为什么Oracle使用跳过扫描进行此查询？
https://stackoverflow.com/questions/2776884
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03-10-2019 - |
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这是一个非常缓慢运行的查询的TKProf输出（警告: ： 它的长 ：-） ）：
SELECT mbr_comment_idn, mbr_crt_dt, mbr_data_source, mbr_dol_bl_rmo_ind, mbr_dxcg_ctl_member, mbr_employment_start_dt, mbr_employment_term_dt, mbr_entity_active, mbr_ethnicity_idn, mbr_general_health_status_code, mbr_hand_dominant_code, mbr_hgt_feet, mbr_hgt_inches, mbr_highest_edu_level, mbr_insd_addr_idn, mbr_insd_alt_id, mbr_insd_name, mbr_insd_ssn_tin, mbr_is_smoker, mbr_is_vip, mbr_lmbr_first_name, mbr_lmbr_last_name, mbr_marital_status_cd, mbr_mbr_birth_dt, mbr_mbr_death_dt, mbr_mbr_expired, mbr_mbr_first_name, mbr_mbr_gender_cd, mbr_mbr_idn, mbr_mbr_ins_type, mbr_mbr_isreadonly, mbr_mbr_last_name, mbr_mbr_middle_name, mbr_mbr_name, mbr_mbr_status_idn, mbr_mpi_id, mbr_preferred_am_pm, mbr_preferred_time, mbr_prv_innetwork, mbr_rep_addr_idn, mbr_rep_name, mbr_rp_mbr_id, mbr_same_mbr_ins, mbr_special_needs_cd, mbr_timezone, mbr_upd_dt, mbr_user_idn, mbr_wgt, mbr_work_status_idn
FROM (SELECT /*+ FIRST_ROWS(1) */ mbr_comment_idn, mbr_crt_dt, mbr_data_source, mbr_dol_bl_rmo_ind, mbr_dxcg_ctl_member, mbr_employment_start_dt, mbr_employment_term_dt, mbr_entity_active, mbr_ethnicity_idn, mbr_general_health_status_code, mbr_hand_dominant_code, mbr_hgt_feet, mbr_hgt_inches, mbr_highest_edu_level, mbr_insd_addr_idn, mbr_insd_alt_id, mbr_insd_name, mbr_insd_ssn_tin, mbr_is_smoker, mbr_is_vip, mbr_lmbr_first_name, mbr_lmbr_last_name, mbr_marital_status_cd, mbr_mbr_birth_dt, mbr_mbr_death_dt, mbr_mbr_expired, mbr_mbr_first_name, mbr_mbr_gender_cd, mbr_mbr_idn, mbr_mbr_ins_type, mbr_mbr_isreadonly, mbr_mbr_last_name, mbr_mbr_middle_name, mbr_mbr_name, mbr_mbr_status_idn, mbr_mpi_id, mbr_preferred_am_pm, mbr_preferred_time, mbr_prv_innetwork, mbr_rep_addr_idn, mbr_rep_name, mbr_rp_mbr_id, mbr_same_mbr_ins, mbr_special_needs_cd, mbr_timezone, mbr_upd_dt, mbr_user_idn, mbr_wgt, mbr_work_status_idn, ROWNUM AS ora_rn
FROM (SELECT mbr.comment_idn AS mbr_comment_idn, mbr.crt_dt AS mbr_crt_dt, mbr.data_source AS mbr_data_source, mbr.dol_bl_rmo_ind AS mbr_dol_bl_rmo_ind, mbr.dxcg_ctl_member AS mbr_dxcg_ctl_member, mbr.employment_start_dt AS mbr_employment_start_dt, mbr.employment_term_dt AS mbr_employment_term_dt, mbr.entity_active AS mbr_entity_active, mbr.ethnicity_idn AS mbr_ethnicity_idn, mbr.general_health_status_code AS mbr_general_health_status_code, mbr.hand_dominant_code AS mbr_hand_dominant_code, mbr.hgt_feet AS mbr_hgt_feet, mbr.hgt_inches AS mbr_hgt_inches, mbr.highest_edu_level AS mbr_highest_edu_level, mbr.insd_addr_idn AS mbr_insd_addr_idn, mbr.insd_alt_id AS mbr_insd_alt_id, mbr.insd_name AS mbr_insd_name, mbr.insd_ssn_tin AS mbr_insd_ssn_tin, mbr.is_smoker AS mbr_is_smoker, mbr.is_vip AS mbr_is_vip, mbr.lmbr_first_name AS mbr_lmbr_first_name, mbr.lmbr_last_name AS mbr_lmbr_last_name, mbr.marital_status_cd AS mbr_marital_status_cd, mbr.mbr_birth_dt AS mbr_mbr_birth_dt, mbr.mbr_death_dt AS mbr_mbr_death_dt, mbr.mbr_expired AS mbr_mbr_expired, mbr.mbr_first_name AS mbr_mbr_first_name, mbr.mbr_gender_cd AS mbr_mbr_gender_cd, mbr.mbr_idn AS mbr_mbr_idn, mbr.mbr_ins_type AS mbr_mbr_ins_type, mbr.mbr_isreadonly AS mbr_mbr_isreadonly, mbr.mbr_last_name AS mbr_mbr_last_name, mbr.mbr_middle_name AS mbr_mbr_middle_name, mbr.mbr_name AS mbr_mbr_name, mbr.mbr_status_idn AS mbr_mbr_status_idn, mbr.mpi_id AS mbr_mpi_id, mbr.preferred_am_pm AS mbr_preferred_am_pm, mbr.preferred_time AS mbr_preferred_time, mbr.prv_innetwork AS mbr_prv_innetwork, mbr.rep_addr_idn AS mbr_rep_addr_idn, mbr.rep_name AS mbr_rep_name, mbr.rp_mbr_id AS mbr_rp_mbr_id, mbr.same_mbr_ins AS mbr_same_mbr_ins, mbr.special_needs_cd AS mbr_special_needs_cd, mbr.timezone AS mbr_timezone, mbr.upd_dt AS mbr_upd_dt, mbr.user_idn AS mbr_user_idn, mbr.wgt AS mbr_wgt, mbr.work_status_idn AS mbr_work_status_idn
FROM mbr JOIN mbr_identfn ON mbr.mbr_idn = mbr_identfn.mbr_idn
WHERE mbr_identfn.mbr_idn = mbr.mbr_idn AND mbr_identfn.identfd_type = :identfd_type_1 AND mbr_identfn.identfd_number = :identfd_number_1 AND mbr_identfn.entity_active = :entity_active_1)
WHERE ROWNUM <= :ROWNUM_1)
WHERE ora_rn > :ora_rn_1
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 9936 0.46 0.49 0 0 0 0
Execute 9936 0.60 0.59 0 0 0 0
Fetch 9936 329.87 404.00 0 136966922 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 29808 330.94 405.09 0 136966922 0 0
Misses in library cache during parse: 0
Optimizer mode: FIRST_ROWS
Parsing user id: 36 (JIVA_DEV)
Rows Row Source Operation
------- ---------------------------------------------------
0 VIEW (cr=102 pr=0 pw=0 time=2180 us)
0 COUNT STOPKEY (cr=102 pr=0 pw=0 time=2163 us)
0 NESTED LOOPS (cr=102 pr=0 pw=0 time=2152 us)
0 INDEX SKIP SCAN IDX_MBR_IDENTFN (cr=102 pr=0 pw=0 time=2140 us)(object id 341053)
0 TABLE ACCESS BY INDEX ROWID MBR (cr=0 pr=0 pw=0 time=0 us)
0 INDEX UNIQUE SCAN PK_CLAIMANT (cr=0 pr=0 pw=0 time=0 us)(object id 334044)
Rows Execution Plan
------- ---------------------------------------------------
0 SELECT STATEMENT MODE: HINT: FIRST_ROWS
0 VIEW
0 COUNT (STOPKEY)
0 NESTED LOOPS
0 INDEX MODE: ANALYZED (SKIP SCAN) OF 'IDX_MBR_IDENTFN'
(INDEX (UNIQUE))
0 TABLE ACCESS MODE: ANALYZED (BY INDEX ROWID) OF 'MBR'
(TABLE)
0 INDEX MODE: ANALYZED (UNIQUE SCAN) OF 'PK_CLAIMANT'
(INDEX (UNIQUE))
********************************************************************************
根据我对Oracle的阅读 跳过扫描的文档, ，当索引的第一列的唯一值数量较低时，跳过扫描是最有用的。问题是该列的第一个索引具有大量的唯一功能。因此，假设跳过扫描是错误的事情，我是否正确？另外，什么样的扫描 应该 它在做吗？我应该为此查询做更多的暗示吗？
编辑: ：我还应该指出查询的子句在其中使用列 IDX_MBR_IDENTFN 除该索引中的内容外，没有其他列。据我所知，我没有跳过任何列。
编辑2: ：我已经做了一些事情来加快此查询。首先，我删除了分页。事实证明，此查询无论如何只返回一排。其次，我添加了一个 LEADING 提示确保桌子按正确的顺序查询。第三，我删除了副本 mbr_idn 谓词。最后，我做了 IDX_MBR_IDENTFN 独特。总的来说，这可以改善性能（尽管这仍然是我正在运行的最昂贵的查询）：
SELECT /*+ LEADING (mbr_identfn, mbr) */ mbr.comment_idn AS mbr_comment_idn, mbr.crt_dt AS mbr_crt_dt, mbr.data_source AS mbr_data_source, mbr.dol_bl_rmo_ind AS mbr_dol_bl_rmo_ind, mbr.dxcg_ctl_member AS mbr_dxcg_ctl_member, mbr.employment_start_dt AS mbr_employment_start_dt, mbr.employment_term_dt AS mbr_employment_term_dt, mbr.entity_active AS mbr_entity_active, mbr.ethnicity_idn AS mbr_ethnicity_idn, mbr.general_health_status_code AS mbr_general_health_status_code, mbr.hand_dominant_code AS mbr_hand_dominant_code, mbr.hgt_feet AS mbr_hgt_feet, mbr.hgt_inches AS mbr_hgt_inches, mbr.highest_edu_level AS mbr_highest_edu_level, mbr.insd_addr_idn AS mbr_insd_addr_idn, mbr.insd_alt_id AS mbr_insd_alt_id, mbr.insd_name AS mbr_insd_name, mbr.insd_ssn_tin AS mbr_insd_ssn_tin, mbr.is_smoker AS mbr_is_smoker, mbr.is_vip AS mbr_is_vip, mbr.lmbr_first_name AS mbr_lmbr_first_name, mbr.lmbr_last_name AS mbr_lmbr_last_name, mbr.marital_status_cd AS mbr_marital_status_cd, mbr.mbr_birth_dt AS mbr_mbr_birth_dt, mbr.mbr_death_dt AS mbr_mbr_death_dt, mbr.mbr_expired AS mbr_mbr_expired, mbr.mbr_first_name AS mbr_mbr_first_name, mbr.mbr_gender_cd AS mbr_mbr_gender_cd, mbr.mbr_idn AS mbr_mbr_idn, mbr.mbr_ins_type AS mbr_mbr_ins_type, mbr.mbr_isreadonly AS mbr_mbr_isreadonly, mbr.mbr_last_name AS mbr_mbr_last_name, mbr.mbr_middle_name AS mbr_mbr_middle_name, mbr.mbr_name AS mbr_mbr_name, mbr.mbr_status_idn AS mbr_mbr_status_idn, mbr.mpi_id AS mbr_mpi_id, mbr.preferred_am_pm AS mbr_preferred_am_pm, mbr.preferred_time AS mbr_preferred_time, mbr.prv_innetwork AS mbr_prv_innetwork, mbr.rep_addr_idn AS mbr_rep_addr_idn, mbr.rep_name AS mbr_rep_name, mbr.rp_mbr_id AS mbr_rp_mbr_id, mbr.same_mbr_ins AS mbr_same_mbr_ins, mbr.special_needs_cd AS mbr_special_needs_cd, mbr.timezone AS mbr_timezone, mbr.upd_dt AS mbr_upd_dt, mbr.user_idn AS mbr_user_idn, mbr.wgt AS mbr_wgt, mbr.work_status_idn AS mbr_work_status_idn
FROM mbr JOIN mbr_identfn ON mbr.mbr_idn = mbr_identfn.mbr_idn
WHERE mbr_identfn.identfd_type = :identfd_type_1 AND mbr_identfn.identfd_number = :identfd_number_1 AND mbr_identfn.entity_active = :entity_active_1
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 10102 0.45 0.42 0 0 0 0
Execute 10102 0.44 0.52 0 0 0 0
Fetch 10102 1.60 1.81 0 218121 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 30306 2.50 2.75 0 218121 0 0
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 36 (JIVA_DEV)
Rows Row Source Operation
------- ---------------------------------------------------
0 NESTED LOOPS (cr=3 pr=0 pw=0 time=96 us)
0 TABLE ACCESS BY INDEX ROWID MBR_IDENTFN (cr=3 pr=0 pw=0 time=88 us)
0 INDEX UNIQUE SCAN UK_CLM_IDFN (cr=3 pr=0 pw=0 time=77 us)(object id 334118)
0 TABLE ACCESS BY INDEX ROWID MBR (cr=0 pr=0 pw=0 time=0 us)
0 INDEX UNIQUE SCAN PK_CLAIMANT (cr=0 pr=0 pw=0 time=0 us)(object id 334044)
Rows Execution Plan
------- ---------------------------------------------------
0 SELECT STATEMENT MODE: ALL_ROWS
0 NESTED LOOPS
0 TABLE ACCESS MODE: ANALYZED (BY INDEX ROWID) OF
'MBR_IDENTFN' (TABLE)
0 INDEX MODE: ANALYZED (UNIQUE SCAN) OF 'UK_CLM_IDFN' (INDEX
(UNIQUE))
0 TABLE ACCESS MODE: ANALYZED (BY INDEX ROWID) OF 'MBR' (TABLE)
0 INDEX MODE: ANALYZED (UNIQUE SCAN) OF 'PK_CLAIMANT' (INDEX
(UNIQUE))
解决方案
索引跳过扫描意味着索引的第一列被忽略。由于Oracle读取了第一列的每个项目，因此此费用性能是您要搜索的第二列（或第三，...）列。这通常比全表扫描快（取决于您的查询），但比 索引范围扫描.
尝试在列上创建一个单独的索引 IDX_MBR_IDENTFN 并在您的查询中使用。
例如，如果 your_table 看起来这样：
id status
1 0
2 0
3 0
4 1
而且您有一个复合索引 (id, status), ，查询 Select * From your_table Where status = 1 可能使用索引，但是为了找到正确的行，它必须读取索引的每一行（ID 1 到 4）并检查 status.
更新: ：以下索引可以进一步提高性能，但是如果确实有帮助，您将不得不尝试：
mbr_identfn( identfd_type, identfd_number, entity_active, mbr_idn )
这甚至可以帮助避免提示。
其他提示
我会将重点从跳过扫描中移开。
TKPROF片段向您展示您的首要任务应该是减少发表此声明的次数。当前，您正在执行此语句9936次。每个执行仅需405/9936秒。合理的快速。但是，如果您执行9936次执行它。
因此，此陈述几乎可以肯定在循环构造中。在每次迭代中，您都提供不同的输入参数集（：IDSIDEFD_TYPE_1，：IDSIDEFD_NUMBER_1，：ENTITY_ACTIVE_1，：RONNUM_1，：ORA_RN_1）。重写此循环构造以使该语句在整个集合中执行一次，并且您的性能问题可能会从过去的一件事。如果没有，请发布新的TKPROF输出。
问候，罗布。
如果您确定索引中的列（PK_Claimant和IDX_MBR_INDENTFN）以及按什么顺序确定。
我怀疑这是一个数据类问题。例如，如果mbr_enterfn.inderfd_type是索引的领先列，是数字的，但是您的：IDSIDEFD_TYPE_1是字符变量（或反之亦然），它变得无法使用。但是，如果类型很少，则可以与跳过扫描一起使用索引。
您还指定谓词“ mbr.mbr_idn = mbr_identfn.mbr_idn”中的where子句和加入子句。
要解释跳过扫描...这似乎是查询的相关谓词部分：
WHERE mbr_identfn.mbr_idn = mbr.mbr_idn
AND mbr_identfn.identfd_type = :identfd_type_1
AND mbr_identfn.identfd_number = :identfd_number_1
AND mbr_identfn.entity_active = :entity_active_1
如果执行以MBR_INDERFN开头，那么我们还没有MBR_IDN在索引中查找的值；这意味着我们不能进行唯一或范围扫描。但是我们为索引的其他三列具有给出的值（作为绑定变量），因此我们可以进行跳过扫描。 Oracle选择这样做是为了避免访问基础表，这似乎是明智的。
MBR_INDENFN的主要键是什么？是单独的mbr_idn吗？
我认为您应该在MBR_INDENFN上具有单独的索引，其中某些或全部sidentfd_type，dissidefd_number和entity_active作为领先列。这将允许进行范围或唯一的扫描，而不是跳过扫描。
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