{"id":72265,"date":"2020-03-03T08:42:17","date_gmt":"2020-03-03T05:42:17","guid":{"rendered":"https:\/\/prohoster.info\/blog\/postgresql-antipatterns-menyaem-dannye-v-obhod-triggera"},"modified":"2020-03-03T16:13:51","modified_gmt":"2020-03-03T13:13:51","slug":"postgresql-antipatterns-menyaem-dannye-v-obhod-triggera","status":"publish","type":"post","link":"https:\/\/prohoster.info\/nl\/blog\/administrirovanie\/postgresql-antipatterns-menyaem-dannye-v-obhod-triggera","title":{"rendered":"PostgreSQL Antipatterns: gegevens wijzigen om triggers te omzeilen","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p>Vroeg of laat wordt iedereen geconfronteerd met de noodzaak om iets in de records van een tabel massaal te corrigeren. Ik heb al <noindex><a rel=\"nofollow\" href=\"https:\/\/habr.com\/post\/481610\/\">verteld hoe je dit het beste kunt doen<\/a><\/noindex>, en hoe vooral niet. Vandaag zal ik het hebben over de tweede kant van massale updates \u2014 <b>de uitvoering van triggers.<\/b>.<\/p>\n<p>Bijvoorbeeld, op de tabel waarin je iets moet corrigeren, hangt er een kwade trigger <code>ON UPDATE<\/code>, die alle wijzigingen naar bepaalde aggregaten verplaatst. En je moet alles bijwerken (bijvoorbeeld een nieuw veld initialiseren) op een manier die deze aggregaten niet raakt.<\/p>\n<h2>Laten we gewoon de triggers uitschakelen!<\/h2>\n<p><\/p>\n<pre><code class=\"sql\">BEGIN;\n  ALTER TABLE ... DISABLE TRIGGER ...;\n  UPDATE ...; -- hier duurt het lang\n  ALTER TABLE ... ENABLE TRIGGER ...;\nCOMMIT;<\/code><\/pre>\n<p>\nEigenlijk is dit alles \u2014 <b>alles hangt al<\/b>.<\/p>\n<p>Want <code>ALTER TABLE<\/code> legt een <b>AccessExclusive<\/b>-de vergrendeling, waaronder niemand die parallel uitgevoerd wordt, zelfs een eenvoudige <code>SELECT<\/code>, kan niets uit de tabel lezen. Dat wil zeggen, totdat deze transactie is voltooid, moeten alle ge\u00efnteresseerden zelfs \"gewoon lezen\" wachten. En we herinneren ons dat <code>UPDATE<\/code> we een ve-e-eel te lange\u2026<br \/>\n<noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><\/p>\n<h4>Laten we dan snel uitschakelen, en dan snel weer inschakelen!<\/h4>\n<p><\/p>\n<pre><code class=\"sql\">BEGIN;\n  ALTER TABLE ... DISABLE TRIGGER ...;\nCOMMIT;\n\nUPDATE ...;\n\nBEGIN;\n  ALTER TABLE ... ENABLE TRIGGER ...;\nCOMMIT;<\/code><\/pre>\n<p>\nHier is de situatie al beter, de wachttijd is aanzienlijk korter. Maar er zijn twee problemen die de hele schoonheid verpesten:<\/p>\n<ul>\n<li><code>ALTER TABLE<\/code> de trigger wacht op alle andere operaties op de tabel, inclusief lange <code>SELECT<\/code><\/li>\n<li>Zolang de trigger uitgeschakeld is, <b>\"ontglipt\" elke wijziging<\/b> in de tabel, zelfs niet onze. En het komt helemaal niet in de aggregaten, hoewel het zou moeten. Een probleem!<\/li>\n<\/ul>\n<p><\/p>\n<h2>Beheer van sessievariabelen<\/h2>\n<p>\nDus, in de vorige optie zijn we tegen een principieel punt aangelopen \u2014 we moeten de trigger leren om \"onze\" wijzigingen in de tabel te onderscheiden van \"niet onze\". \"Onze\" ongewijzigd doorlaten, en voor \"niet onze\" activeren. Hiervoor kunnen we gebruikmaken van <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client\">sessievariabelen<\/a><\/noindex>.<\/p>\n<h4>session_replication_role<\/h4>\n<p>\nLees meer <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/sql-altertable\">handleiding<\/a><\/noindex>:<\/p>\n<blockquote><p>Het mechanisme voor het activeren van triggers wordt ook be\u00efnvloed door de configuratievariabele <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client#GUC-SESSION-REPLICATION-ROLE\">session_replication_role<\/a><\/noindex>. Triggers die zonder aanvullende instructies (standaard) zijn ingeschakeld, worden geactiveerd wanneer de replicatierol \u2014 \"origin\" (standaard) of \"local\". Triggers die met de aanwijzing <code>ENABLE REPLICA<\/code>, worden alleen geactiveerd als <b>de huidige sessiemodus<\/b> \u2014 \"replica\", en triggers die met de aanwijzing <code>ENABLE ALWAYS<\/code>, worden geactiveerd ongeacht de huidige replicatiemodus.<\/p><\/blockquote>\n<p>Ik benadruk dat deze instelling niet voor iedereen geldt, zoals <code>ALTER TABLE<\/code>, maar alleen naar onze aparte speciale verbinding. Dus, om te voorkomen dat er enige trigger-activering is:<\/p>\n<pre><code class=\"sql\">SET session_replication_role = replica; -- triggers uitgeschakeld\nUPDATE ...;\nSET session_replication_role = DEFAULT; -- in oorspronkelijke staat teruggebracht<\/code><\/pre>\n<p><\/p>\n<h4>Voorwaarde binnen de trigger<\/h4>\n<p>\nMaar de bovenstaande variant werkt voor alle triggers tegelijk (of je moet van tevoren de triggers 'alteren' die je niet wilt uitschakelen). En als we moeten <b>\u00e9\u00e9n specifieke trigger \"uitschakelen\"<\/b>?<\/p>\n<p>Daarbij helpt ons <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-custom\">\"gebruikers\" sessievariabele<\/a><\/noindex>:<\/p>\n<blockquote><p>De namen van de extensieparameters worden als volgt genoteerd: naam van de extensie, punt en dan de naam van de parameter, vergelijkbaar met de volledige namen van objecten in SQL. Bijvoorbeeld: plpgsql.variable_conflict.<br \/>\nAangezien niet-systeemparameters kunnen worden ingesteld in processen die de overeenkomstige extensiemodule niet laden, accepteert PostgreSQL <b>waarden voor elke naam met twee componenten<\/b>.<\/p><\/blockquote>\n<p>Eerst werken we de trigger als volgt bij:<\/p>\n<pre><code class=\"sql\">BEGIN\n    -- in het conversieproces kan alles gedaan worden\n    IF current_setting('mycfg.my_table_convert_process') = 'TRUE' THEN\n        IF TG_OP IN ('INSERT', 'UPDATE') THEN\n            RETURN NEW;\n        ELSE\n            RETURN OLD;\n        END IF;\n    END IF;\n...<\/code><\/pre>\n<p>\nOverigens, dit kan \"live\" gedaan worden, zonder blokkades, via <code>CREATE OR REPLACE<\/code> voor de triggerfunctie. En dan in de speciale verbinding zetten we onze \"variabele\":<\/p>\n<pre><code class=\"sql\">\nSET mycfg.my_table_convert_process = 'TRUE';\nUPDATE ...;\nSET mycfg.my_table_convert_process = ''; -- in oorspronkelijke staat teruggebracht\n<\/code><\/pre>\n<p>\nKen je andere manieren? Deel ze in de opmerkingen.<br \/>\n<br \/>Bron: <a content=\"nofollow\" rel=\"nofollow\" href=\"https:\/\/habr.com\/ru\/company\/tensor\/blog\/489900\/\">habr.com<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"<p>\u0420\u0430\u043d\u043e \u0438\u043b\u0438 \u043f\u043e\u0437\u0434\u043d\u043e \u043c\u043d\u043e\u0433\u0438\u0435 \u0441\u0442\u0430\u043b\u043a\u0438\u0432\u0430\u044e\u0442\u0441\u044f \u0441 \u043d\u0435\u043e\u0431\u0445\u043e\u0434\u0438\u043c\u043e\u0441\u0442\u044c\u044e \u0447\u0442\u043e-\u0442\u043e \u043c\u0430\u0441\u0441\u043e\u0432\u043e \u0438\u0441\u043f\u0440\u0430\u0432\u0438\u0442\u044c \u0432 \u0437\u0430\u043f\u0438\u0441\u044f\u0445 \u0442\u0430\u0431\u043b\u0438\u0446\u044b. \u042f \u0443\u0436\u0435 \u0440\u0430\u0441\u0441\u043a\u0430\u0437\u044b\u0432\u0430\u043b, \u043a\u0430\u043a \u044d\u0442\u043e \u0434\u0435\u043b\u0430\u0442\u044c \u043b\u0443\u0447\u0448\u0435, \u0430 \u043a\u0430\u043a \u2014 \u043b\u0443\u0447\u0448\u0435 \u043d\u0435 \u0434\u0435\u043b\u0430\u0442\u044c. \u0421\u0435\u0433\u043e\u0434\u043d\u044f \u0440\u0430\u0441\u0441\u043a\u0430\u0436\u0443 \u043e \u0432\u0442\u043e\u0440\u043e\u043c \u0430\u0441\u043f\u0435\u043a\u0442\u0435 \u043c\u0430\u0441\u0441\u043e\u0432\u043e\u0433\u043e \u043e\u0431\u043d\u043e\u0432\u043b\u0435\u043d\u0438\u044f \u2014 \u043e \u0441\u0440\u0430\u0431\u043e\u0442\u043a\u0435 \u0442\u0440\u0438\u0433\u0433\u0435\u0440\u043e\u0432. \u041d\u0430\u043f\u0440\u0438\u043c\u0435\u0440, \u043d\u0430 \u0442\u0430\u0431\u043b\u0438\u0446\u0435, \u0432 \u043a\u043e\u0442\u043e\u0440\u043e\u0439 \u0432\u0430\u043c \u043d\u0430\u0434\u043e \u0447\u0442\u043e-\u0442\u043e \u043f\u043e\u043f\u0440\u0430\u0432\u0438\u0442\u044c, \u0432\u0438\u0441\u0438\u0442 \u0437\u043b\u043e\u0431\u043d\u044b\u0439 \u0442\u0440\u0438\u0433\u0433\u0435\u0440 ON UPDATE, \u043f\u0435\u0440\u0435\u043d\u043e\u0441\u044f\u0449\u0438\u0439 \u0432\u0441\u0435 \u0438\u0437\u043c\u0435\u043d\u0435\u043d\u0438\u044f \u0432 [&hellip;]<\/p>\n","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[688],"tags":[],"class_list":["post-72265","post","type-post","status-publish","format-standard","hentry","category-administrirovanie"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 5.0.3 - aioseo.com -->\n\t<meta name=\"description\" content=\"\u0420\u0430\u043d\u043e \u0438\u043b\u0438 \u043f\u043e\u0437\u0434\u043d\u043e \u043c\u043d\u043e\u0433\u0438\u0435 \u0441\u0442\u0430\u043b\u043a\u0438\u0432\u0430\u044e\u0442\u0441\u044f \u0441 \u043d\u0435\u043e\u0431\u0445\u043e\u0434\u0438\u043c\u043e\u0441\u0442\u044c\u044e \u0447\u0442\u043e-\u0442\u043e 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