<!--
Documentation of the system catalogs, directed toward PostgreSQL developers
- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.85 2004/04/01 21:28:43 tgl Exp $
+ $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.86 2004/06/07 04:04:47 tgl Exp $
-->
<chapter id="catalogs">
The catalog <structname>pg_class</structname> catalogs tables and most
everything else that has columns or is otherwise similar to a
table. This includes indexes (but see also
- <structname>pg_index</structname>), sequences, views, and some
- kinds of special relation; see <structfield>relkind</>.
+ <structname>pg_index</structname>), sequences, views, composite types,
+ and some kinds of special relation; see <structfield>relkind</>.
Below, when we mean all of these
kinds of objects we speak of <quote>relations</quote>. Not all
columns are meaningful for all relation types.
<entry><type>oid</type></entry>
<entry><literal><link linkend="catalog-pg-type"><structname>pg_type</structname></link>.oid</literal></entry>
<entry>
- The OID of the data type that corresponds to this table, if any
- (zero for indexes, which have no <structname>pg_type</> entry)
+ The OID of the data type that corresponds to this table's rowtype,
+ if any (zero for indexes, which have no <structname>pg_type</> entry)
</entry>
</row>
<entry></entry>
<entry>
<structfield>typtype</structfield> is <literal>b</literal> for
- a base type, <literal>c</literal> for a composite type (i.e., a
+ a base type, <literal>c</literal> for a composite type (e.g., a
table's row type), <literal>d</literal> for a domain, or
<literal>p</literal> for a pseudo-type. See also
<structfield>typrelid</structfield> and
<structname>pg_class</structname> entry doesn't really represent
a table, but it is needed anyway for the type's
<structname>pg_attribute</structname> entries to link to.)
- Zero for base types.
+ Zero for non-composite types.
</entry>
</row>
--- /dev/null
+<!-- $PostgreSQL: pgsql/doc/src/sgml/rowtypes.sgml,v 2.1 2004/06/07 04:04:47 tgl Exp $ -->
+
+<sect1 id="rowtypes">
+ <title>Composite Types</title>
+
+ <indexterm>
+ <primary>composite type</primary>
+ </indexterm>
+
+ <indexterm>
+ <primary>row type</primary>
+ </indexterm>
+
+ <para>
+ A <firstterm>composite type</> describes the structure of a row or record;
+ it is in essence just a list of field names and their datatypes.
+ <productname>PostgreSQL</productname> allows values of composite types to be
+ used in many of the same ways that simple types can be used. For example, a
+ column of a table can be declared to be of a composite type.
+ </para>
+
+ <sect2>
+ <title>Declaration of Composite Types</title>
+
+ <para>
+ Here are two simple examples of defining composite types:
+<programlisting>
+CREATE TYPE complex AS (
+ r double precision,
+ i double precision
+);
+
+CREATE TYPE inventory_item AS (
+ name text,
+ supplier_id integer,
+ price numeric
+);
+</programlisting>
+ The syntax is comparable to <command>CREATE TABLE</>, except that only
+ field names and types can be specified; no constraints (such as <literal>NOT
+ NULL</>) can presently be included. Note that the <literal>AS</> keyword
+ is essential; without it, the system will think a quite different kind
+ of <command>CREATE TYPE</> command is meant, and you'll get odd syntax
+ errors.
+ </para>
+
+ <para>
+ Having defined the types, we can use them to create tables:
+
+<programlisting>
+CREATE TABLE on_hand (
+ item inventory_item,
+ count integer
+);
+
+INSERT INTO on_hand VALUES (ROW('fuzzy dice', 42, 1.99), 1000);
+</programlisting>
+
+ or functions:
+
+<programlisting>
+CREATE FUNCTION price_extension(inventory_item, integer) RETURNS numeric
+AS 'SELECT $1.price * $2' LANGUAGE SQL;
+
+SELECT price_extension(item, 10) FROM on_hand;
+</programlisting>
+
+ </para>
+ </sect2>
+
+ <sect2>
+ <title>Composite Value Input</title>
+
+ <indexterm>
+ <primary>composite type</primary>
+ <secondary>constant</secondary>
+ </indexterm>
+
+ <para>
+ To write a composite value as a literal constant, enclose the field
+ values within parentheses and separate them by commas. You may put double
+ quotes around any field value, and must do so if it contains commas or
+ parentheses. (More details appear below.) Thus, the general format of a
+ composite constant is the following:
+<synopsis>
+'( <replaceable>val1</replaceable> , <replaceable>val2</replaceable> , ... )'
+</synopsis>
+ An example is
+<programlisting>
+'("fuzzy dice",42,1.99)'
+</programlisting>
+ which would be a valid value of the <literal>inventory_item</> type
+ defined above. To make a field be NULL, write no characters at all
+ in its position in the list. For example, this constant specifies
+ a NULL third field:
+<programlisting>
+'("fuzzy dice",42,)'
+</programlisting>
+ If you want an empty string rather than NULL, write double quotes:
+<programlisting>
+'("",42,)'
+</programlisting>
+ Here the first field is a non-NULL empty string, the third is NULL.
+ </para>
+
+ <para>
+ (These constants are actually only a special case of
+ the generic type constants discussed in <xref
+ linkend="sql-syntax-constants-generic">. The constant is initially
+ treated as a string and passed to the composite-type input conversion
+ routine. An explicit type specification might be necessary.)
+ </para>
+
+ <para>
+ The <literal>ROW</literal> expression syntax may also be used to
+ construct composite values. In most cases this is considerably
+ simpler to use than the string-literal syntax, since you don't have
+ to worry about multiple layers of quoting. We already used this
+ method above:
+<programlisting>
+ROW('fuzzy dice', 42, 1.99)
+ROW('', 42, NULL)
+</programlisting>
+ The ROW keyword is actually optional as long as you have more than one
+ field in the expression, so these can simplify to
+<programlisting>
+('fuzzy dice', 42, 1.99)
+('', 42, NULL)
+</programlisting>
+ The <literal>ROW</> expression syntax is discussed in more detail in <xref
+ linkend="sql-syntax-row-constructors">.
+ </para>
+ </sect2>
+
+ <sect2>
+ <title>Accessing Composite Types</title>
+
+ <para>
+ To access a field of a composite column, one writes a dot and the field
+ name, much like selecting a field from a table name. In fact, it's so
+ much like selecting from a table name that you often have to use parentheses
+ to keep from confusing the parser. For example, you might try to select
+ some subfields from our <literal>on_hand</> example table with something
+ like:
+
+<programlisting>
+SELECT item.name FROM on_hand WHERE item.price > 9.99;
+</programlisting>
+
+ This will not work since the name <literal>item</> is taken to be a table
+ name, not a field name, per SQL syntax rules. You must write it like this:
+
+<programlisting>
+SELECT (item).name FROM on_hand WHERE (item).price > 9.99;
+</programlisting>
+
+ or if you need to use the table name as well (for instance in a multi-table
+ query), like this:
+
+<programlisting>
+SELECT (on_hand.item).name FROM on_hand WHERE (on_hand.item).price > 9.99;
+</programlisting>
+
+ Now the parenthesized object is correctly interpreted as a reference to
+ the <literal>item</> column, and then the subfield can be selected from it.
+ </para>
+
+ <para>
+ Similar syntactic issues apply whenever you select a field from a composite
+ value. For instance, to select just one field from the result of a function
+ that returns a composite value, you'd need to write something like
+
+<programlisting>
+SELECT (my_func(...)).field FROM ...
+</programlisting>
+
+ Without the extra parentheses, this will provoke a syntax error.
+ </para>
+ </sect2>
+
+ <sect2>
+ <title>Composite Type Input and Output Syntax</title>
+
+ <para>
+ The external text representation of a composite value consists of items that
+ are interpreted according to the I/O conversion rules for the individual
+ field types, plus decoration that indicates the composite structure.
+ The decoration consists of parentheses (<literal>(</> and <literal>)</>)
+ around the whole value, plus commas (<literal>,</>) between adjacent
+ items. Whitespace outside the parentheses is ignored, but within the
+ parentheses it is considered part of the field value, and may or may not be
+ significant depending on the input conversion rules for the field datatype.
+ For example, in
+<programlisting>
+'( 42)'
+</programlisting>
+ the whitespace will be ignored if the field type is integer, but not if
+ it is text.
+ </para>
+
+ <para>
+ As shown previously, when writing a composite value you may write double
+ quotes around any individual field value.
+ You <emphasis>must</> do so if the field value would otherwise
+ confuse the composite-value parser. In particular, fields containing
+ parentheses, commas, double quotes, or backslashes must be double-quoted.
+ To put a double quote or backslash in a quoted composite field value,
+ precede it with a backslash. (Also, a pair of double quotes within a
+ double-quoted field value is taken to represent a double quote character,
+ analogously to the rules for single quotes in SQL literal strings.)
+ Alternatively, you can use backslash-escaping to protect all data characters
+ that would otherwise be taken as composite syntax.
+ </para>
+
+ <para>
+ A completely empty field value (no characters at all between the commas
+ or parentheses) represents a NULL. To write a value that is an empty
+ string rather than NULL, write <literal>""</>.
+ </para>
+
+ <para>
+ The composite output routine will put double quotes around field values
+ if they are empty strings or contain parentheses, commas,
+ double quotes, backslashes, or white space. (Doing so for white space
+ is not essential, but aids legibility.) Double quotes and backslashes
+ embedded in field values will be doubled.
+ </para>
+
+ <note>
+ <para>
+ Remember that what you write in an SQL command will first be interpreted
+ as a string literal, and then as a composite. This doubles the number of
+ backslashes you need. For example, to insert a <type>text</> field
+ containing a double quote and a backslash in a composite
+ value, you'd need to write
+<programlisting>
+INSERT ... VALUES ('("\\"\\\\")');
+</programlisting>
+ The string-literal processor removes one level of backslashes, so that
+ what arrives at the composite-value parser looks like
+ <literal>("\"\\")</>. In turn, the string
+ fed to the <type>text</> data type's input routine
+ becomes <literal>"\</>. (If we were working
+ with a data type whose input routine also treated backslashes specially,
+ <type>bytea</> for example, we might need as many as eight backslashes
+ in the command to get one backslash into the stored composite field.)
+ </para>
+ </note>
+
+ <tip>
+ <para>
+ The <literal>ROW</> constructor syntax is usually easier to work with
+ than the composite-literal syntax when writing composite values in SQL
+ commands.
+ In <literal>ROW</>, individual field values are written the same way
+ they would be written when not members of a composite.
+ </para>
+ </tip>
+ </sect2>
+
+</sect1>
<!--
-$PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.92 2004/05/16 23:22:07 neilc Exp $
+$PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.93 2004/06/07 04:04:47 tgl Exp $
-->
<chapter id="sql-syntax">
<title>Row Constructors</title>
<indexterm>
- <primary>row</primary>
+ <primary>composite type</primary>
+ <secondary>constructor</secondary>
+ </indexterm>
+
+ <indexterm>
+ <primary>row type</primary>
<secondary>constructor</secondary>
</indexterm>
<para>
- A row constructor is an expression that builds a row value from values
+ A row constructor is an expression that builds a row value (also
+ called a composite value) from values
for its member fields. A row constructor consists of the key word
<literal>ROW</literal>, a left parenthesis <literal>(</>, zero or more
expressions (separated by commas) for the row field values, and finally
a right parenthesis <literal>)</>. For example,
<programlisting>
-SELECT myfunc(ROW(1,2.5,'this is a test'));
+SELECT ROW(1,2.5,'this is a test');
</programlisting>
The key word <literal>ROW</> is optional when there is more than one
expression in the list.
</para>
<para>
- Row constructors have only limited uses, other than creating an argument
- value for a user-defined function that accepts a rowtype parameter, as
- illustrated above.
- It is possible to compare two row values or test a row with
+ Row constructors can be used to build composite values to be stored
+ in a composite-type table column, or to be passed to a function that
+ accepts a composite parameter. Also,
+ it is possible to compare two row values or test a row with
<literal>IS NULL</> or <literal>IS NOT NULL</>, for example
<programlisting>
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');
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