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Commit6889537

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Some small docs improvements motivated by reading the comments for the 7.4
7.4 interactive docs.
1 parentcb6eab7 commit6889537

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-780
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‎doc/src/sgml/datatype.sgml‎

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<!--
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$PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.152 2004/12/23 05:37:39 tgl Exp $
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$PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.153 2005/01/08 05:19:18 tgl Exp $
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-->
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<chapter id="datatype">
@@ -446,9 +446,9 @@ $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.152 2004/12/23 05:37:39 tgl Ex
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The type <type>numeric</type> can store numbers with up to 1000
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digits of precision and perform calculations exactly. It is
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especially recommended for storing monetary amounts and other
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quantities where exactness is required. However,the
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<type>numeric</type>type is very slow compared to the
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floating-point types described in the next section.
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quantities where exactness is required. However,arithmetic on
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<type>numeric</type>values is very slow compared to the integer
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types, or to thefloating-point types described in the next section.
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</para>
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<para>
@@ -464,7 +464,8 @@ $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.152 2004/12/23 05:37:39 tgl Ex
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</para>
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<para>
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Both the precision and the scale of the numeric type can be
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Both the maximum precision and the maximum scale of a
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<type>numeric</type> column can be
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configured. To declare a column of type <type>numeric</type> use
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the syntax
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<programlisting>
@@ -492,10 +493,19 @@ NUMERIC
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</para>
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<para>
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If the precision or scale of a value is greater than the declared
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precision or scale of a column, the system will attempt to round
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the value. If the value cannot be rounded so as to satisfy the
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declared limits, an error is raised.
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If the scale of a value to be stored is greater than the declared
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scale of the column, the system will round the value to the specified
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number of fractional digits. Then, if the number of digits to the
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left of the decimal point exceeds the declared precision minus the
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declared scale, an error is raised.
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</para>
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<para>
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Numeric values are physically stored without any extra leading or
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trailing zeroes. Thus, the declared precision and scale of a column
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are maximums, not fixed allocations. (In this sense the <type>numeric</>
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type is more akin to <type>varchar(<replaceable>n</>)</type>
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than to <type>char(<replaceable>n</>)</type>.)
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</para>
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<para>
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<para>
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A binary string is a sequence of octets (or bytes). Binary
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strings are distinguished fromcharacters strings by two
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strings are distinguished fromcharacter strings by two
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characteristics: First, binary strings specifically allow storing
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octets of value zero and other <quote>non-printable</quote>
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octets (defined as octets outside the range 32 to 126).
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octets (usually, octets outside the range 32 to 126).
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Character strings disallow zero octets, and also disallow any
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other octet values and sequences of octet values that are invalid
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according to the database's selected character set encoding.
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Second, operations on binary strings process the actual bytes,
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whereas the encoding and processing of character strings depends
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on locale settings.
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whereas the processing of character strings depends on locale settings.
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In short, binary strings are appropriate for storing data that the
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programmer thinks of as <quote>raw bytes</>, whereas character
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strings are appropriate for storing text.
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</para>
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<para>
@@ -1254,7 +1269,7 @@ SELECT b, char_length(b) FROM test2;
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<para>
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The <acronym>SQL</acronym> standard defines a different binary
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string type, called <type>BLOB</type> or <type>BINARY LARGE
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OBJECT</type>. The input format is differentcompared to
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OBJECT</type>. The input format is differentfrom
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<type>bytea</type>, but the provided functions and operators are
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mostly the same.
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</para>
@@ -1295,7 +1310,9 @@ SELECT b, char_length(b) FROM test2;
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<para>
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<productname>PostgreSQL</productname> supports the full set of
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<acronym>SQL</acronym> date and time types, shown in <xref
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linkend="datatype-datetime-table">.
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linkend="datatype-datetime-table">. The operations available
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on these data types are described in
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<xref linkend="functions-datetime">.
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</para>
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<table id="datatype-datetime-table">
@@ -1842,8 +1859,10 @@ January 8 04:05:06 1999 PST
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are specially represented inside the system and will be displayed
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the same way; but the others are simply notational shorthands
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that will be converted to ordinary date/time values when read.
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All of these values are treated as normal constants and need to be
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written in single quotes.
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(In particular, <literal>now</> and related strings are converted
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to a specific time value as soon as they are read.)
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All of these values need to be written in single quotes when used
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as constants in SQL commands.
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</para>
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<table id="datatype-datetime-special-table">
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<literal>CURRENT_DATE</literal>, <literal>CURRENT_TIME</literal>,
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<literal>CURRENT_TIMESTAMP</literal>, <literal>LOCALTIME</literal>,
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<literal>LOCALTIMESTAMP</literal>. The latter four accept an
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optional precision specification. (Seealso<xref
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optional precision specification. (See <xref
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linkend="functions-datetime-current">.) Note however that these are
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SQL functions and are <emphasis>not</> recognized as data input strings.
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</para>
@@ -2265,7 +2284,7 @@ SELECT * FROM test1 WHERE a;
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not work). This can be accomplished using the
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<literal>CASE</literal> expression: <literal>CASE WHEN
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<replaceable>boolval</replaceable> THEN 'value if true' ELSE
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'value if false' END</literal>. Seealso<xref
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'value if false' END</literal>. See <xref
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linkend="functions-conditional">.
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</para>
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</tip>
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<para>
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Paths are represented by lists of connected points. Paths can be
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<firstterm>open</firstterm>, where
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the first and last points in the list are not connected, or
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the first and last points in the list are notconsideredconnected, or
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<firstterm>closed</firstterm>,
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where the first and last points are connected.
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where the first and last points areconsideredconnected.
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</para>
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<para>
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is preferable to use these types instead of plain text types to store
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network addresses, because
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these types offer input error checking and several specialized
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operators and functions.
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operators and functions (see <xref linkend="functions-net">).
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</para>
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<table tocentry="1" id="datatype-net-types-table">
@@ -3006,12 +3025,25 @@ SELECT * FROM test;
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for specialized input and output routines. These routines are able
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to accept and display symbolic names for system objects, rather than
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the raw numeric value that type <type>oid</> would use. The alias
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types allow simplified lookup of OID values for objects: for example,
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one may write <literal>'mytable'::regclass</> to get the OID of table
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<literal>mytable</>, rather than <literal>SELECT oid FROM pg_class WHERE
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relname = 'mytable'</>. (In reality, a much more complicated <command>SELECT</> would
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be needed to deal with selecting the right OID when there are multiple
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tables named <literal>mytable</> in different schemas.)
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types allow simplified lookup of OID values for objects. For example,
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to examine the <structname>pg_attribute</> rows related to a table
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<literal>mytable</>, one could write
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<programlisting>
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SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass;
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</programlisting>
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rather than
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<programlisting>
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SELECT * FROM pg_attribute
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WHERE attrelid = (SELECT oid FROM pg_class WHERE relname = 'mytable');
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</programlisting>
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While that doesn't look all that bad by itself, it's still oversimplified.
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A far more complicated sub-select would be needed to
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select the right OID if there are multiple tables named
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<literal>mytable</> in different schemas.
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The <type>regclass</> input converter handles the table lookup according
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to the schema path setting, and so it does the <quote>right thing</>
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automatically. Similarly, casting a table's OID to
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<type>regclass</> is handy for symbolic display of a numeric OID.
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</para>
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<table id="datatype-oid-table">

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