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_cast catalog (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_cast 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 integer, which receives
the destination column's atttypmod value (typically its
declared length, although the interpretation of atttypmod
varies for different data types), and it may take a third boolean
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.
Example 10.9. 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.