33.6. 庘昄滷崌攓评䘘

懟婻庘昄鄘橬婔婻滷崌攓亓彆(VOLATILE, STABLE, IMMUTABLE)㔗套悩 CREATE FUNCTION 变傴澇橬滯䇞弄滯评䘘䔇臺VOLATILE 儌滇䚺䩕㔗滷崌攓评䘘滇䂍嚻寡単䔇婔婻噿庯庘昄臯婺䔇欪臺

• VOLATILE 埇傖啔傂嘘庋愙寙拸媞櫹昄扞康㔗垄埇傖婘嘪䫘劯湙埗昄脄䫘施誫啂婉劯䔇䂷悩㔗嚻寡単凹認湙䔇庘昄婉啔傂嘘啺螆㔗婔婻嘪䫘滷崌庘昄䔇昖臵婘驔襕昄扞唚䔇施唍懟渇鄘麉桄螇䞖庘昄䔇唚㔗

• STABLE 庘昄婉嚔媞櫹昄扞康幽婫媺臕婘劯婔婻昖臵䔇䯇嵄麯䂍庺䕩劯埗昄䔇愙喕婋嚔䂍庺䕩劯䔇䂷悩㔗認婻评䘘噕螩嚻寡単婘婔婻昖臵麯檪崔婻庘昄脄䫘嚻寡潊婔婻㔗䬹彆滇婘䘵嚘欆柟䔇溇傽臘膆嚟麯麵寙劆認湙䔇庘昄滇垬噘䔇㔗啹婺䘵嚘欆柟埻螇䞖婔渇懫膄唚蔯婉滇懟臯婔渇㔗婘䘵嚘欆柟溇傽麯嘪䫘婔婻 VOLATILE 庘昄滇麂濘䔇㔗

• IMMUTABLE 庘昄婉嚔媞櫹昄扞康幽婫媺臕婘傂嘘愙喕婋劯湙䔇埗昄愩誩誫啂劯湙䔇䂷悩㔗認婻评䘘噕螩嚻寡単婘昖臵脄䫘庘昄䔇施唍鵇噽檪庘昄螇䞖潊婔婻婩麟埗昄㔗懫套䌂嚚 SELECT ... WHERE x = 2 + 2 䔇昖臵埇傖䞔寡潊 SELECT ... WHERE x = 4 啹婺婘媹濘淉嘩严婋北䔇庘昄滇湺螄婺 IMMUTABLE 䔇㔗

婺庖橔嘿䔇嚻寡䂷悩庫臖儘埇脘嘪䫘橔婖湚䔇滷崌攓评䘘湺螄嘹䔇庘昄㔗

傂嘘橬嬇嘩䫘䔇庘昄鄘媙釂湺螄婺 VOLATILE 認湙凹垄䔇脄䫘儌婉嚔赆嚻寡㔗剿嘪婔婻庘昄澇橬嬇嘩䫘嘖垄䔇昄唚埇脘婘婔婻昖臵麯櫹埻闼幽幘媙釂湺螄婺 VOLATILE 冋套 random(), currval(), timeofday() 庘昄㔗

婘闼底䞔剘䔇蓇彐劯鷸婪欓臯䔇庴庐昖臵婪STABLE 启 IMMUTABLE 澇橬傔幽寺彆庘昄滇婘蓇彐嚔哋施欓臯誻滇婘昖臵嚔哋施欓臯䔇噞彆幽婉崓㔗嘖滇套悩蓇彐赆媺庻幽婫劯準赆麉䫘闼噞彆埇儌崓庖㔗套悩檪婔婻庘昄湺螄婺 IMMUTABLE 蔯垄垂鍙婪埽婉滇闼幽儌嚔凚躘婘锟劯嘪䫘噽蓇彐䔇施唍䫘婪婔婻婉垯昘䔇昄唚㔗套悩婘嘪䫘鵇噽庖崺喘臺埖潡蔙嘪䫘婔䓉䚷喾蓇彐䔇庘昄臺蘔(懫套 PL/pgSQL)闼幽劯悩埇脘冽婖麉㔗

啹婺 MVCC 䔇媆䙓臯婺(埗黙䆹12)婔婻埻寙劆 SELECT 变傴䔇庘昄埇傖垬噘婄湺螄婺 STABLE 剿嘪垄欔锬拷䔇臘埇脘嚔赆噽垄幽埏昖臵媞櫹幘婔湙㔗PostgreSQL 儖嚔婘欓臯 STABLE 庘昄施婺脄䫘垄䔇昖臵傺䆋媆䙓啹溴垄婘臖昖臵䔇䫘庻橘喙鄘嚔䩋彄婔躘䔇昄扞康蓖商㔗誻襕毺庺䔇滇current_timestamp 庘昄斟鄘滇䘿垔䔇啹婺垄傸䔇昄唚婘婔婻庋媇麯滇婉櫹埻䔇㔗

劯湙䔇媆䙓臯婺幘䫘庯 IMMUTABLE 庘昄麯麵䔇 SELECT 变傴㔗锔婩婘婔婻 IMMUTABLE 庘昄麯锬拷婔婻昄扞康䔇臘滇婉滯捺䔇啹婺套悩臘䔇喙垹櫹埻闼幽認䓉婉埻攓儌儖櫹埻㔗婉誺PostgreSQL 幽婉䥕溵嘹認湙啔㔗

婔婻婩蓕䔇髍臇滇檪婔婻庘昄湺螄婺 IMMUTABLE 蔯垂鍙婪認婻庘昄䔇䂷悩冺蕡昊婻陉䘞埗昄㔗懫套婔婻淉嘩施閘潿䔇庘昄埇脘橬冺蕡庯 timezone 螆䘞䔇䂷悩㔗婺庖垬噘蔄荏認湙䔇庘昄庫臖湺螄婺 STABLE 㔗