What are few Considerations about Trigger ?

• upsert triggers fire both before and after insert or before and after update triggers as appropriate.
• merge triggers fire both before and after delete triggers for the losing records and before update triggers for the winning record only.
• Triggers that execute after a record has been undeleted only work with specific objects.
• Field history is not recorded until the end of a trigger. If you query field history in a trigger, you will not see any history for the current transaction.
• You can only use the webService keyword in a trigger when it is in a method defined as asynchronous; that is, when the method is defined with the @future keyword.
• A trigger invoked by an insert, delete, or update of a recurring event or recurring task results in a runtime error when the trigger is called in bulk from the Force.com API.
• Merge trigger doesn’t fire there own trigger instead they fire delete and update of loosing and winning records respectively.