When it is done/advised ?
Any patient suspected of having coronary artery disease undergoes angiography. Angiography shows the obstruction, along with its severity, location and condition of the blood vessel. A cardiologist after a thorough review of the film will advise which patients will benefit from surgery.

There are various consideration in taking this decision, like age and general condition of the patient, the number of vessels involved, various locations of the obstructions, the quality of vessel beyond the obstruction, presence of diabetes, and of course patients' own preference.