Simplify custom type autoloading with pgxpool

Provide a backwards-compatible configuration option for pgxpool
which streamlines the use of the bulk loading and registration of types:
- ReuseTypeMaps: if enabled, pgxpool will cache the typemap information,
  avoiding the need to perform any further queries as new connections
  are created.

ReuseTypeMaps is disabled by default as in some situations, a
connection string might resolve to a pool of servers which do not share
the same type name -> OID mapping.

conn.go

@@ -44,6 +44,12 @@ type ConnConfig struct {
 
 	// automatically call LoadTypes with these values
 	AutoLoadTypes []string
+
+	// TypeRegistrationMap is used to register types which require special operations.
+	// The type name is the key, the value is a function which will be called for each
+	// connection, providing the OID of that type name for that connection.
+	// The function will manipulate conn.TypeMap() in some way.
+	TypeRegistrationMap map[string]CustomRegistrationFunction
 }
 
 // ParseConfigOptions contains options that control how a config is built such as getsslpassword.

derived_types.go

@@ -10,6 +10,10 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+// CustomRegistrationFunction is capable of registering whatever is necessary for
+// a custom type. It is provided with the backend's OID for this type.
+type CustomRegistrationFunction func(ctx context.Context, m *pgtype.Map, oid uint32) error
+
 /*
 buildLoadDerivedTypesSQL generates the correct query for retrieving type information.
 
@@ -254,3 +258,29 @@ func serverVersion(c *Conn) (int64, error) {
 	}
 	return serverVersion, nil
 }
+
+func fetchOidMapForCustomRegistration(ctx context.Context, conn *Conn) (map[string]uint32, error) {
+	sql := `
+SELECT oid, typname
+FROM pg_type
+WHERE typname = ANY($1)`
+	result := make(map[string]uint32)
+	typeNames := make([]string, 0, len(conn.config.TypeRegistrationMap))
+	for typeName := range conn.config.TypeRegistrationMap {
+		typeNames = append(typeNames, typeName)
+	}
+	rows, err := conn.Query(ctx, sql, typeNames)
+	if err != nil {
+		return nil, fmt.Errorf("While collecting OIDs for custom registrations: %w", err)
+	}
+	defer rows.Close()
+	var typeName string
+	var oid uint32
+	for rows.Next() {
+		if err := rows.Scan(&typeName, &oid); err != nil {
+			return nil, fmt.Errorf("While scanning a row for custom registrations: %w", err)
+		}
+		result[typeName] = oid
+	}
+	return result, nil
+}