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+$Header: /cvsroot/pgsql/doc/src/sgml/datatype.sgml,v 1.59 2001/08/24 20:03:41 petere Exp $
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<chapter id="datatype">
</row>
<row>
+ <entry><type>bigserial</type></entry>
+ <entry><type>serial8</type></entry>
+ <entry>autoincrementing eight-byte integer</entry>
+ </row>
+
+ <row>
<entry><type>bit</type></entry>
<entry></entry>
<entry>fixed-length bit string</entry>
</row>
<row>
- <entry><type>serial8</type></entry>
- <entry></entry>
- <entry>autoincrementing eight-byte integer</entry>
- </row>
-
- <row>
<entry><type>text</type></entry>
<entry></entry>
<entry>variable-length character string</entry>
<tgroup cols="4">
<thead>
<row>
- <entry>Type Name</entry>
- <entry>Storage</entry>
+ <entry>Type name</entry>
+ <entry>Storage size</entry>
<entry>Description</entry>
<entry>Range</entry>
</row>
<entry>bigint</entry>
<entry>8 bytes</entry>
<entry>Very large range fixed-precision</entry>
- <entry>about 18 decimal places</entry>
+ <entry>about 18 decimal digits</entry>
</row>
<row>
<entry>decimal</entry>
<entry>variable</entry>
- <entry>User-specified precision</entry>
+ <entry>user-specified precision, exact</entry>
<entry>no limit</entry>
</row>
<row>
<entry>numeric</entry>
<entry>variable</entry>
- <entry>User-specified precision</entry>
+ <entry>user-specified precision, exact</entry>
<entry>no limit</entry>
</row>
<row>
<entry>real</entry>
<entry>4 bytes</entry>
- <entry>Variable-precision</entry>
- <entry>6 decimal places</entry>
+ <entry>variable-precision, inexact</entry>
+ <entry>6 decimal digits precision</entry>
</row>
<row>
<entry>double precision</entry>
<entry>8 bytes</entry>
- <entry>Variable-precision</entry>
- <entry>15 decimal places</entry>
+ <entry>variable-precision, inexact</entry>
+ <entry>15 decimal digits precision</entry>
</row>
<row>
- <entry>serial4</entry>
+ <entry>serial</entry>
<entry>4 bytes</entry>
- <entry>Identifier or cross-reference</entry>
+ <entry>autoincrementing integer</entry>
<entry>1 to 2147483647</entry>
</row>
<row>
- <entry>serial8</entry>
+ <entry>bigserial</entry>
<entry>8 bytes</entry>
- <entry>Identifier or cross-reference</entry>
+ <entry>autoincrementing integer</entry>
<entry>1 to 9223372036854775807</entry>
</row>
</tbody>
<xref linkend="sql-syntax-constants">. The numeric types have a
full set of corresponding arithmetic operators and
functions. Refer to <xref linkend="functions"> for more
- information.
+ information. The following sections describe the types in detail.
</para>
- <para>
- The <type>bigint</type> type may not function correctly on all platforms,
- since it relies on compiler support for eight-byte integers. On a machine
- without such support, <type>bigint</type> acts the same
- as <type>integer</type> (but still takes up eight bytes of storage).
- </para>
+ <sect2 id="datatype-int">
+ <title>The Integer Types</title>
+
+ <para>
+ The types <type>smallint</type>, <type>integer</type>,
+ <type>bigint</type> store whole numbers, that is, numbers without
+ fractional components, of various ranges. Attempts to store
+ values outside of the allowed range will result in an error.
+ </para>
+
+ <para>
+ The type <type>integer</type> is the usual choice, as it offers
+ the best balance between range, storage size, and performance.
+ The <type>smallint</type> type is generally only used if disk
+ space is at a premium. The <type>bigint</type> type should only
+ be used if the <type>integer</type> range is not sufficient,
+ because the latter is definitely faster.
+ </para>
+
+ <para>
+ The <type>bigint</type> type may not function correctly on all
+ platforms, since it relies on compiler support for eight-byte
+ integers. On a machine without such support, <type>bigint</type>
+ acts the same as <type>integer</type> (but still takes up eight
+ bytes of storage). However, we are not aware of any reasonable
+ platform where this is actually the case.
+ </para>
+
+ <note>
+ <para>
+ If you have a column of type <type>smallint</type> or
+ <type>bigint</type> with an index, you may encounter problems
+ getting the system to use that index. For instance, a clause of
+ the form
+<programlisting>
+... WHERE smallint_column = 42
+</programlisting>
+ will not use an index, because the system assigns type
+ <type>integer</type> to the 42, and PostgreSQL currently cannot
+ use an index when two different data types are involved. A
+ workaround is to single-quote the constant, thus:
+<programlisting>
+... WHERE smallint_column = '42'
+</programlisting>
+ This will cause the system to delay the type resolution and will
+ assign the right type to the constant.
+ </para>
+ </note>
+
+ <para>
+ SQL only specifies the integer types <type>integer</type> (or
+ <type>int</type>) and <type>smallint</type>. The type
+ <type>bigint</type>, and the type names <type>int2</type>,
+ <type>int4</type>, and <type>int8</type> are extensions, which
+ are shared with various other RDBMS products.
+ </para>
+
+ </sect2>
+
+ <sect2 id="datatype-numeric-decimal">
+ <title>Arbitrary Precision Numbers</title>
+
+ <para>
+ The type <type>numeric</type> can store numbers of practically
+ unlimited size and precision, while being able to store all
+ numbers and carry out all calculations exactly. It is especially
+ recommended for storing monetary amounts and other quantities
+ where exactness is required. However, the <type>numeric</type>
+ type is very slow compared to the floating point types described
+ in the next section.
+ </para>
+
+ <para>
+ In what follows we use these terms: The
+ <firstterm>scale</firstterm> of a <type>numeric</type> is the
+ count of decimal digits in the fractional part, to the right of
+ the decimal point. The <firstterm>precision</firstterm> of a
+ <type>numeric</type> is the total count of significant digits in
+ the whole number, that is, the number of digits to both sides of
+ the decimal point. So the number 23.5141 has a precision of 6
+ and a scale of 4. Integers can be considered to have a scale of
+ zero.
+ </para>
+
+ <para>
+ Both the precision and the scale of the numeric type can be
+ configured. To declare a column of type <type>numeric</type> use
+ the syntax
+<programlisting>
+NUMERIC(<replaceable>precision</replaceable>, <replaceable>scale</replaceable>)
+</programlisting>
+ The precision must be positive, the scale zero or positive.
+ Alternatively,
+<programlisting>
+NUMERIC(<replaceable>precision</replaceable>)
+</programlisting>
+ selects a scale of 0. Merely specifying
+<programlisting>
+NUMERIC
+</programlisting>
+ uses a default precision and scale, which is currently (30,6).
+ (The SQL standard requires a default scale of 0. We find this a
+ bit useless. If you're concerned about portability, always
+ specify the precision and scale explicitly.)
+ </para>
+
+ <para>
+ If the precision or scale of a value is greater than the declared
+ precision or scale of a column, the system will attempt to round
+ the value. If the value cannot be rounded so as to satisfy the
+ declared limits, an error is raised.
+ </para>
+
+ <para>
+ The types <type>decimal</type> and <type>numeric</type> are
+ equivalent. Both types are part of the SQL standard.
+ </para>
+ </sect2>
+
+
+ <sect2 id="datatype-float">
+ <title>Floating Point Types</title>
+
+ <para>
+ The data types <type>real</type> and <type>double
+ precision</type> are inexact, variable precision numeric types.
+ In practice, these types are usually implementations of IEEE 754
+ binary floating point (single and double precision,
+ respectively), to the extent that the underlying processor,
+ operating system, and compiler support it.
+ </para>
+
+ <para>
+ Inexact means that some values cannot be converted exactly to the
+ internal format and are stored as approximations, so that storing
+ and printing back out a value may show slight discrepancies.
+ Managing these errors and how they propagate through calculations
+ is the subject of an entire branch of mathematics and computer
+ science and will not be discussed further here, except for the
+ following points:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If you require exact storage and calculations (such as for
+ monetary amounts), use the <type>numeric</type> type instead.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ If you want to do complicated calculations with these types
+ for anything important, especially if you rely on certain
+ behavior in boundary cases (infinity, underflow), you should
+ evaluate the implementation carefully.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Comparing two floating point values for equality may or may
+ not work as expected.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ Normally, the <type>real</type> type has a range of at least
+ -1E+37 to +1E+37 with a precision of at least 6. The
+ <type>double precision</type> type normally has a range of around
+ -1E+308 to +1E+308 with a precision of at least 15. Values that
+ are too large or too small will cause an error. Rounding may
+ take place if the precision of an input number is too high.
+ Numbers too close to zero that are not representable as distinct
+ from zero will cause an underflow error.
+ </para>
+
+ </sect2>
<sect2 id="datatype-serial">
<title>The Serial Types</title>
in tables.
In the current implementation, specifying
+<programlisting>
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (<replaceable class="parameter">colname</replaceable> SERIAL);
- </programlisting>
+</programlisting>
is equivalent to specifying:
+<programlisting>
CREATE SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq;
CREATE TABLE <replaceable class="parameter">tablename</replaceable>
- (<replaceable class="parameter">colname</replaceable> integer DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq') UNIQUE NOT NULL;
- </programlisting>
+ (<replaceable class="parameter">colname</replaceable> integer DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq') UNIQUE NOT NULL);
+</programlisting>
Thus, we have created an integer column and arranged for its default
values to be assigned from a sequence generator. UNIQUE and NOT NULL
<para>
The type names <type>serial</type> and <type>serial4</type> are
equivalent: both create <type>integer</type> columns. The type
- name <type>serial8</type> works just the same way, except that it
- creates a <type>bigint</type> column. <type>serial8</type> should
- be used if you anticipate use of more than 2^31 identifiers over
- the lifetime of the table.
+ names <type>bigserial</type> and <type>serial8</type> works just
+ the same way, except that it creates a <type>bigint</type>
+ column. <type>serial8</type> should be used if you anticipate
+ use of more than 2^31 identifiers over the lifetime of the table.
</para>
<para>
not automatically dropped when a table containing a serial type
is dropped. So, the following commands executed in order will likely fail:
+<programlisting>
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (<replaceable class="parameter">colname</replaceable> SERIAL);
DROP TABLE <replaceable class="parameter">tablename</replaceable>;
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (<replaceable class="parameter">colname</replaceable> SERIAL);
- </programlisting>
+</programlisting>
The sequence will remain in the database until explicitly dropped using
<command>DROP SEQUENCE</command>.
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