Sophie: postgresql8.4-docs-8.4.12-0.1mdv2010.2 i586

postgresql8.4-docs-8.4.12-0.1mdv2010.2.i586.rpm

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><A
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>10.4. Value Storage</A
></H1
><P
>   Values to be inserted into a table are converted to the destination
   column's data type according to the
   following steps.
  </P
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><B
>Value Storage Type Conversion</B
></P
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>Check for an exact match with the target.</P
></LI
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CLASS="STEP"
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>Otherwise, try to convert the expression to the target type.  This will succeed
if there is a registered cast between the two types.
If the expression is an unknown-type literal, the contents of
the literal string will be fed to the input conversion routine for the target
type.</P
></LI
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>Check to see if there is a sizing cast for the target type.  A sizing
cast is a cast from that type to itself.  If one is found in the
<TT
CLASS="STRUCTNAME"
>pg_cast</TT
> catalog, apply it to the expression before storing
into the destination column.  The implementation function for such a cast
always takes an extra parameter of type <TT
CLASS="TYPE"
>integer</TT
>, which receives
the destination column's <TT
CLASS="STRUCTFIELD"
>atttypmod</TT
> value (typically its
declared length, although the interpretation of <TT
CLASS="STRUCTFIELD"
>atttypmod</TT
>
varies for different data types), and it may take a third <TT
CLASS="TYPE"
>boolean</TT
>
parameter that says whether the cast is explicit or implicit.  The cast
function
is responsible for applying any length-dependent semantics such as size
checking or truncation.</P
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>Example 10-6. <TT
CLASS="TYPE"
>character</TT
> Storage Type Conversion</B
></P
><P
>For a target column declared as <TT
CLASS="TYPE"
>character(20)</TT
> the following
statement shows that the stored value is sized correctly:

</P><PRE
CLASS="SCREEN"
>CREATE TABLE vv (v character(20));
INSERT INTO vv SELECT 'abc' || 'def';
SELECT v, octet_length(v) FROM vv;

          v           | octet_length
----------------------+--------------
 abcdef               |           20
(1 row)</PRE
><P></P
><P
>What has really happened here is that the two unknown literals are resolved
to <TT
CLASS="TYPE"
>text</TT
> by default, allowing the <TT
CLASS="LITERAL"
>||</TT
> operator
to be resolved as <TT
CLASS="TYPE"
>text</TT
> concatenation.  Then the <TT
CLASS="TYPE"
>text</TT
>
result of the operator is converted to <TT
CLASS="TYPE"
>bpchar</TT
> (<SPAN
CLASS="QUOTE"
>"blank-padded
char"</SPAN
>, the internal name of the <TT
CLASS="TYPE"
>character</TT
> data type) to match the target
column type.  (Since the conversion from <TT
CLASS="TYPE"
>text</TT
> to
<TT
CLASS="TYPE"
>bpchar</TT
> is binary-coercible, this conversion does
not insert any real function call.)  Finally, the sizing function
<TT
CLASS="LITERAL"
>bpchar(bpchar, integer, boolean)</TT
> is found in the system catalog
and applied to the operator's result and the stored column length.  This
type-specific function performs the required length check and addition of
padding spaces.</P
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