PostgreSQLで翌営業日を取得する方法

t_holiday
holiday	holiday_name
2023-03-21	春分の日
2023-05-03	憲法記念日
2023-05-04	みどりの日
2023-05-05	こどもの日

  SELECT 
    lag(SEQDAY ,-(1 + cnt)) OVER(ORDER BY SEQDAY)  as NEXT_DAY
  FROM 
  (SELECT 
      CAST(A AS DATE) SEQDAY
   FROM
      (SELECT GENERATE_SERIES('2023/05/01'::date, '2023/05/01'::date + 15, '1 day') AS A) DAYS
  )DAYS
    LEFT JOIN t_holiday KYU
      ON DAYS.SEQDAY = KYU.holiday 
  CROSS JOIN 
    --指定日が土日祝かチェック
    (SELECT CASE WHEN COUNT(*) = 0 THEN -1 ELSE 0 END cnt 
    FROM 
      (SELECT 
          CAST(A AS DATE) SEQDAY
       FROM
          (SELECT GENERATE_SERIES('2023/05/01'::date,'2023/05/01'::date + 15, '1 day') AS A) DAYS
      )DAYS
        LEFT JOIN t_holiday KYU
          ON DAYS.SEQDAY = KYU.holiday 
      WHERE
          EXTRACT(DOW FROM CAST(DAYS.SEQDAY AS DATE)) NOT IN (6,0)
          AND holiday IS NULL
          AND SEQDAY = '2023/05/01' 
    )b
  WHERE
    EXTRACT(DOW FROM CAST(DAYS.SEQDAY AS DATE)) NOT IN (6,0)
    AND holiday IS NULL
  limit 1;

CREATE OR REPLACE FUNCTION "public"."NEXT_DAY"("datdate" date, "nissu" int4)
  RETURNS "pg_catalog"."date" AS $BODY$
BEGIN
  --平日か土日祝で指定日が異なる
  return lag(SEQDAY ,-(Nissu + cnt)) OVER(ORDER BY SEQDAY)  as 次営業日
  FROM 
  (SELECT 
      CAST(A AS DATE) SEQDAY
   FROM
      (SELECT GENERATE_SERIES(datDate, datDate + 15, '1 day') AS A) DAYS
  )DAYS
    LEFT JOIN t_holiday KYU
      ON DAYS.SEQDAY = KYU.holiday 
  CROSS JOIN 
    --指定日が土日祝かチェック
    (SELECT CASE WHEN COUNT(*) = 0 THEN -1 ELSE 0 END cnt 
    FROM 
      (SELECT 
          CAST(A AS DATE) SEQDAY
       FROM
          (SELECT GENERATE_SERIES(datDate,datDate + 15, '1 day') AS A) DAYS
      )DAYS
        LEFT JOIN t_holiday KYU
          ON DAYS.SEQDAY = KYU.日付 
      WHERE
          EXTRACT(DOW FROM CAST(DAYS.SEQDAY AS DATE)) NOT IN (6,0)
          AND holiday IS NULL
          AND SEQDAY = datDate 
    )b
  WHERE
    EXTRACT(DOW FROM CAST(DAYS.SEQDAY AS DATE)) NOT IN (6,0)
    AND holiday IS NULL
  limit 1;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100

SELECT NEXT_DAY('2023/05/02',1)

結果
2023/05/08