10.4. Value Storage
Values to be inserted into a table are converted to the destination column's data type according to the following steps.
Value Storage Type Conversion
-
Check for an exact match with the target.
-
Otherwise, try to convert the expression to the target type. This is possible if an assignment cast between the two types is registered in the
pg_castcatalog (see CREATE CAST ). Alternatively, 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. -
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
pg_castcatalog, apply it to the expression before storing into the destination column. The implementation function for such a cast always takes an extra parameter of typeinteger, which receives the destination column'satttypmodvalue (typically its declared length, although the interpretation ofatttypmodvaries for different data types), and it may take a thirdbooleanparameter 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.
Example 10.8.
character
Storage Type Conversion
For a target column declared as
character(20)
the following
statement shows that the stored value is sized correctly:
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)
What has really happened here is that the two unknown literals are resolved
to
text
by default, allowing the
||
operator
to be resolved as
text
concatenation. Then the
text
result of the operator is converted to
bpchar
(
"
blank-padded
char
"
, the internal name of the
character
data type) to match the target
column type. (Since the conversion from
text
to
bpchar
is binary-coercible, this conversion does
not insert any real function call.) Finally, the sizing function
bpchar(bpchar, integer, boolean)
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.