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Markup examples as examples. Fix formatting of examples.

This commit is contained in:
Peter Eisentraut 2001-09-15 00:48:59 +00:00
parent 184c4afcd6
commit 4284002d35
3 changed files with 170 additions and 125 deletions
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3
doc/src/sgml/stylesheet.css
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@ -1,4 +1,4 @@
/* $Header: /cvsroot/pgsql/doc/src/sgml/stylesheet.css,v 1.1 2001/09/14 20:37:55 petere Exp $ */
/* $Header: /cvsroot/pgsql/doc/src/sgml/stylesheet.css,v 1.2 2001/09/15 00:48:59 petere Exp $ */
/* color scheme similar to www.postgresql.org */
@ -38,6 +38,7 @@ DIV.example {
border-width: 0px;
border-left-width: 2px;
border-color: black;
margin: 0.5ex;
}
/* less dense spacing of TOC */

38
doc/src/sgml/stylesheet.dsl
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@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/stylesheet.dsl,v 1.9 2001/09/14 20:37:55 petere Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/stylesheet.dsl,v 1.10 2001/09/15 00:48:59 petere Exp $ -->
<!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN" [
<!-- must turn on one of these with -i on the jade command line -->
@ -68,6 +68,42 @@
(define html-index #t)
;; Block elements are allowed in PARA in DocBook, but not in P in
;; HTML. With %fix-para-wrappers% turned on, the stylesheets attempt
;; to avoid putting block elements in HTML P tags by outputting
;; additional end/begin P pairs around them.
(define %fix-para-wrappers% #t)
;; ...but we need to do some extra work to make the above apply to PRE
;; as well. (mostly pasted from dbverb.dsl)
(define ($verbatim-display$ indent line-numbers?)
(let ((content (make element gi: "PRE"
attributes: (list
(list "CLASS" (gi)))
(if (or indent line-numbers?)
($verbatim-line-by-line$ indent line-numbers?)
(process-children)))))
(if %shade-verbatim%
(make element gi: "TABLE"
attributes: ($shade-verbatim-attr$)
(make element gi: "TR"
(make element gi: "TD"
content)))
(make sequence
(para-check)
content
(para-check 'restart)))))
;; ...and for notes.
(element note
(make sequence
(para-check)
($admonition$)
(para-check 'restart)))
;;; XXX The above is very ugly. It might be better to run 'tidy' on
;;; the resulting *.html files.
]]> <!-- %output-html -->
<![ %output-print; [

254
doc/src/sgml/typeconv.sgml
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@ -1,34 +1,38 @@
<chapter Id="typeconv">
<title>Type Conversion</title>
<sect1 id="typeconv-intro">
<title>Introduction</title>
<para>
<acronym>SQL</acronym> queries can, intentionally or not, require
mixing of different data types in the same expression.
<productname>Postgres</productname> has extensive facilities for
<productname>PostgreSQL</productname> has extensive facilities for
evaluating mixed-type expressions.
</para>
<para>
In many cases a user will not need
to understand the details of the type conversion mechanism.
However, the implicit conversions done by <productname>Postgres</productname>
However, the implicit conversions done by <productname>PostgreSQL</productname>
can affect the results of a query. When necessary, these results
can be tailored by a user or programmer
using <emphasis>explicit</emphasis> type coercion.
</para>
<para>
This chapter introduces the <productname>Postgres</productname>
This chapter introduces the <productname>PostgreSQL</productname>
type conversion mechanisms and conventions.
Refer to the relevant sections in the User's Guide and Programmer's Guide
Refer to the relevant sections in the <xref linkend="datatype"> and <xref linkend="functions">
for more information on specific data types and allowed functions and
operators.
</para>
<para>
The Programmer's Guide has more details on the exact algorithms used for
The <citetitle>Programmer's Guide</citetitle> has more details on the exact algorithms used for
implicit type conversion and coercion.
</para>
</sect1>
<sect1 id="typeconv-overview">
<title>Overview</title>
@ -36,29 +40,29 @@ implicit type conversion and coercion.
<para>
<acronym>SQL</acronym> is a strongly typed language. That is, every data item
has an associated data type which determines its behavior and allowed usage.
<productname>Postgres</productname> has an extensible type system that is
<productname>PostgreSQL</productname> has an extensible type system that is
much more general and flexible than other <acronym>RDBMS</acronym> implementations.
Hence, most type conversion behavior in <productname>Postgres</productname>
Hence, most type conversion behavior in <productname>PostgreSQL</productname>
should be governed by general rules rather than by ad-hoc heuristics to allow
mixed-type expressions to be meaningful, even with user-defined types.
</para>
<para>
The <productname>Postgres</productname> scanner/parser decodes lexical
The <productname>PostgreSQL</productname> scanner/parser decodes lexical
elements into only five fundamental categories: integers, floats, strings,
names, and keywords. Most extended types are first tokenized into
strings. The <acronym>SQL</acronym> language definition allows specifying type
names with strings, and this mechanism can be used in
<productname>Postgres</productname> to start the parser down the correct
<productname>PostgreSQL</productname> to start the parser down the correct
path. For example, the query
<programlisting>
<screen>
tgl=> SELECT text 'Origin' AS "Label", point '(0,0)' AS "Value";
Label | Value
--------+-------
Origin | (0,0)
(1 row)
</programlisting>
</screen>
has two strings, of type <type>text</type> and <type>point</type>.
If a type is not specified for a string, then the placeholder type
@ -68,7 +72,7 @@ stages as described below.
<para>
There are four fundamental <acronym>SQL</acronym> constructs requiring
distinct type conversion rules in the <productname>Postgres</productname>
distinct type conversion rules in the <productname>PostgreSQL</productname>
parser:
</para>
@ -79,8 +83,8 @@ Operators
</term>
<listitem>
<para>
<productname>Postgres</productname> allows expressions with
left- and right-unary (one argument) operators,
<productname>PostgreSQL</productname> allows expressions with
prefix and postfix unary (one argument) operators,
as well as binary (two argument) operators.
</para>
</listitem>
@ -91,12 +95,12 @@ Function calls
</term>
<listitem>
<para>
Much of the <productname>Postgres</productname> type system is built around a
Much of the <productname>PostgreSQL</productname> type system is built around a
rich set of functions. Function calls have one or more arguments which, for
any specific query, must be matched to the functions available in the system
catalog. Since <productname>Postgres</productname> permits function
catalog. Since <productname>PostgreSQL</productname> permits function
overloading, the function name alone does not uniquely identify the function
to be called --- the parser must select the right function based on the data
to be called; the parser must select the right function based on the data
types of the supplied arguments.
</para>
</listitem>
@ -107,7 +111,7 @@ Query targets
</term>
<listitem>
<para>
<acronym>SQL</acronym> INSERT and UPDATE statements place the results of
<acronym>SQL</acronym> <command>INSERT</command> and <command>UPDATE</command> statements place the results of
expressions into a table. The expressions in the query must be matched up
with, and perhaps converted to, the types of the target columns.
</para>
@ -115,15 +119,15 @@ with, and perhaps converted to, the types of the target columns.
</varlistentry>
<varlistentry>
<term>
UNION and CASE constructs
<literal>UNION</literal> and <literal>CASE</literal> constructs
</term>
<listitem>
<para>
Since all select results from a UNION SELECT statement must appear in a single
Since all select results from a unionized <literal>SELECT</literal> statement must appear in a single
set of columns, the types of the results
of each SELECT clause must be matched up and converted to a uniform set.
Similarly, the result expressions of a CASE construct must be coerced to
a common type so that the CASE expression as a whole has a known output type.
of each <literal>SELECT</> clause must be matched up and converted to a uniform set.
Similarly, the result expressions of a <literal>CASE</> construct must be coerced to
a common type so that the <literal>CASE</> expression as a whole has a known output type.
</para>
</listitem>
</varlistentry>
@ -131,14 +135,14 @@ a common type so that the CASE expression as a whole has a known output type.
<para>
Many of the general type conversion rules use simple conventions built on
the <productname>Postgres</productname> function and operator system tables.
the <productname>PostgreSQL</productname> function and operator system tables.
There are some heuristics included in the conversion rules to better support
conventions for the <acronym>SQL92</acronym> standard native types such as
<type>smallint</type>, <type>integer</type>, and <type>float</type>.
conventions for the <acronym>SQL</acronym> standard native types such as
<type>smallint</type>, <type>integer</type>, and <type>real</type>.
</para>
<para>
The <productname>Postgres</productname> parser uses the convention that all
The <productname>PostgreSQL</productname> parser uses the convention that all
type conversion functions take a single argument of the source type and are
named with the same name as the target type. Any function meeting these
criteria is considered to be a valid conversion function, and may be used
@ -162,9 +166,6 @@ they will raise an error when there are multiple choices for user-defined
types.
</para>
<sect2>
<title>Guidelines</title>
<para>
All type conversion rules are designed with several principles in mind:
@ -185,7 +186,7 @@ be converted to a user-defined type (of course, only if conversion is necessary)
<listitem>
<para>
User-defined types are not related. Currently, <productname>Postgres</productname>
User-defined types are not related. Currently, <productname>PostgreSQL</productname>
does not have information available to it on relationships between types, other than
hardcoded heuristics for built-in types and implicit relationships based on available functions
in the catalog.
@ -209,18 +210,25 @@ should use this new function and will no longer do the implicit conversion using
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="typeconv-oper">
<title>Operators</title>
<para>
The operand types of an operator invocation are resolved following
to the procedure below. Note that this procedure is indirectly affected
by the precedence of the involved operators. See <xref
linkend="sql-precedence"> for more information.
</para>
<procedure>
<title>Operator Type Resolution</title>
<title>Operand Type Resolution</title>
<step performance="required">
<para>
Check for an exact match in the pg_operator system catalog.
Check for an exact match in the <classname>pg_operator</classname> system catalog.
</para>
<substeps>
@ -299,46 +307,45 @@ then fail.
</step>
</procedure>
<sect2>
<title>Examples</title>
<bridgehead renderas="sect2">Examples</bridgehead>
<sect3>
<title>Exponentiation Operator</title>
<example>
<title>Exponentiation Operator Type Resolution</title>
<para>
There is only one exponentiation
operator defined in the catalog, and it takes arguments of type
<type>double precision</type>.
The scanner assigns an initial type of <type>int4</type> to both arguments
The scanner assigns an initial type of <type>integer</type> to both arguments
of this query expression:
<programlisting>
<screen>
tgl=> select 2 ^ 3 AS "Exp";
Exp
-----
8
(1 row)
</programlisting>
</screen>
So the parser does a type conversion on both operands and the query
is equivalent to
<programlisting>
<screen>
tgl=> select CAST(2 AS double precision) ^ CAST(3 AS double precision) AS "Exp";
Exp
-----
8
(1 row)
</programlisting>
</screen>
or
<programlisting>
<screen>
tgl=> select 2.0 ^ 3.0 AS "Exp";
Exp
-----
8
(1 row)
</programlisting>
</screen>
<note>
<para>
@ -348,10 +355,10 @@ have an impact on the performance of queries involving large tables.
</para>
</note>
</para>
</sect3>
</example>
<sect3>
<title>String Concatenation</title>
<example>
<title>String Concatenation Operator Type Resolution</title>
<para>
A string-like syntax is used for working with string types as well as for
@ -361,13 +368,13 @@ Strings with unspecified type are matched with likely operator candidates.
<para>
One unspecified argument:
<programlisting>
<screen>
tgl=> SELECT text 'abc' || 'def' AS "Text and Unknown";
Text and Unknown
------------------
abcdef
(1 row)
</programlisting>
</screen>
</para>
<para>
@ -378,13 +385,13 @@ be interpreted as of type <type>text</type>.
<para>
Concatenation on unspecified types:
<programlisting>
<screen>
tgl=> SELECT 'abc' || 'def' AS "Unspecified";
Unspecified
-------------
abcdef
(1 row)
</programlisting>
</screen>
</para>
<para>
@ -396,26 +403,26 @@ that category is selected, and then the
<quote>preferred type</quote> for strings, <type>text</type>, is used as the specific
type to resolve the unknown literals to.
</para>
</sect3>
</example>
<sect3>
<title>Factorial</title>
<example>
<title>Factorial Operator Type Resolution</title>
<para>
This example illustrates an interesting result. Traditionally, the
factorial operator is defined for integers only. The <productname>Postgres</productname>
factorial operator is defined for integers only. The <productname>PostgreSQL</productname>
operator catalog has only one entry for factorial, taking an integer operand.
If given a non-integer numeric argument, <productname>Postgres</productname>
If given a non-integer numeric argument, <productname>PostgreSQL</productname>
will try to convert that argument to an integer for evaluation of the
factorial.
<programlisting>
<screen>
tgl=> select (4.3 !);
?column?
----------
24
(1 row)
</programlisting>
</screen>
<note>
<para>
@ -423,20 +430,25 @@ Of course, this leads to a mathematically suspect result,
since in principle the factorial of a non-integer is not defined.
However, the role of a database is not to teach mathematics, but
to be a tool for data manipulation. If a user chooses to take the
factorial of a floating point number, <productname>Postgres</productname>
factorial of a floating point number, <productname>PostgreSQL</productname>
will try to oblige.
</para>
</note>
</para>
</sect3>
</sect2>
</example>
</sect1>
<sect1 id="typeconv-func">
<title>Functions</title>
<para>
The argument types of function calls are resolved according to the
following steps.
</para>
<procedure>
<title>Function Call Type Resolution</title>
<title>Function Argument Type Resolution</title>
<step performance="required">
<para>
@ -520,38 +532,37 @@ to the named data type.
</step>
</procedure>
<sect2>
<title>Examples</title>
<bridgehead renderas="sect2">Examples</bridgehead>
<sect3>
<title>Factorial Function</title>
<example>
<title>Factorial Function Argument Type Resolution</title>
<para>
There is only one factorial function defined in the <classname>pg_proc</classname> catalog.
So the following query automatically converts the <type>int2</type> argument
to <type>int4</type>:
<programlisting>
<screen>
tgl=> select int4fac(int2 '4');
int4fac
---------
24
(1 row)
</programlisting>
</screen>
and is actually transformed by the parser to
<programlisting>
<screen>
tgl=> select int4fac(int4(int2 '4'));
int4fac
---------
24
(1 row)
</programlisting>
</screen>
</para>
</sect3>
</example>
<sect3>
<title>Substring Function</title>
<example>
<title>Substring Function Type Resolution</title>
<para>
There are two <function>substr</function> functions declared in <classname>pg_proc</classname>. However,
@ -561,34 +572,35 @@ only one takes two arguments, of types <type>text</type> and <type>int4</type>.
<para>
If called with a string constant of unspecified type, the type is matched up
directly with the only candidate function type:
<programlisting>
<screen>
tgl=> select substr('1234', 3);
substr
--------
34
(1 row)
</programlisting>
</screen>
</para>
<para>
If the string is declared to be of type <type>varchar</type>, as might be the case
if it comes from a table, then the parser will try to coerce it to become <type>text</type>:
<programlisting>
<screen>
tgl=> select substr(varchar '1234', 3);
substr
--------
34
(1 row)
</programlisting>
</screen>
which is transformed by the parser to become
<programlisting>
<screen>
tgl=> select substr(text(varchar '1234'), 3);
substr
--------
34
(1 row)
</programlisting>
</screen>
</para>
<para>
<note>
<para>
Actually, the parser is aware that <type>text</type> and <type>varchar</type>
@ -597,30 +609,31 @@ accepts the other without doing any physical conversion. Therefore, no
explicit type conversion call is really inserted in this case.
</para>
</note>
</para>
<para>
And, if the function is called with an <type>int4</type>, the parser will
try to convert that to <type>text</type>:
<programlisting>
<screen>
tgl=> select substr(1234, 3);
substr
--------
34
(1 row)
</programlisting>
</screen>
actually executes as
<programlisting>
<screen>
tgl=> select substr(text(1234), 3);
substr
--------
34
(1 row)
</programlisting>
</screen>
This succeeds because there is a conversion function text(int4) in the
system catalog.
</para>
</sect3>
</sect2>
</example>
</sect1>
<sect1 id="typeconv-query">
@ -654,17 +667,14 @@ passing the column's declared length as the second parameter.
</procedure>
<sect2>
<title>Examples</title>
<sect3>
<title><type>varchar</type> Storage</title>
<example>
<title><type>varchar</type> Storage Type Conversion</title>
<para>
For a target column declared as <type>varchar(4)</type> the following query
ensures that the target is sized correctly:
<programlisting>
<screen>
tgl=> CREATE TABLE vv (v varchar(4));
CREATE
tgl=> INSERT INTO vv SELECT 'abc' || 'def';
@ -674,33 +684,32 @@ tgl=> SELECT * FROM vv;
------
abcd
(1 row)
</programlisting>
</screen>
What's really happened here is that the two unknown literals are resolved
to text by default, allowing the <literal>||</literal> operator to be
resolved as text concatenation. Then the text result of the operator
What has really happened here is that the two unknown literals are resolved
to <type>text</type> by default, allowing the <literal>||</literal> operator to be
resolved as <type>text</type> concatenation. Then the <type>text</type> result of the operator
is coerced to <type>varchar</type> to match the target column type. (But, since the
parser knows that text and <type>varchar</type> are binary-compatible, this coercion
parser knows that <type>text</type> and <type>varchar</type> are binary-compatible, this coercion
is implicit and does not insert any real function call.) Finally, the
sizing function <literal>varchar(varchar,int4)</literal> is found in the system
sizing function <literal>varchar(varchar, integer)</literal> is found in the system
catalogs and applied to the operator's result and the stored column length.
This type-specific function performs the desired truncation.
</para>
</sect3>
</sect2>
</example>
</sect1>
<sect1 id="typeconv-union-case">
<title>UNION and CASE Constructs</title>
<title><literal>UNION</> and <literal>CASE</> Constructs</title>
<para>
The UNION and CASE constructs must match up possibly dissimilar types to
The <literal>UNION</> and <literal>CASE</> constructs must match up possibly dissimilar types to
become a single result set. The resolution algorithm is applied separately to
each output column of a UNION. CASE uses the identical algorithm to match
each output column of a union. <literal>CASE</> uses the identical algorithm to match
up its result expressions.
</para>
<procedure>
<title>UNION and CASE Type Resolution</title>
<title><literal>UNION</> and <literal>CASE</> Type Resolution</title>
<step performance="required">
<para>
@ -731,48 +740,47 @@ Coerce all inputs to the selected type.
</para></step>
</procedure>
<sect2>
<title>Examples</title>
<bridgehead renderas="sect2">Examples</bridgehead>
<sect3>
<title>Underspecified Types</title>
<example>
<title>Underspecified Types in a Union</title>
<para>
<programlisting>
<screen>
tgl=> SELECT text 'a' AS "Text" UNION SELECT 'b';
Text
------
a
b
(2 rows)
</programlisting>
</screen>
Here, the unknown-type literal <literal>'b'</literal> will be resolved as type text.
</para>
</sect3>
</example>
<sect3>
<title>Simple UNION</title>
<example>
<title>Type Conversion in a Simple Union</title>
<para>
<programlisting>
<screen>
tgl=> SELECT 1.2 AS "Double" UNION SELECT 1;
Double
--------
1
1.2
(2 rows)
</programlisting>
</screen>
</para>
</sect3>
</example>
<sect3>
<title>Transposed UNION</title>
<example>
<title>Type Conversion in a Transposed Union</title>
<para>
Here the output type of the union is forced to match the type of
the first/top clause in the union:
<programlisting>
<screen>
tgl=> SELECT 1 AS "All integers"
tgl-> UNION SELECT CAST('2.2' AS REAL);
All integers
@ -780,7 +788,7 @@ tgl-> UNION SELECT CAST('2.2' AS REAL);
1
2
(2 rows)
</programlisting>
</screen>
</para>
<para>
Since <type>REAL</type> is not a preferred type, the parser sees no reason
@ -788,11 +796,11 @@ to select it over <type>INTEGER</type> (which is what the 1 is), and instead
falls back on the use-the-first-alternative rule.
This example demonstrates that the preferred-type mechanism doesn't encode
as much information as we'd like. Future versions of
<productname>Postgres</productname> may support a more general notion of
<productname>PostgreSQL</productname> may support a more general notion of
type preferences.
</para>
</sect3>
</sect2>
</example>
</sect1>
</chapter>