <para>
    <productname>PostgreSQL</> actually treats every SQL statement as being
    executed within a transaction.  If you do not issue a <command>BEGIN</>
    command, 
    then each individual statement has an implicit <command>BEGIN</> and
    (if successful) <command>COMMIT</> wrapped around it.  A group of