ORACLE - Data Guard : Real-Time Query

Real-time Query Restrictions:
The following additional restrictions apply if STANDBY_MAX_DATA_DELAY is set to 0 or if the ALTER SESSION SYNC WITH PRIMARY SQL statement is used:

1.The standby database must receive redo data via the SYNC transport
2.The redo transport status at the standby database must be SYNCHRONIZED and the primary database must be running in either maximum protection mode or maximum availability mode.
3.Real-time apply must be enabled.

Configuring Apply Lag Tolerance in a Real-time Query:
The STANDBY_MAX_DATA_DELAY session parameter can be used to specify a session-specific apply lag tolerance, measured in seconds, for queries issued by non-administrative users to a physical standby database that is in real-time query mode.

If STANDBY_MAX_DATA_DELAY is set to the default value of NONE, queries issued to a physical standby database will be executed regardless of the apply lag on that database.

If STANDBY_MAX_DATA_DELAY is set to a non-zero value, a query issued to a physical standby database will be executed only if the apply lag is less than or equal to STANDBY_MAX_DATA_DELAY. Otherwise, an ORA-3172 error is returned to alert the client that the apply lag is too large.

If STANDBY_MAX_DATA_DELAY is set to 0, a query issued to a physical standby database is guaranteed to return the exact same result as if the query were issued on the primary database, unless the standby database is lagging behind the primary database, in which case an ORA-3172 error is returned.

Forcing Redo Apply Synchronization in a Real-time Query Environment:
ALTER SESSION SYNC WITH PRIMARY;This statement will block until all redo data received by the standby database at the time that this command is issued has been applied to the physical standby database. An ORA-3173 error is returned immediately, and synchronization will not occur, if the redo transport status at the standby database is not SYNCHRONIZED or if Redo Apply is not active.

or

For example, you could create the following trigger that would execute the ALTER SESSION SYNC WITH PRIMARY statement for a specific user connection at logon:
CREATE TRIGGER adg_logon_sync_trigger
 AFTER LOGON ON user.schema
  begin
    if (SYS_CONTEXT('USERENV', 'DATABASE_ROLE')  IN ('PHYSICAL STANDBY')) then
      execute immediate 'alter session sync with primary';
    end if;
  end;

Ref. ORACLE Docs/Metalink