用存储过程修改，把下边这个存储过程里边的USERA替换成你的只读用户A，USERB替换成你的B用户

DECLARE
    SQLSTMT STRING;
    CURSOR CUR FOR SELECT ID,NAME  FROM SYSOBJECTS  WHERE  TYPE$ = 'SCHOBJ' AND SUBTYPE$ IN ('STAB','UTAB') AND (PID=-1 OR PID=0) AND  SCHID=(SELECT ID FROM SYSOBJECTS WHERE TYPE$='SCH' AND NAME='USERB' );
    TYPE MYREC IS CUR%ROWTYPE;
    REC_V MYREC;
    OBJ_PRIV CURSOR;
    TYPE R_PRIV IS RECORD (CUR_PRIV_NAME varchar(128),CUR_BE_GRANT CHAR(1),CUR_CAN_GRANT CHAR(1),CUR_IS_GRANT CHAR(1),ORG_PRIV_NAME varchar(128),ORG_CAN_GRANT CHAR(1)); 
    TYPE T_PRIV IS TABLE OF R_PRIV;
    t_p T_PRIV;
BEGIN
    FOR REC_V IN CUR LOOP
    OPEN OBJ_PRIV FOR  WITH
                       CUR_PRIVS(PRIV_NAME, BE_GRANT,CAN_GRANT,ISGRANT) AS
                       (
                         SELECT 'SELECT','Y','N','Y' FROM DUAL 
                       )
                       SELECT CUR_PRIVS.*,SF_GET_SYS_PRIV(ORG_PRIV. PRIVID),ORG_PRIV.GRANTABLE from CUR_PRIVS  LEFT JOIN  (SELECT * FROM SYSGRANTS WHERE OBJID=REC_V.ID AND COLID =-1 AND URID=50331751) ORG_PRIV   ON CUR_PRIVS.PRIV_NAME=SF_GET_SYS_PRIV(ORG_PRIV.PRIVID);
  FETCH OBJ_PRIV BULK COLLECT INTO  t_p;
  CLOSE OBJ_PRIV;
  FOR I IN 1..t_p.COUNT LOOP
    SQLSTMT:=NULL;
  IF(t_p(I).CUR_IS_GRANT ='Y' ) THEN
     IF (t_p(I).CUR_CAN_GRANT='Y' ) THEN
       SQLSTMT='GRANT '|| t_p(I).CUR_PRIV_NAME ||' on "USERB"."'|| REC_V.NAME || '"' ||' TO  "USERA" WITH grant option ';
     ELSE 
       IF(t_p(I).ORG_PRIV_NAME is null) THEN
         SQLSTMT= 'GRANT '|| t_p(I).CUR_PRIV_NAME ||' on "USERB"."'|| REC_V.NAME || '"' ||' TO  "USERA"';
       ELSE
         IF(t_p(I).ORG_CAN_GRANT='Y')  THEN
           SQLSTMT='REVOKE grant option for '|| t_p(I).CUR_PRIV_NAME ||' on "USERB"."'|| REC_V.NAME || '"' ||' from  "USERA" cascade;';
         END IF;
       END IF;
     END IF;
  ELSE
     IF(t_p(I).CUR_BE_GRANT ='Y' && t_p(I).ORG_PRIV_NAME is not null) THEN
       SQLSTMT= 'REVOKE '|| t_p(I).CUR_PRIV_NAME ||' on "USERB"."'|| REC_V.NAME || '"' ||' from  "USERA" cascade;';
     ELSE
       IF(t_p(I).CUR_CAN_GRANT ='Y' && t_p(I).ORG_CAN_GRANT ='Y') THEN
         SQLSTMT='REVOKE grant option for '|| t_p(I).CUR_PRIV_NAME ||' on "USERB"."'|| REC_V.NAME || '"' ||' from  "USERA" cascade;';
        END IF;
     END IF;
  END IF;
  IF(SQLSTMT IS NOT NULL) THEN
     EXECUTE IMMEDIATE SQLSTMT;
  END IF;
  END LOOP; 
    END LOOP;
    COMMIT;
END;

1.设置 INI 参数 GRANT_SCHEMA=1
2.将模式 B 下所有对象的查询权限授予用户 A
GRANT SELECT ON SCHEMA B TO A;

补充一个方法
--创建普通用户
CREATE USER "RDOLUSER" IDENTIFIED BY "XXXXXXXXX";
--生成批量赋权SQL
SELECT 'GRANT SELECT ON 模式名.'||TABLE_NAME||' TO RDOLUSER;' FROM SYS.DBA_TABLES WHERE OWNER = '模式名';
再执行结果集的SQL。