Values to be inserted into a table are converted to the destination column's data type according to the following steps.
Example 10-6. character Storage Type Conversion
For a target column declared as character(20) the following statement ensures that the stored value is sized correctly:
CREATE TABLE vv (v character(20));
INSERT INTO vv SELECT 'abc' || 'def';
SELECT v, length(v) FROM vv;
v | length
abcdef | 20
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 types text and bpchar are binary-compatible, this conversion does not insert any real function call.) Finally, the sizing function bpchar(bpchar, integer) 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.