Allow the planner's estimate of the fraction of a cursor's rows that will be

[postgresql] / doc / src / sgml / advanced.sgml

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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 might 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
    statements surrounded by <command>BEGIN</> and <command>COMMIT</>
    is sometimes called a <firstterm>transaction block</>.
   </para>

   <note>
    <para>
     Some client libraries issue <command>BEGIN</> and <command>COMMIT</>
     commands automatically, so that you might get the effect of transaction
     blocks without asking.  Check the documentation for the interface
     you are using.
    </para>
   </note>

   <para>
    It's possible to control the statements in a transaction in a more
    granular fashion through the use of <firstterm>savepoints</>.  Savepoints
    allow you to selectively discard parts of the transaction, while
    committing the rest.  After defining a savepoint with
    <command>SAVEPOINT</>, you can if needed roll back to the savepoint
    with <command>ROLLBACK TO</>.  All the transaction's database changes
    between defining the savepoint and rolling back to it are discarded, but
    changes earlier than the savepoint are kept.
   </para> 

   <para>
    After rolling back to a savepoint, it continues to be defined, so you can
    roll back to it several times.  Conversely, if you are sure you won't need
    to roll back to a particular savepoint again, it can be released, so the
    system can free some resources.  Keep in mind that either releasing or
    rolling back to a savepoint
    will automatically release all savepoints that were defined after it.
   </para> 

   <para>
    All this is happening within the transaction block, so none of it
    is visible to other database sessions.  When and if you commit the
    transaction block, the committed actions become visible as a unit
    to other sessions, while the rolled-back actions never become visible
    at all.
   </para> 

   <para>
    Remembering the bank database, suppose we debit $100.00 from Alice's
    account, and credit Bob's account, only to find later that we should
    have credited Wally's account.  We could do it using savepoints like
    this:

<programlisting>
BEGIN;
UPDATE accounts SET balance = balance - 100.00
    WHERE name = 'Alice';
SAVEPOINT my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = 'Bob';
-- oops ... forget that and use Wally's account
ROLLBACK TO my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = 'Wally';
COMMIT;
</programlisting>
   </para>

   <para>
    This example is, of course, oversimplified, but there's a lot of control
    to be had over a transaction block through the use of savepoints.
    Moreover, <command>ROLLBACK TO</> is the only way to regain control of a
    transaction block that was put in aborted state by the
    system due to an error, short of rolling it back completely and starting
    again.
   </para>

  </sect1>


  <sect1 id="tutorial-inheritance">
   <title>Inheritance</title>

   <indexterm zone="tutorial-inheritance">
    <primary>inheritance</primary>
   </indexterm>

   <para>
    Inheritance is a concept from object-oriented databases.  It opens
    up interesting new possibilities of database design.
   </para>

   <para>
    Let's create two tables:  A table <classname>cities</classname>
    and a table <classname>capitals</classname>.  Naturally, capitals
    are also cities, so you want some way to show the capitals
    implicitly when you list all cities.  If you're really clever you
    might invent some scheme like this:

<programlisting>
CREATE TABLE capitals (
  name       text,
  population real,
  altitude   int,    -- (in ft)
  state      char(2)
);

CREATE TABLE non_capitals (
  name       text,
  population real,
  altitude   int     -- (in ft)
);

CREATE VIEW cities AS
  SELECT name, population, altitude FROM capitals
    UNION
  SELECT name, population, altitude FROM non_capitals;
</programlisting>

    This works OK as far as querying goes, but it gets ugly when you
    need to update several rows, for one thing.
   </para>

   <para>
    A better solution is this:

<programlisting>
CREATE TABLE cities (
  name       text,
  population real,
  altitude   int     -- (in ft)
);

CREATE TABLE capitals (
  state      char(2)
) INHERITS (cities);
</programlisting>
   </para>

   <para>
    In this case, a row of <classname>capitals</classname>
    <firstterm>inherits</firstterm> all columns (<structfield>name</>,
    <structfield>population</>, and <structfield>altitude</>) from its
    <firstterm>parent</firstterm>, <classname>cities</classname>.  The
    type of the column <structfield>name</structfield> is
    <type>text</type>, a native <productname>PostgreSQL</productname>
    type for variable length character strings.  State capitals have
    an extra column, <structfield>state</>, that shows their state.  In
    <productname>PostgreSQL</productname>, a table can inherit from
    zero or more other tables.
   </para>

   <para>
    For example, the  following  query finds the  names  of  all  cities,
    including  state capitals, that are located at an altitude 
    over 500 feet:

<programlisting>
SELECT name, altitude
  FROM cities
  WHERE altitude &gt; 500;
</programlisting>

    which returns:

<screen>
   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
 Madison   |      845
(3 rows)
</screen>
   </para>

   <para>
    On the other hand, the  following  query  finds
    all  the cities that are not state capitals and
    are situated at an altitude of 500 feet or higher:

<programlisting>
SELECT name, altitude
    FROM ONLY cities
    WHERE altitude &gt; 500;
</programlisting>

<screen>
   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
(2 rows)
</screen>
   </para>

   <para>
    Here the <literal>ONLY</literal> before <literal>cities</literal>
    indicates that the query should be run over only the
    <classname>cities</classname> table, and not tables below
    <classname>cities</classname> in the inheritance hierarchy.  Many
    of the commands that we have already discussed &mdash;
    <command>SELECT</command>, <command>UPDATE</command>, and
    <command>DELETE</command> &mdash; support this <literal>ONLY</literal>
    notation.
   </para>

   <note>
    <para>
     Although inheritance is frequently useful, it has not been integrated
     with unique constraints or foreign keys, which limits its usefulness.
     See <xref linkend="ddl-inherit"> for more detail.
    </para>
   </note>
  </sect1>


  <sect1 id="tutorial-conclusion">
   <title>Conclusion</title>
 
   <para>
    <productname>PostgreSQL</productname> has many features not
    touched upon in this tutorial introduction, which has been
    oriented toward newer users of <acronym>SQL</acronym>.  These
    features are discussed in more detail in the remainder of this
    book.
   </para>

   <para>
    If you feel you need more introductory material, please visit the PostgreSQL
    <ulink url="http://www.postgresql.org">web site</ulink>
    for links to more resources.
   </para>
  </sect1>
 </chapter>