Re: Where are the detoast function called in select * from table_name case?

On 05/06/2015 04:00 PM, Rui Hai Jiang wrote:
> I've been reading the PostgreSQL code for weeks to figure out how TOAST
> works.
> I couldn't find where are the TOAST function called to detoast a tuple
> comes from a select query, for example, select * from table_name.
> Does anyone know this? Can you give me some help? And any help would
> save me a lot of time.

The tuple isn't detoasted immediately when it's read. Individual datums
of the tuple are detoasted lazily, when needed, in whatever function
it's passed to. If you just do "select * from table", the first function
it's passed to is the datatype's output function, so that detoasts it.
For example, if it's a text column, the toasted Datum is passed to
textout(), which calls text_to_cstring(). text_to_cstring() calls
pg_detoast_datum_packed(), which finally detoasts it.

You can launch a debugger on the backend process, and put a breakpoint
on e.g. heap_tuple_untoast_attr() to see it in action.

- Heikki