最近BA用户反馈有两句看似很像的语句返回的结果数不一样，比较奇怪，怀疑是不是Hive的Bug
Query 1 返回结果数6071
select count(distinct reviewid) as dis_reviewcnt from (select a.reviewid from bi.dpods_dp_reviewreport a left outer join bi.dpods_dp_reviewlog b on a.reviewid=b.reviewid and b.hp_statdate='2013-07-24' where to_date(a.feedadddate) >= '2013-07-01' and a.hp_statdate='2013-07-24' ) a
Query 2 返回结果数6443
select count(distinct reviewid) as dis_reviewcnt from (select a.reviewid from bi.dpods_dp_reviewreport a left outer join bi.dpods_dp_reviewlog b on a.reviewid=b.reviewid and b.hp_statdate='2013-07-24' and a.hp_statdate='2013-07-24' where to_date(a.feedadddate) >= '2013-07-01' ) a
第二条query比第一条多了372条数据，而且在子查询的左表中并不存在
两条语句唯一的区别是dpods_dp_reviewreport的分区过滤条件（hp_statdate是partition column）一个在where后面，另一个在on后面
粗看感觉出来的数据应该是一样的，但是玄机其实就在where和on的区别。
where 后面跟的是过滤条件，query 1 中的a.hp_statdate='2013-07-24', 在table scan之前就会Partition Pruner 过滤分区，所以只有'2013-07-24'下的数据会和dpods_dp_reviewlog进行join。
而query 2中会读入所有partition下的数据，再和dpods_dp_reviewlog join，并且根据join的关联条件只有a.hp_statdate='2013-07-24'的时候才会真正执行join，其余情况下又由于是left outer join, join不上右面会留NULL，query 2中其实是取出了所有的reviewid，所以会和query 1 结果不一样
可以做一个实验，query2去掉on后面的a.hp_statdate='2013-07-24'，其余不动，执行语句，出来的distinct reviewcnt 也是 6443
select count(distinct reviewid) as dis_reviewcnt from (select a.reviewid from bi.dpods_dp_reviewreport a left outer join bi.dpods_dp_reviewlog b on a.reviewid=b.reviewid and b.hp_statdate='2013-07-24' where to_date(a.feedadddate) >= '2013-07-01' ) a
更多精彩内容：http://www.bianceng.cnhttp://www.bianceng.cn/database/extra/
query 1的query plan
ABSTRACT SYNTAX TREE:
(TOK_QUERY (TOK_FROM (TOK_SUBQUERY (TOK_QUERY (TOK_FROM (TOK_LEFTOUTERJOIN (TOK_TABREF (TOK_TABNAME bi dpods_dp_reviewreport) a) (TOK_TABREF (TOK_TABNAME bi dpods_dp_reviewlog) b) (and (= (. (TOK_TABLE_OR_COL a) reviewid) (. (TOK_TABLE_OR_COL b) reviewid)) (= (. (TOK_TABLE_OR_COL b) hp_statdate) '2013-07-24')))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (. (TOK_TABLE_OR_COL a) reviewid))) (TOK_WHERE (and (>= (TOK_FUNCTION to_date (. (TOK_TABLE_OR_COL a) feedadddate)) '2013-07-01') (= (. (TOK_TABLE_OR_COL a) hp_statdate) '2013-07-24'))))) a)) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (TOK_FUNCTIONDI count (TOK_TABLE_OR_COL reviewid)) dis_reviewcnt))))
STAGE DEPENDENCIES:
Stage-5 is a root stage , consists of Stage-1
Stage-1
Stage-2 depends on stages: Stage-1
Stage-0 is a root stage
STAGE PLANS:
Stage: Stage-5
Conditional Operator
Stage: Stage-1
Map Reduce
Alias -> Map Operator Tree:
a:a
TableScan
alias: a
Filter Operator
predicate:
expr: (to_date(feedadddate) >= '2013-07-01')
type: boolean
Reduce Output Operator
key expressions:
expr: reviewid
type: int
sort order: +
Map-reduce partition columns:
expr: reviewid
type: int
tag: 0
value expressions:
expr: feedadddate
type: string
expr: reviewid
type: int
expr: hp_statdate
type: string
a:b
TableScan
alias: b
Reduce Output Operator
key expressions:
expr: reviewid
type: int
sort order: +
Map-reduce partition columns:
expr: reviewid
type: int
tag: 1
Reduce Operator Tree:
Join Operator
condition map:
Left Outer Join0 to 1
condition expressions:
0 {VALUE._col5} {VALUE._col8} {VALUE._col17}
1
handleSkewJoin: false
outputColumnNames: _col5, _col8, _col17
Select Operator
expressions:
expr: _col8
type: int
outputColumnNames: _col0
Select Operator
expressions:
expr: _col0
type: int
outputColumnNames: _col0
Group By Operator
aggregations:
expr: count(DISTINCT _col0)
bucketGroup: false
keys:
expr: _col0
type: int
mode: hash
outputColumnNames: _col0, _col1
File Output Operator
compressed: true
GlobalTableId: 0
table:
input format: org.apache.hadoop.mapred.SequenceFileInputFormat
output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat
Stage: Stage-2
Map Reduce
Alias -> Map Operator Tree:
hdfs://10.2.6.102/tmp/hive-hadoop/hive_2013-07-26_18-10-59_408_7272696604651905662/-mr-10002
Reduce Output Operator
key expressions:
expr: _col0
type: int
sort order: +
tag: -1
value expressions:
expr: _col1
type: bigint
Reduce Operator Tree:
Group By Operator
aggregations:
expr: count(DISTINCT KEY._col0:0._col0)
bucketGroup: false
mode: mergepartial
outputColumnNames: _col0
Select Operator
expressions:
expr: _col0
type: bigint
outputColumnNames: _col0
File Output Operator
compressed: false
GlobalTableId: 0
table:
input format: org.apache.hadoop.mapred.TextInputFormat
output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
Stage: Stage-0
Fetch Operator
limit: -1
Query 2的query plan
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