MySQL 存储过程替换 JSON 内容时，为什么会出现“大字段信息不存在”的错误？

delimiter //
drop procedure if exists `replacename`//
create procedure replacename()

begin 
declare c int default 0;
declare r json ;
declare id int default 0;
declare i int default 0;

declare result cursor for select id,slider_image from `eb_store_product`;

select count(*) into c from eb_store_product;
open  result;

repeat 
    set i = i + 1;
    fetch result into id,r;

    set @t = replace(json_extract(r,'$[0]'),'ceshi','chenggong');

until i >= c
end repeat; 
close result;
select @t;

end//

call replacename();//
delimiter ;

DELIMITER //
DROP PROCEDURE IF EXISTS `replacename`//
CREATE PROCEDURE replacename()
BEGIN
    DECLARE c INT DEFAULT 0;
    DECLARE r JSON;
    DECLARE id INT DEFAULT 0;
    DECLARE i INT DEFAULT 0;

    DECLARE result CURSOR FOR SELECT id, slider_image FROM `eb_store_product`;

    SELECT COUNT(*) INTO c FROM eb_store_product;
    OPEN result;

    REPEAT
        SET i = i + 1;
        FETCH result INTO id, r;

        SET @t = REPLACE(JSON_EXTRACT(r, '$[0]'), 'ceshi', 'chenggong');

        -- 更新 eb_store_product 表中的 slider_image 字段
        UPDATE eb_store_product SET slider_image = JSON_REPLACE(r, '$[0]', @t) WHERE id = id;

    UNTIL i >= c
    END REPEAT;

    CLOSE result;
    SELECT @t;
END//
DELIMITER ;

CALL replacename();