-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.65 2006/10/16 17:28:03 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.66 2006/10/22 03:03:40 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
</programlisting>
Attempting to drop a table that does not exist is an error.
Nevertheless, it is common in SQL script files to unconditionally
- try to drop each table before creating it, ignoring the error
- messages.
+ try to drop each table before creating it, ignoring any error
+ messages, so that the script works whether or not the table exists.
+ (If you like, you can use the <literal>DROP TABLE IF EXISTS</> variant
+ to avoid the error messages, but this is not standard SQL.)
</para>
<para>
<para>
A column can be assigned a default value. When a new row is
- created and no values are specified for some of the columns, the
+ created and no values are specified for some of the columns, those
columns will be filled with their respective default values. A
data manipulation command can also request explicitly that a column
be set to its default value, without having to know what that value is.
standard data type that accepts only positive numbers. Another issue is
that you might want to constrain column data with respect to other
columns or rows. For example, in a table containing product
- information, there should only be one row for each product number.
+ information, there should be only one row for each product number.
</para>
<para>
ensure that a column does not contain null values, the not-null
constraint described in the next section can be used.
</para>
-
- <para>
- Check constraints can be useful for enhancing the performance of
- partitioned tables. For details see <xref linkend="ddl-partitioning">.
- </para>
</sect2>
<sect2>
<literal>NULL</literal> constraint. This does not mean that the
column must be null, which would surely be useless. Instead, this
simply selects the default behavior that the column may be null.
- The <literal>NULL</literal> constraint is not defined in the SQL
+ The <literal>NULL</literal> constraint is not present in the SQL
standard and should not be used in portable applications. (It was
only added to <productname>PostgreSQL</productname> to be
compatible with some other database systems.) Some users, however,
In general, a unique constraint is violated when there are two or
more rows in the table where the values of all of the
columns included in the constraint are equal.
- However, null values are not considered equal in this
+ However, two null values are not considered equal in this
comparison. That means even in the presence of a
unique constraint it is possible to store duplicate
rows that contain a null value in at least one of the constrained
</para>
<para>
- A table can have at most one primary key (while it can have many
- unique and not-null constraints). Relational database theory
+ A table can have at most one primary key. (There can be any number
+ of unique and not-null constraints, which are functionally the same
+ thing, but only one can be identified as the primary key.)
+ Relational database theory
dictates that every table must have a primary key. This rule is
not enforced by <productname>PostgreSQL</productname>, but it is
usually best to follow it.
The object identifier (object ID) of a row. This column is only
present if the table was created using <literal>WITH
OIDS</literal>, or if the <xref linkend="guc-default-with-oids">
- configuration variable was set. This column is of type
+ configuration variable was set at the time. This column is of type
<type>oid</type> (same name as the column); see <xref
linkend="datatype-oid"> for more information about the type.
</para>
</listitem>
<listitem>
<para>
- The tables in question should be created using <literal>WITH
+ Of course, the tables in question must be created <literal>WITH
OIDS</literal>. As of <productname>PostgreSQL</productname> 8.1,
<literal>WITHOUT OIDS</> is the default.
</para>
All these actions are performed using the
<xref linkend="sql-altertable" endterm="sql-altertable-title">
- command.
+ command, which see for details beyond those given here.
</para>
<sect2>
constraints later (see below) after you've filled in the new column
correctly.
</para>
+
+ <tip>
+ <para>
+ Adding a column with a default requires updating each row of the
+ table (to store the new column value). However, if no default is
+ specified, <productname>PostgreSQL</productname> is able to avoid
+ the physical update. So if you intend to fill the column with
+ mostly nondefault values, it's best to add the column with no default,
+ insert the correct values using <command>UPDATE</>, and then add any
+ desired default as described below.
+ </para>
+ </tip>
</sect2>
<sect2>
<programlisting>
GRANT UPDATE ON accounts TO joe;
</programlisting>
- To grant a privilege to a group, use this syntax:
-<programlisting>
-GRANT SELECT ON accounts TO GROUP staff;
-</programlisting>
- The special <quote>user</quote> name <literal>PUBLIC</literal> can
- be used to grant a privilege to every user on the system. Writing
- <literal>ALL</literal> in place of a specific privilege grants all
+ Writing <literal>ALL</literal> in place of a specific privilege grants all
privileges that are relevant for the object type.
</para>
<para>
+ The special <quote>user</quote> name <literal>PUBLIC</literal> can
+ be used to grant a privilege to every user on the system. Also,
+ <quote>group</> roles can be set up to help manage privileges when
+ there are many users of a database — for details see
+ <xref linkend="user-manag">.
+ </para>
+
+ <para>
To revoke a privilege, use the fittingly named
<command>REVOKE</command> command:
<programlisting>
</indexterm>
<para>
- <productname>PostgreSQL</productname> implements table inheritance
+ <productname>PostgreSQL</productname> implements table inheritance,
which can be a useful tool for database designers. (SQL:1999 and
later define a type inheritance feature, which differs in many
respects from the features described here.)
Table inheritance is typically established when the child table is
created, using the <literal>INHERITS</> clause of the
<xref linkend="sql-createtable" endterm="sql-createtable-title">
- statement. However the related statement <command>CREATE TABLE AS</command>
- does not allow inheritance to be specified.
- </para>
-
- <para>
+ statement.
Alternatively, a table which is already defined in a compatible way can
have a new parent relationship added, using the <literal>INHERIT</literal>
variant of <xref linkend="sql-altertable" endterm="sql-altertable-title">.
</listitem>
</varlistentry>
</variablelist>
-
- Hash partitioning is not currently supported.
</para>
</sect2>