更新の数を数える

たとえば、RETURNINGを使用します ：

DROP TABLE IF EXISTS t;
CREATE TABLE t(i INT);

INSERT INTO t(i) VALUES (1),(2),(3);

WITH cte AS
(
    UPDATE t
    SET i = i * 10
    WHERE i > 1
    RETURNING *   
)
SELECT COUNT(*) AS num_of_rows_updated
FROM cte;

Rextesterデモ