What is the purpose of the input output functions in Postgresql 9.2 user defined types?

Question

I have been implementing user defined types in Postgresql 9.2 and got confused. In the PostgreSQL 9.2 documentation, there is a section (35.11) on user defined types. In the third paragraph of that section, the documentation refers to input and output functions that are used to construct a type. I am confused about the purpose of these functions. Are they concerned with on-disk representation or only in-memory representation? In the section referred to above, after defining the input and output functions, it states that:

If we want to do anything more with the type than merely store it, we must provide additional functions to implement whatever operations we'd like to have for the type.

Do the input and output functions deal with serialization? As I understand it, the input function is the one which will be used to perform INSERT INTO and the output function to perform SELECT on the type so basically if we want to perform an INSERT INTO then we need a serialization function embedded or invoked in the input or output function. Can anyone help explain this to me?

Daniel Vérité · Accepted Answer · 2013-01-29 19:41:50Z

1

Types must have a text representation, so that values of this type can be expressed as literals in a SQL query, and returned as results in output columns.

For example, '2013-20-01' is a text representation of a date. It's possible to write VALUES('2013-20-01'::date) in a SQL statement, because the input function of the date type recognizes this string as a date and transforms it into an internal representation (for both using it in memory and storing to disk).

Conversely, when client code issues SELECT date_field FROM table, the values inside date_field are returned in their text representation, which is produced by the type's output function from the internal representation (unless the client requested a binary format for this column).

answered Jan 29, 2013 at 19:41

Daniel Vérité

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