• 回表的代价：回表的代价跟回表的行数也也是正相关的，回表的行数越多(回表的行数是指满足所有能在索引上执行的谓词的行数)，执行时间就会越久。回表的扫描是随机 IO，所以回表一行的代价会比 query range 扫描一行的代价高很多。
    2. Query OK, 0 rows affected (0.26 sec)
    4. OceanBase (root@test)> explain extended_noaddr select/*+index(t1 k1)*/ * from t1 where c2 > 20 and c2 < 800 and c3 < 200;
    5. +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    6. | Query Plan                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
    7. +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    8. | =====================================
    9. |ID|OPERATOR  |NAME  |EST. ROWS|COST|
    10. -------------------------------------
    11. =====================================
    13. Outputs & filters:
    14. -------------------------------------
    15.   0 - output([t1.c1], [t1.c2], [t1.c3], [t1.c4], [t1.c5]), filter([t1.c3 < 200]),
    16.       access([t1.c2], [t1.c3], [t1.c1], [t1.c4], [t1.c5]), partitions(p0),
    17.       is_index_back=true, filter_before_indexback[true],
    18.       range_key([t1.c2], [t1.c3], [t1.c1]), range(20,MAX,MAX ; 800,MIN,MIN),
    19.       range_cond([t1.c2 > 20], [t1.c2 < 800])
    20. -- query range 扫描的行数
    21. OceanBase (root@test)> select/*+index(t1 k1)*/ count(*) from t1 where c2 > 20 and c2 < 800;
    22. | count(*) |
    24. |      779 |
    25. +----------+
    26. 1 row in set (0.02 sec)
    27. -- 回表的行数
    28. OceanBase (root@test)> select/*+index(t1 k1)*/ count(*) from t1 where c2 > 20 and c2 < 800 and c3 < 200;
    29. +----------+
    30. | count(*) |
    31. +----------+
    32. |      179 |
    33. 1 row in set (0.01 sec)