这篇文章主要为大家展示了“Oracle 12CR2查询转换之星型转换的方法”，内容简而易懂，条理清晰，希望能够帮助大家解决疑惑，下面让小编带领大家一起研究并学习一下“Oracle 12CR2查询转换之星型转换的方法”这篇文章吧。

星型转换是一种优化转换它用来避免对星型方案中的事实表进行全表扫描。一个星型方案将数据分成事实与维度表。事实是对一个事件比如销售的测量通常是数字。维度是标识事实的分类，比如日期，位置与产品。一个事实表有一个由方案中维度表主键所组成的复合键。维度表实际上充当查找或引用表能让你选择你查询所要请求的值。

星型转换的目的
在连接事实表与维度表时，星型转换可能避免对事实表执行完全扫描。星型转换通过只获取连接到约束维度行记录的相关事实行记录来提高性能。在有些情况下，查询已经在维度表的其它列上有限制性过滤了。过滤组合可以大大减少数据库从事实表中要处理的数据集大小。

星型转换的工作原理
星型转换增加了子查询谓词，叫作位图半连接谓词，关联到约束维度表。当在实际连接列上存在索引时优化器执行转换。通过驱动位图and和or来操作由子查询所提供的键值，数据库只需要从事实表中检索相关行记录。如果维度表上的谓词过滤掉了大量数据，那么星型转换比对事实表完全扫描更有效。

在数据库从事实表中检索相关行记录之后，数据库可能需要使用原始谓词连接这些行记录回维度表。当以下条件满足时数据库可以消除连接回维度表:
.维度表上的所有谓词是半连接子查询谓词的一部分
.从子查询中所选择的列具有唯一性
.维度列不在select列，group by子句中等等

控制星型转换
star_transformation_enabled参数控制着星型转换。这个参数有以下参数值:
.true
优化器通过自动识别事实与约束维度表来执行星型转换。只有转换后的执行计划成本比原始执行计划成本低时优化器才执行星型转换。当物化提高性能时优化器也会尝试临时表转换。

.false(缺省值)
优化器不执行星型转换

.temp_disable
这个值与true相同，只是优化器不会尝试临时表转换

星型转换:应用场景
下面的查询找出1999年Q1和Q2季度在California的所有城市中的总的网络销售额:

SELECTc.cust_city,t.calendar_quarter_desc,SUM(s.amount_sold)sales_amountFROMsaless,timest,customersc,channelschWHEREs.time_id=t.time_idANDs.cust_id=c.cust_idANDs.channel_id=ch.channel_idANDc.cust_state_province='CA'ANDch.channel_desc='Internet'ANDt.calendar_quarter_descIN('1999-01','1999-02')GROUPBYc.cust_city,t.calendar_quarter_desc;

示例输出如下:

SQL>showparameterstar_transformation_enabledNAMETYPEVALUE-----------------------------------------------------------------------------star_transformation_enabledstringFALSESQL>SELECTc.cust_city,2t.calendar_quarter_desc,3SUM(s.amount_sold)sales_amount4FROMsaless,5timest,6customersc,7channelsch8WHEREs.time_id=t.time_id9ANDs.cust_id=c.cust_id10ANDs.channel_id=ch.channel_id11ANDc.cust_state_province='CA'12ANDch.channel_desc='Internet'13ANDt.calendar_quarter_descIN('1999-01','1999-02')14GROUPBYc.cust_city,t.calendar_quarter_desc;CUST_CITYCALENDASALES_AMOUNT-------------------------------------------------Montara1999-021618.01Pala1999-013263.93Cloverdale1999-0152.64Cloverdale1999-02266.28SanFrancisco1999-013058.27SanMateo1999-018754.59LosAngeles1999-011886.19SanMateo1999-0221399.42Pala1999-02936.62ElSobrante1999-023744.03ElSobrante1999-015392.34Quartzhill1999-01987.3Legrand1999-0126.32Pescadero1999-0126.32Arbuckle1999-02241.2Quartzhill1999-02412.83Montara1999-01289.07Arbuckle1999-01270.08SanFrancisco1999-0211257LosAngeles1999-022128.59Pescadero1999-02298.44Legrand1999-0218.6622rowsselected.SQL>select*fromtable(dbms_xplan.display_cursor(null,null,'advancedallstatslastrunstats_lastpeeked_binds'));SQL_IDa069wzk60bbqd,childnumber1-------------------------------------SELECTc.cust_city,t.calendar_quarter_desc,SUM(s.amount_sold)sales_amountFROMsaless,timest,customersc,channelschWHEREs.time_id=t.time_idANDs.cust_id=c.cust_idANDs.channel_id=ch.channel_idANDc.cust_state_province='CA'ANDch.channel_desc='Internet'ANDt.calendar_quarter_descIN('1999-01','1999-02')GROUPBYc.cust_city,t.calendar_quarter_descPlanhashvalue:1865285285---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|Id|Operation|Name|Starts|E-Rows|E-Bytes|Cost(%CPU)|E-Time|Pstart|Pstop|A-Rows|A-Time|Buffers|OMem|1Mem|Used-Mem|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|0|SELECTSTATEMENT||1|||957(100)||||22|00:00:00.18|1812|||||1|HASHGROUPBY||1|22|1672|957(2)|00:00:01|||22|00:00:00.18|1812|1022K|1022K|1382K(0)||*2|HASHJOIN||1|138|10488|956(2)|00:00:01|||964|00:00:00.14|1812|1538K|1538K|1588K(0)||3|PARTJOINFILTERCREATE|:BF0000|1|183|2928|18(0)|00:00:01|||181|00:00:00.01|65|||||*4|TABLEACCESSFULL|TIMES|1|183|2928|18(0)|00:00:01|||181|00:00:00.01|65|||||*5|HASHJOIN||1|964|57840|938(2)|00:00:01|||964|00:00:00.11|1747|1448K|1448K|1521K(0)||6|MERGEJOINCARTESIAN||1|3341|127K|426(1)|00:00:01|||3341|00:00:00.02|1531|||||*7|TABLEACCESSFULL|CHANNELS|1|1|13|3(0)|00:00:01|||1|00:00:00.01|9|||||8|BUFFERSORT||1|3341|86866|423(1)|00:00:01|||3341|00:00:00.02|1522|178K|178K|158K(0)||*9|TABLEACCESSFULL|CUSTOMERS|1|3341|86866|423(1)|00:00:01|||3341|00:00:00.01|1522|||||10|PARTITIONRANGEJOIN-FILTER||1|819K|16M|510(2)|00:00:01|:BF0000|:BF0000|118K|00:00:00.02|216|||||11|TABLEACCESSFULL|SALES|2|819K|16M|510(2)|00:00:01|:BF0000|:BF0000|118K|00:00:00.02|216||||---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

从上面的执行中可以看到，在没有使用星型转换时id=11，对表sales执行的是全表扫描。在这个例子中，sales表是事实表，并且其它的表是维度表。sales表对于每一个销售的产品都有一行记录，因此它可以包含上十亿行销售记录。然而，只有少量产品在指定的季度通过网络销售到了California的客户手中。

使用星形转换

SQL>altersessionsetstar_transformation_enabled='true';Sessionaltered.SQL>SELECTc.cust_city,2t.calendar_quarter_desc,3SUM(s.amount_sold)sales_amount4FROMsaless,5timest,6customersc,7channelsch8WHEREs.time_id=t.time_id9ANDs.cust_id=c.cust_id10ANDs.channel_id=ch.channel_id11ANDc.cust_state_province='CA'12ANDch.channel_desc='Internet'13ANDt.calendar_quarter_descIN('1999-01','1999-02')14GROUPBYc.cust_city,t.calendar_quarter_desc;Montara1999-021618.01Pala1999-013263.93Cloverdale1999-0152.64Cloverdale1999-02266.28SanFrancisco1999-013058.27SanMateo1999-018754.59LosAngeles1999-011886.19SanMateo1999-0221399.42Pala1999-02936.62ElSobrante1999-023744.03ElSobrante1999-015392.34Quartzhill1999-01987.3Legrand1999-0126.32Pescadero1999-0126.32Arbuckle1999-02241.2Quartzhill1999-02412.83Montara1999-01289.07Arbuckle1999-01270.08SanFrancisco1999-0211257LosAngeles1999-022128.59Pescadero1999-02298.44Legrand1999-0218.6622rowsselected.

从10053跟踪文件中找到的星型转换后的语句如下:

ST:Queryafterstarxformation:*******UNPARSEDQUERYIS*******SELECT/*+CACHE(T1)*/T1.C1CUST_CITY,T.CALENDAR_QUARTER_DESCCALENDAR_QUARTER_DESC,SUM(S.AMOUNT_SOLD)SALES_AMOUNTFROMSH.SALESS,SH.TIMEST,SYS.SYS_TEMP_0FD9D6684_63D6F82T1WHERES.CUST_ID=ANY(SELECT/*+SEMIJOIN_DRIVERCACHE(T1)*/T1.C0C0FROMSYS.SYS_TEMP_0FD9D6684_63D6F82T1)ANDS.CHANNEL_ID=ANY(SELECT/*+SEMIJOIN_DRIVER*/CH.CHANNEL_IDITEM_1FROMSH.CHANNELSCHWHERECH.CHANNEL_DESC='Internet')ANDS.TIME_ID=ANY(SELECT/*+SEMIJOIN_DRIVER*/T.TIME_IDITEM_1FROMSH.TIMESTWHERET.CALENDAR_QUARTER_DESC='1999-01'ORT.CALENDAR_QUARTER_DESC='1999-02')ANDS.TIME_ID=T.TIME_IDANDS.CUST_ID=T1.C0AND(T.CALENDAR_QUARTER_DESC='1999-01'ORT.CALENDAR_QUARTER_DESC='1999-02')GROUPBYT1.C1,T.CALENDAR_QUARTER_DESC

其执行计划如下:

SQL>select*fromtable(dbms_xplan.display_cursor(null,null,'advancedallstatslastrunstats_lastpeeked_binds'));SQL_IDa069wzk60bbqd,childnumber3-------------------------------------SELECTc.cust_city,t.calendar_quarter_desc,SUM(s.amount_sold)sales_amountFROMsaless,timest,customersc,channelschWHEREs.time_id=t.time_idANDs.cust_id=c.cust_idANDs.channel_id=ch.channel_idANDc.cust_state_province='CA'ANDch.channel_desc='Internet'ANDt.calendar_quarter_descIN('1999-01','1999-02')GROUPBYc.cust_city,t.calendar_quarter_descPlanhashvalue:2164696140------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|Id|Operation|Name|Starts|E-Rows|E-Bytes|Cost(%CPU)|E-Time|Pstart|Pstop|A-Rows|A-Time|Buffers|Reads|Writes|OMem|1Mem|Used-Mem|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|0|SELECTSTATEMENT||1|||573(100)||||22|00:00:01.41|9083|96|10|||||1|TEMPTABLETRANSFORMATION||1|||||||22|00:00:01.41|9083|96|10|||||2|LOADASSELECT|SYS_TEMP_0FD9D667F_63D6F82|1|||||||0|00:00:01.07|1539|2|10|1042K|1042K|||*3|TABLEACCESSFULL|CUSTOMERS|1|383|9958|423(1)|00:00:01|||3341|00:00:00.01|1522|0|0|||||4|HASHGROUPBY||1|542|30894|150(1)|00:00:01|||22|00:00:00.33|7538|93|0|1022K|1022K|1346K(0)||*5|HASHJOIN||1|1681|95817|149(0)|00:00:01|||964|00:00:00.31|7538|93|0|1572K|1572K|1677K(0)||6|TABLEACCESSFULL|SYS_TEMP_0FD9D667F_63D6F82|1|383|5745|2(0)|00:00:01|||3341|00:00:00.01|18|10|0|||||*7|HASHJOIN||1|1681|70602|147(0)|00:00:01|||964|00:00:00.29|7520|83|0|1538K|1538K|1686K(0)||*8|TABLEACCESSFULL|TIMES|1|183|2928|18(0)|00:00:01|||181|00:00:00.01|65|0|0|||||9|VIEW|VW_ST_A3F94988|1|1685|43810|129(0)|00:00:01|||964|00:00:00.23|7455|83|0|||||10|NESTEDLOOPS||1|1685|96045|106(0)|00:00:01|||964|00:00:00.23|7455|83|0|||||11|PARTITIONRANGESUBQUERY||1|1684|47167|52(0)|00:00:01|KEY(SQ)|KEY(SQ)|964|00:00:00.22|7271|83|0|||||12|BITMAPCONVERSIONTOROWIDS||2|1684|47167|52(0)|00:00:01|||964|00:00:00.21|7204|83|0|||||13|BITMAPAND||2|||||||2|00:00:00.21|7204|83|0|||||14|BITMAPMERGE||2|||||||2|00:00:00.05|15|10|0|1024K|512K|4096(0)||15|BITMAPKEYITERATION||2|||||||2|00:00:00.04|15|10|0|||||16|BUFFERSORT||2|||||||2|00:00:00.01|9|0|0|73728|73728|||*17|TABLEACCESSFULL|CHANNELS|1|1|13|3(0)|00:00:01|||1|00:00:00.01|9|0|0|||||*18|BITMAPINDEXRANGESCAN|SALES_CHANNEL_BIX|2|||||KEY(SQ)|KEY(SQ)|2|00:00:00.04|6|10|0|||||19|BITMAPMERGE||2|||||||2|00:00:00.03|445|10|0|1024K|512K|39936(0)||20|BITMAPKEYITERATION||2|||||||181|00:00:00.02|445|10|0|||||21|BUFFERSORT||2|||||||362|00:00:00.01|65|0|0|73728|73728|||*22|TABLEACCESSFULL|TIMES|1|183|2928|18(0)|00:00:01|||181|00:00:00.01|65|0|0|||||*23|BITMAPINDEXRANGESCAN|SALES_TIME_BIX|362|||||KEY(SQ)|KEY(SQ)|181|00:00:00.01|380|10|0|||||24|BITMAPMERGE||2|||||||2|00:00:00.14|6744|63|0|1024K|512K|45056(0)||25|BITMAPKEYITERATION||2|||||||403|00:00:00.14|6744|63|0|||||26|BUFFERSORT||2|||||||6682|00:00:00.01|18|0|0|5512K|964K|174K(0)||27|TABLEACCESSFULL|SYS_TEMP_0FD9D667F_63D6F82|1|383|1915|2(0)|00:00:01|||3341|00:00:00.01|18|0|0|||||*28|BITMAPINDEXRANGESCAN|SALES_CUST_BIX|6682|||||KEY(SQ)|KEY(SQ)|403|00:00:00.10|6726|63|0|||||29|TABLEACCESSBYUSERROWID|SALES|964|1|29|77(2)|00:00:01|ROWID|ROWID|964|00:00:00.01|184|0|0||||------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------QueryBlockName/ObjectAlias(identifiedbyoperationid):-------------------------------------------------------------1-SEL$D5EF75992-SEL$F6045C7B3-SEL$F6045C7B/C@SEL$F6045C7B6-SEL$D5EF7599/T1@SEL$9C741BEB8-SEL$D5EF7599/T@SEL$19-SEL$5E9A798F/VW_ST_A3F94988@SEL$D5EF759910-SEL$5E9A798F12-SEL$5E9A798F/S@SEL$117-SEL$6EE793B7/CH@SEL$6EE793B722-SEL$ACF30367/T@SEL$ACF3036727-SEL$E1F9C76C/T1@SEL$E1F9C76C29-SEL$5E9A798F/SYS_CP_S@SEL$5E9A798FOutlineData-------------/*+BEGIN_OUTLINE_DATAIGNORE_OPTIM_EMBEDDED_HINTSOPTIMIZER_FEATURES_ENABLE('12.2.0.1')DB_VERSION('12.2.0.1')OPT_PARAM('star_transformation_enabled''true')ALL_ROWSNO_PARALLELOUTLINE_LEAF(@"SEL$F6045C7B")OUTLINE_LEAF(@"SEL$ACF30367")OUTLINE_LEAF(@"SEL$6EE793B7")OUTLINE_LEAF(@"SEL$E1F9C76C")OUTLINE_LEAF(@"SEL$5E9A798F")TABLE_LOOKUP_BY_NL(@"SEL$0E028FD0""S"@"SEL$1")OUTLINE_LEAF(@"SEL$D5EF7599")OUTLINE(@"SEL$1")OUTLINE(@"SEL$0E028FD0")OUTLINE(@"SEL$C3AF6D21")ELIMINATE_JOIN(@"SEL$1""CH"@"SEL$1")OUTLINE(@"SEL$5208623C")STAR_TRANSFORMATION(@"SEL$1""S"@"SEL$1"SUBQUERIES(("T"@"SEL$1")("CH"@"SEL$1")TEMP_TABLE("C"@"SEL$1")))FULL(@"SEL$D5EF7599""T"@"SEL$1")NO_ACCESS(@"SEL$D5EF7599""VW_ST_A3F94988"@"SEL$D5EF7599")FULL(@"SEL$D5EF7599""T1"@"SEL$9C741BEB")LEADING(@"SEL$D5EF7599""T"@"SEL$1""VW_ST_A3F94988"@"SEL$D5EF7599""T1"@"SEL$9C741BEB")USE_HASH(@"SEL$D5EF7599""VW_ST_A3F94988"@"SEL$D5EF7599")USE_HASH(@"SEL$D5EF7599""T1"@"SEL$9C741BEB")SWAP_JOIN_INPUTS(@"SEL$D5EF7599""T1"@"SEL$9C741BEB")USE_HASH_AGGREGATION(@"SEL$D5EF7599")BITMAP_AND(@"SEL$5E9A798F""S"@"SEL$1"("SALES"."CHANNEL_ID")1)BITMAP_AND(@"SEL$5E9A798F""S"@"SEL$1"("SALES"."TIME_ID")2)BITMAP_AND(@"SEL$5E9A798F""S"@"SEL$1"("SALES"."CUST_ID")3)ROWID(@"SEL$5E9A798F""SYS_CP_S"@"SEL$5E9A798F")LEADING(@"SEL$5E9A798F""S"@"SEL$1""SYS_CP_S"@"SEL$5E9A798F")SUBQUERY_PRUNING(@"SEL$5E9A798F""S"@"SEL$1"PARTITION)USE_NL(@"SEL$5E9A798F""SYS_CP_S"@"SEL$5E9A798F")FULL(@"SEL$E1F9C76C""T1"@"SEL$E1F9C76C")SEMIJOIN_DRIVER(@"SEL$E1F9C76C")FULL(@"SEL$6EE793B7""CH"@"SEL$6EE793B7")SEMIJOIN_DRIVER(@"SEL$6EE793B7")FULL(@"SEL$ACF30367""T"@"SEL$ACF30367")SEMIJOIN_DRIVER(@"SEL$ACF30367")FULL(@"SEL$F6045C7B""C"@"SEL$F6045C7B")SEMIJOIN_DRIVER(@"SEL$F6045C7B")END_OUTLINE_DATA*/PredicateInformation(identifiedbyoperationid):---------------------------------------------------3-filter("C"."CUST_STATE_PROVINCE"='CA')5-access("ITEM_1"="C0")7-access("ITEM_2"="T"."TIME_ID")8-filter(("T"."CALENDAR_QUARTER_DESC"='1999-01'OR"T"."CALENDAR_QUARTER_DESC"='1999-02'))17-filter("CH"."CHANNEL_DESC"='Internet')18-access("S"."CHANNEL_ID"="CH"."CHANNEL_ID")22-filter(("T"."CALENDAR_QUARTER_DESC"='1999-01'OR"T"."CALENDAR_QUARTER_DESC"='1999-02'))23-access("S"."TIME_ID"="T"."TIME_ID")28-access("S"."CUST_ID"="C0")ColumnProjectionInformation(identifiedbyoperationid):-----------------------------------------------------------1-"C1"[VARCHAR2,30],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7],SUM("ITEM_3")[22]2-SYSDEF[4],SYSDEF[0],SYSDEF[1],SYSDEF[120],SYSDEF[0]3-"C"."CUST_ID"[NUMBER,22],"C"."CUST_CITY"[VARCHAR2,30],"C"."CUST_STATE_PROVINCE"[VARCHAR2,40]4-"C1"[VARCHAR2,30],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7],SUM("ITEM_3")[22]5-(#keys=1;rowset=256)"C0"[NUMBER,22],"ITEM_1"[NUMBER,22],"C1"[VARCHAR2,30],"T"."TIME_ID"[DATE,7],"ITEM_2"[DATE,7],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7],"ITEM_3"[NUMBER,22]6-(rowset=256)"C0"[NUMBER,22],"C1"[VARCHAR2,30]7-(#keys=1;rowset=256)"T"."TIME_ID"[DATE,7],"ITEM_2"[DATE,7],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7],"ITEM_1"[NUMBER,22],"ITEM_3"[NUMBER,22]8-(rowset=256)"T"."TIME_ID"[DATE,7],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7]9-"ITEM_1"[NUMBER,22],"ITEM_2"[DATE,7],"ITEM_3"[NUMBER,22]10-ROWID[ROWID,10],ROWID[ROWID,10],"S"."CUST_ID"[NUMBER,22],"S"."TIME_ID"[DATE,7],"S"."AMOUNT_SOLD"[NUMBER,22]11-ROWID[ROWID,10]12-ROWID[ROWID,10]13-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,32496]14-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,32496]15-STRDEF[10],STRDEF[10],STRDEF[7920],"S"."CHANNEL_ID"[NUMBER,22]16-(#keys=2)"CH"."CHANNEL_ID"[NUMBER,22],"CH"."CHANNEL_DESC"[VARCHAR2,20]17-(rowset=256)"CH"."CHANNEL_ID"[NUMBER,22],"CH"."CHANNEL_DESC"[VARCHAR2,20]18-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,7920],"S"."CHANNEL_ID"[NUMBER,22]19-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,32496]20-STRDEF[10],STRDEF[10],STRDEF[7920],"S"."TIME_ID"[DATE,7]21-(#keys=2)"T"."TIME_ID"[DATE,7],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7]22-(rowset=256)"T"."TIME_ID"[DATE,7],"T"."CALENDAR_QUARTER_DESC"[CHARACTER,7]23-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,7920],"S"."TIME_ID"[DATE,7]24-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,32496]25-STRDEF[10],STRDEF[10],STRDEF[7920],"S"."CUST_ID"[NUMBER,22]26-(#keys=1)"C0"[NUMBER,22]27-(rowset=256)"C0"[NUMBER,22]28-STRDEF[BMVAR,10],STRDEF[BMVAR,10],STRDEF[BMVAR,7920],"S"."CUST_ID"[NUMBER,22]29-ROWID[ROWID,10],"S"."CUST_ID"[NUMBER,22],"S"."TIME_ID"[DATE,7],"S"."AMOUNT_SOLD"[NUMBER,22]Note------automaticDOP:ComputedDegreeofParallelismis1becauseofparallelthreshold-cbqtstartransformationusedforthisstatement-thisisanadaptiveplan

从Note部分的cbqt star transformation used for this statement信息可知执行了星型转换，从执行计划中的ID=29这个步骤可知对表sales使用了索引扫描而不是全表扫描。对于子查询中的times(第22行),customers(第3行),channels（第17行）表中的每个键值，数据库使用事实表sales(第23,28,18行)上索引检索位图。

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