delete all but top 26 rows from records grouped by opp_id

CREATE TEMPORARY TABLE tmp SELECT i1.*
FROM decision_process i1
LEFT OUTER JOIN decision_process i2
  ON (i1.opp_id = i2.opp_id AND i1.id < i2.id)
GROUP BY i1.id
HAVING COUNT(*) < 26
ORDER BY opp_id;

DELETE FROM decision_process;

INSERT INTO decision_process SELECT * FROM tmp;