Uniquely identifies queries by their type for the purpose of prepared statement caching.
All types which implement
QueryFragment should also implement this trait
(It is not an actual supertrait of
QueryFragment for boxing purposes).
This trait can be automatically derived by Diesel. For example, given this struct:
If the SQL generated by a struct is not uniquely identifiable by its type,
HAS_STATIC_QUERY_ID should always be false,
you should not derive this trait.
In that case you should manually implement it instead.
A type which uniquely represents
Self in a SQL query.
Typically this will be a re-construction of
Self using the
type of each of your type parameters. For example, the type
And<Left, Right> would have
type QueryId = And<Left::QueryId, Right::QueryId>.
The exception to this is when one of your type parameters does not
affect whether the same prepared statement can be used or not. For
example, a bind parameter is represented as
The actual Rust type we are serializing does not matter for the purposes
of prepared statement reuse, but a query which has identical SQL but
different types for its bind parameters requires a new prepared
statement. For this reason,
Bound would have
type QueryId = Bound<SqlType::QueryId, ()>.
false, you can put any type here
Can the SQL generated by
Self be uniquely identified by its type?
Typically this question can be answered by looking at whether
unsafe_to_cache_prepared is called in your implementation of
QueryFragment::walk_ast. In Diesel itself, the only type which has
false here, but is potentially safe to store in the prepared statement
cache is a boxed query.
Returns the type id of
You should never need to override this method.