How to understand the return type?

I'm building a framework for rust-postgres.

I need to know what value type will be returned from a row.try_get, to get the value in a variable of the appropriate type.

I can get the sql type from row.columns()[index].type, but not if the value is nullable , so i can't decide to put the value in a normal type or a Option<T>.

I can use just the content of the row to understand it, i can't do things like "get the table structure from Postgresql".

is there a way?

The reason that the Column type does not expose any way to find out if a result column is nullable is because the database does not return this information.

Remember that result columns are derived from running a query, and that query may contain arbitrary expressions. If the query was a simple SELECT of columns from a table, then it would be reasonably simple to determine if a column could be nullable.

But it could also be a very complex expression, derived from multiple columns, subselects or even custom functions. Postgres can figure out the data type of each column, but in the general case it doesn't know if a result column may contain nulls.

If your application is only performing simple queries, and you know which table column each result column comes from, then you can find out if that table column is nullable like this:

SELECT is_nullable 
FROM information_schema.columns 
WHERE table_schema='myschema' 
  AND table_name='mytable' 
  AND column_name='mycolumn';

If your queries are not that simple then I recommend you always get the result as an Option<T> and handle the possibility that the result might be None.