如果我使用UNION，则测试查询的成本为910，如果使用UNION ALL，则成本为46726。我使用PL / SQLDeveloper。据我了解，UNION ALL应该花费更少，因为它不会花费时间删除重复的行。有人可以向我解释为什么用UNION查询更快吗？
My test query is:
SELECT RELATED_ORDR.ORDR_ID ID
,RELATED_ORDR.ORDR_ID
,EXT_TYPE TYPE
,DECODE(RELATION, 1, GOR_CONTINGENCY, CONTINGENCY) CONTINGENCY_TYPE
,RELATION
,DECODE(SIGN(ORDR_QTY),1,'B',-1,'S') BSS
,ORDR_QTY QUANTITY
,FXRE2_ORDER_AUDIT_REPORT.getOrderStatus(EXT_STATUS) STATUS
,STATUS_DT DT
,TRADE_ID TICKET
,CASE WHEN EXT_TYPE IN ('OR', 'CR', 'RE', 'RTE') THEN RATE2
ELSE RATE
END RATE
,OFFER_ID
FROM
(SELECT DISTINCT RO.ID ORDR_ID
,RO.CONTINGENCY_TYPE CONTINGENCY
,DECODE(RO.ID, GOR.ORDR_ID_PRIMARY, 1,
DECODE(RO.CONTINGENCY_ID, GOR.CONTINGENCY_ID, 3,
DECODE(RO.CONTINGENCY_ID, GOR.ORDR_ID, 2,
DECODE(RO.ORDR_ID_PRIMARY, GOR.ORDR_ID, 2)))) RELATION
,GOR.CONTINGENCY_TYPE GOR_CONTINGENCY
FROM /*GIVEN_ORDR GOR --GIVEN*/
(SELECT to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,1)) as ORDR_ID
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,2)) as ORDR_ID_PRIMARY
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,3)) as CONTINGENCY_ID
, regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,4) as CONTINGENCY_TYPE
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,5)) as ACCT_ID
FROM dual t) GOR --GIVEN
,ORDR_HIST RO --RELATED/LINKED
WHERE GOR.ACCT_ID = RO.ACCT_ID
AND RO.ID != GOR.ORDR_ID-- NOT GIVEN ORDR
AND RO.STATUS_DT IS NOT NULL
AND (RO.ID = GOR.ORDR_ID_PRIMARY
OR RO.CONTINGENCY_ID = GOR.CONTINGENCY_ID
OR RO.CONTINGENCY_ID = GOR.ORDR_ID
OR RO.ORDR_ID_PRIMARY = GOR.ORDR_ID)) RELATED_ORDR
,ORDR_HIST
WHERE ORDR_HIST.ID = (SELECT MAX(ID) FROM ORDR_HIST WHERE ID = RELATED_ORDR.ORDR_ID)
UNION ALL
SELECT RELATED_ORDR.ORDR_ID ID -- just for compatibility, it is not used
,RELATED_ORDR.ORDR_ID
,EXT_TYPE TYPE
,DECODE(RELATION, 1, GOR_CONTINGENCY, CONTINGENCY) CONTINGENCY_TYPE
,RELATION
,DECODE(SIGN(ORDR_QTY),1,'B',-1,'S') BSS
,ORDR_QTY QUANTITY
,FXRE2_ORDER_AUDIT_REPORT.getOrderStatus(EXT_STATUS) STATUS
,STATUS_DT DT
,TRADE_ID TICKET
,CASE WHEN EXT_TYPE IN ('OR', 'CR', 'RE', 'RTE') THEN RATE2
ELSE RATE
END RATE
,OFFER_ID
FROM
(SELECT DISTINCT RO.ID ORDR_ID
,RO.CONTINGENCY_TYPE CONTINGENCY
,DECODE(RO.ID, GOR.ORDR_ID_PRIMARY, 1,
DECODE(RO.CONTINGENCY_ID, GOR.CONTINGENCY_ID, 3,
DECODE(RO.CONTINGENCY_ID, GOR.ORDR_ID, 2,
DECODE(RO.ORDR_ID_PRIMARY, GOR.ORDR_ID, 2)))) RELATION
,GOR.CONTINGENCY_TYPE GOR_CONTINGENCY
FROM /*GIVEN_ORDR GOR --GIVEN*/
(SELECT to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,1)) as ORDR_ID
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,2)) as ORDR_ID_PRIMARY
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,3)) as CONTINGENCY_ID
, regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,4) as CONTINGENCY_TYPE
,to_number(regexp_substr('21153473;21151187;21151187;3;5240435','[^;]+',1,5)) as ACCT_ID
FROM dual t) GOR --GIVEN
,ORDR_HIST RO --RELATED/LINKED
WHERE GOR.ACCT_ID = RO.ACCT_ID
AND RO.ID != GOR.ORDR_ID-- NOT GIVEN ORDR
AND RO.STATUS_DT IS NOT NULL
AND (RO.ID = GOR.ORDR_ID_PRIMARY
OR RO.CONTINGENCY_ID = GOR.CONTINGENCY_ID
OR RO.CONTINGENCY_ID = GOR.ORDR_ID
OR RO.ORDR_ID_PRIMARY = GOR.ORDR_ID)) RELATED_ORDR
,ORDR
WHERE ORDR.ID = (SELECT MAX(ID) FROM ORDR_HIST WHERE ID = RELATED_ORDR.ORDR_ID)
ORDER BY ID;}
The plans of 2 queries are:
UNION 0.078sec COST CARD. BYTES CPU COST IO COST TEMP SPACE
SELECT STATEMENT 910 4528 271695 214479428 905
UNION ALL 0.032sec
SELECT STATEMENT 46726 3194117 198026200 970740819 46626
SELECT STATEMENT 910 4528 271695 214479428 905
SORT UNIQUE 909 4528 271695 172294190 905
UNION-ALL
HASH JOIN 551 1 75 45895432 550
NESTED LOOPS 551 1 75 45895432 550
NESTED LOOPS 551 1 75 45895432 550
STATISTICS COLLECTOR
NESTED LOOPS 452 1 33 45187910 451
VIEW U1R2_DEV 353 1 20 44482687 352
HASH UNIQUE 353 1 18 44482687 352
NESTED LOOPS 352 1 18 4775046 352
FAST DUAL 2 1 7271 2
PARTITION RANGE ALL 350 1 18 4767775 350
TABLE ACCESS BY LOCAL INDEX ROWID BATCHED U1R2 ORDR_HIST 350 1 18 4767775 350
INDEX RANGE SCAN U1R2 OH_ACCT_ID_IDX 101 1285 977115 101
VIEW PUSHED PREDICATE SYS VW_SQ_1 99 1 13 705223 99
FILTER
SORT AGGREGATE 1 6
PARTITION RANGE ALL 99 1 6 705223 99
INDEX RANGE SCAN U1R2 ORDR_HIST2_ACCT_ID 99 1 6 705223 99
PARTITION RANGE ALL 98 1 698951 98
INDEX RANGE SCAN U1R2 ORDR_HIST2_ACCT_ID 98 1 698951 98
TABLE ACCESS BY LOCAL INDEX ROWID U1R2 ORDR_HIST 99 1 42 707523 99
PARTITION RANGE ALL 99 1 42 707523 99
TABLE ACCESS FULL U1R2 ORDR_HIST 99 1 42 707523 99
NESTED LOOPS 356 1 60 44506492 355
NESTED LOOPS 356 1 60 44506492 355
VIEW U1R2_DEV 353 1 20 44482687 352
HASH UNIQUE 353 1 18 44482687 352
NESTED LOOPS 352 1 18 4775046 352
FAST DUAL 2 1 7271 2
PARTITION RANGE ALL 350 1 18 4767775 350
TABLE ACCESS BY LOCAL INDEX ROWID BATCHED U1R2 ORDR_HIST 350 1 18 4767775 350
INDEX RANGE SCAN U1R2 OH_ACCT_ID_IDX 101 1285 977115 101
INDEX RANGE SCAN U1R2 ORDR_AIM 2 1 15293 2
PARTITION RANGE ALL MIN/MAX 1 6
SORT AGGREGATE 1 6
FIRST ROW 99 1 6 705223 99
INDEX RANGE SCAN (MIN/MAX) U1R2 ORDR_HIST2_ACCT_ID 99 1 6 705223 99
TABLE ACCESS BY INDEX ROWID U1R2 ORDR 3 1 40 23804 3
UNION ALL 0.032sec
SELECT STATEMENT 46726 3194117 198026200 3970740819 46626
SORT ORDER BY 808 3194117 198026200 89696701 806 269771000
UNION-ALL
NESTED LOOPS 452 1 62 45190210 451
NESTED LOOPS 452 1 62 45190210 451
VIEW U1R2_DEV 353 1 20 44482687 352
HASH UNIQUE 353 1 18 44482687 352
NESTED LOOPS 352 1 18 4775046 352
FAST DUAL 2 1 7271 2
PARTITION RANGE ALL 350 1 18 4767775 350
TABLE ACCESS BY LOCAL INDEX ROWID BATCHED U1R2 ORDR_HIST 350 1 18 4767775 350
INDEX RANGE SCAN U1R2 OH_ACCT_ID_IDX 101 1285 977115 101
PARTITION RANGE ALL 98 1 698951 98
INDEX RANGE SCAN U1R2 ORDR_HIST2_ACCT_ID 98 1 698951 98
PARTITION RANGE ALL MIN/MAX 1 6
SORT AGGREGATE 1 6
FIRST ROW 99 1 6 705223 99
INDEX RANGE SCAN (MIN/MAX) U1R2 ORDR_HIST2_ACCT_ID 99 1 6 705223 99
TABLE ACCESS BY LOCAL INDEX ROWID U1R2 ORDR_HIST 99 1 42 707523 99
NESTED LOOPS 356 1 60 44506492 355
NESTED LOOPS 356 1 60 44506492 355
VIEW U1R2_DEV 353 1 20 44482687 352
HASH UNIQUE 353 1 18 44482687 352
NESTED LOOPS 352 1 18 4775046 352
FAST DUAL 2 1 7271 2
PARTITION RANGE ALL 350 1 18 4767775 350
TABLE ACCESS BY LOCAL INDEX ROWID BATCHED U1R2 ORDR_HIST 350 1 18 4767775 350
INDEX RANGE SCAN U1R2 OH_ACCT_ID_IDX 101 1285 977115 101
INDEX RANGE SCAN U1R2 ORDR_AIM 2 1 15293 2
PARTITION RANGE ALL MIN/MAX 1 6
SORT AGGREGATE 1 6
FIRST ROW 99 1 6 705223 99
INDEX RANGE SCAN (MIN/MAX) U1R2 ORDR_HIST2_ACCT_ID 99 1 6 705223 99
TABLE ACCESS BY INDEX ROWID U1R2 ORDR 3 1 40 23804 3
答案 0 :(得分：0)
Union和Union all之间的区别如下。
UNION确保您从两个表中都获得DISTINCT记录。 UNION ALL从两个表中提取出所有重复的记录。
您的全部查询成本较高的原因可能是Oracle正在获取更多行， 请检查两个查询的解释计划，您可能会发现“并集所有查询”可能比并集查询做更多的处理。
答案 1 :(得分：0)
UNION 0.078sec --->执行需要更多时间。
UNION ---->使用的资源较少。
成本卡。字节CPU成本IO成本温度空间
选择语句910 4528 271695 214479428 905
UNION ALL 0.032sec --->减少执行时间。 UNION ALL --->使用了更多的资源。 成本卡。字节CPU成本IO成本温度空间 选择语句46726 3194117 198026200 970740819 46626
希望这可以澄清您的疑问。