What is the average attendance for all events held at Palmerston Park venue?
CREATE TABLE "2008_09_partick_thistle_f_c_season" (
  "date" text,
  "opponent" text,
  "venue" text,
  "result" text,
  "attendance" real
);
SELECT AVG("attendance") FROM "2008_09_partick_thistle_f_c_season" WHERE "venue"='palmerston park';