背景1：
当我们需要查询很多客户的，离当前时间最近订单时
参考网址：
http://bonesmoses.org/2014/05/08/trumping-the-postgresql-query-planner/
创建测试表
CREATETABLEtest_order ( client_idINTNOTNULL,order_dateTIMESTAMPNOTNULL,fillerTEXTNOTNULL );
插入测试数据
INSERTINTOtest_order
SELECTs1.id,(CURRENT_DATE-INTERVAL'1000days')::DATE
+generate_series(1,s1.id%1000),repeat('',20)
FROMgenerate_series(1,10000)s1(id);
CREATEINDEXidx_test_order_client_id_order_date ONtest_order(client_id,order_dateDESC);
执行普通sql
不走索引
EXPLAINANALYZE SELECTclient_id,max(order_date) FROMtest_order GROUPBYclient_id;
"Executiontime:5741.682ms"
使用索引
EXPLAIN ANALYZE SELECT DISTINCT ON (client_id) client_id,order_date FROM test_order ORDER BY client_id,order_date DESC;
"Executiontime:4628.510ms"
优化后sql
EXPLAINANALYZE WITHRECURSIVEskipAS ( (SELECTclient_id,order_date FROMtest_order ORDERBYclient_id,order_dateDESC LIMIT1) UNIONALL (SELECT(SELECTmin(client_id) FROMtest_order WHEREclient_id>skip.client_id )ASclient_id,(SELECTmax(order_date) FROMtest_order WHEREclient_id=( SELECTmin(client_id) FROMtest_order WHEREclient_id>skip.client_id ) )ASorder_date FROMskip WHEREskip.client_idISNOTNULL) ) SELECT* FROMskip;
"Executiontime:865.889ms"
查询结果
client_id;order_date 1;"2014-03-0900:00:00" 2;"2014-03-1000:00:00" 3;"2014-03-1100:00:00" 4;"2014-03-1200:00:00" 5;"2014-03-1300:00:00" 6;"2014-03-1400:00:00" 7;"2014-03-1500:00:00" 8;"2014-03-1600:00:00" 9;"2014-03-1700:00:00" 10;"2014-03-1800:00:00" 11;"2014-03-1900:00:00" 12;"2014-03-2000:00:00" 13;"2014-03-2100:00:00" 14;"2014-03-2200:00:00" 15;"2014-03-2300:00:00" 16;"2014-03-2400:00:00" 17;"2014-03-2500:00:00" 18;"2014-03-2600:00:00" 19;"2014-03-2700:00:00" 20;"2014-03-2800:00:00" 21;"2014-03-2900:00:00" 22;"2014-03-3000:00:00" 23;"2014-03-3100:00:00" 24;"2014-04-0100:00:00"
背景2：
参考网址：
https://yq.aliyun.com/articles/65202?spm=5176.8091938.0.0.tZZBTS
有一个这样的场景，一张小表A，里面存储了一些ID，大约几百个。
（比如说巡逻车辆ID，环卫车辆的ID，公交车，微公交的ID）。
另外有一张日志表B，每条记录中的ID是来自前面那张小表的，但不是每个ID都出现在这张日志表中，比如说一天可能只有几十个ID会出现在这个日志表的当天的数据中。
（比如车辆的行车轨迹数据，每秒上报轨迹，数据量就非常庞大）。
那么我怎么快速的找出今天没有出现的ID呢。
（哪些巡逻车辆没有出现在这个片区，是不是偷懒了？哪些环卫车辆没有出行，哪些公交或微公交没有出行）？
select id from A where id not in (select id from B where time between ? and ?);
这个QUERY会很慢，有什么优化方法呢。
当然，你还可以让车辆签到的方式来解决这个问题，但是总有未签到的，或者没有这种设计的时候，那么怎么解决呢？
--A createtablea(idintprimarykey,infotext); --B createtableb(idintprimarykey,aidint,crt_timetimestamp); createindexb_aidonb(aid); --a表插入1000条 insertintoaselectgenerate_series(1,1000),md5(random()::text); --b表插入500万条，只包含aid的500个id。 insertintobselectgenerate_series(1,5000000),generate_series(1,500),clock_timestamp(); 优化前： select*fromawhereidnotin(selectaidfromb); 执行时间：大于1min 优化后： select*fromawhereidnotin(withrecursiveskipas( ( selectmin(aid)aidfrombwhereaidisnotnull ) unionall ( select(selectmin(aid)aidfrombwhereb.aid>s.aidandb.aidisnotnull) fromskipswheres.aidisnotnull )--这里的wheres.aidisnotnull一定要加,否则就死循环了. ) selectaidfromskipwhereaidisnotnull); 执行时间：46msec
情景3:
生成树形结构
参考网址：
http://blog.databasepatterns.com/2014/02/trees-paths-recursive-cte-postgresql.html
createtablesubregions( idsmallintprimarykey,nametextnotnull,parent_idsmallintnullreferencessubregions(id) ); insertintosubregionsvalues (1,'World',null),(2,'Africa',1),(5,'SouthAmerica',419),(9,'Oceania',(11,'WesternAfrica',2),(13,'CentralAmerica',(14,'EasternAfrica',(15,'NorthernAfrica',(17,'MiddleAfrica',(18,'SouthernAfrica',(19,'Americas',(21,'NorthernAmerica',19),(29,'Caribbean',(30,'EasternAsia',142),(34,'SouthernAsia',(35,'South-EasternAsia',(39,'SouthernEurope',150),(53,'AustraliaandNewZealand',9),(54,'Melanesia',(57,'Micronesia',(61,'Polynesia',(142,'Asia',(143,'CentralAsia',(145,'WesternAsia',(150,'Europe',(151,'EasternEurope',(154,'NorthernEurope',(155,'WesternEurope',(419,'LatinAmericaandtheCaribbean',19); Andyouwantedtomakeaprettytreelikethis: World Africa EasternAfrica MiddleAfrica NorthernAfrica SouthernAfrica WesternAfrica Americas LatinAmericaandtheCaribbean Caribbean CentralAmerica SouthAmerica NorthernAmerica Asia CentralAsia EasternAsia South-EasternAsia SouthernAsia WesternAsia Europe EasternEurope NorthernEurope SouthernEurope WesternEurope Oceania AustraliaandNewZealand Melanesia Micronesia Polynesia Here'showyou'ddoit: withrecursivemy_expressionas( --startwiththe"anchor",i.e.allofthenodeswhoseparent_idisnull: select id,nameaspath,nameastree,0aslevel fromsubregions where parent_idisnull unionall --thentherecursivepart: select current.idasid,prevIoUs.path||'>'||current.nameaspath,prevIoUs.level+1)||current.nameastree,prevIoUs.level+1aslevel fromsubregionscurrent joinmy_expressionasprevIoUsoncurrent.parent_id=prevIoUs.id ) select tree frommy_expression orderby path 路径间加入父节点和分割 select path frommy_expression orderby path 输出结果： World World>Africa World>Africa>EasternAfrica World>Africa>MiddleAfrica World>Africa>NorthernAfrica World>Africa>SouthernAfrica World>Africa>WesternAfrica World>Americas World>Americas>LatinAmericaandtheCaribbean World>Americas>LatinAmericaandtheCaribbean>Caribbean World>Americas>LatinAmericaandtheCaribbean>CentralAmerica World>Americas>LatinAmericaandtheCaribbean>SouthAmerica World>Americas>NorthernAmerica World>Asia World>Asia>CentralAsia World>Asia>EasternAsia World>Asia>South-EasternAsia World>Asia>SouthernAsia World>Asia>WesternAsia World>Europe World>Europe>EasternEurope World>Europe>NorthernEurope World>Europe>SouthernEurope World>Europe>WesternEurope World>Oceania World>Oceania>AustraliaandNewZealand World>Oceania>Melanesia World>Oceania>Micronesia World>Oceania>Polynesia
