{"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\/pt\/blog\/administrirovanie\/postgresql-antipatterns-menyaem-dannye-v-obhod-triggera","title":{"rendered":"Antipadr\u00f5es do PostgreSQL: alterando dados contornando o gatilho","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p>Mais cedo ou mais tarde, muitos se deparam com a necessidade de corrigir algo em massa nas entradas de uma tabela. Eu j\u00e1 <noindex><a rel=\"nofollow\" href=\"https:\/\/habr.com\/post\/481610\/\">falei sobre como fazer isso melhor<\/a><\/noindex>, e como \u2014 melhor n\u00e3o fazer. Hoje falarei sobre o segundo aspecto da atualiza\u00e7\u00e3o em massa \u2014 <b>o acionamento de gatilhos<\/b>.<\/p>\n<p>Por exemplo, em uma tabela, onde voc\u00ea precisa corrigir algo, h\u00e1 um gatilho maligno <code>ON UPDATE<\/code>, que transfere todas as altera\u00e7\u00f5es para alguns agregados. E voc\u00ea precisa atualizar tudo (inicializar um novo campo, por exemplo) de forma cuidadosa para que esses agregados n\u00e3o sejam afetados.<\/p>\n<h2>Vamos simplesmente desativar os gatilhos!<\/h2>\n<p><\/p>\n<pre><code class=\"sql\">BEGIN;\n  ALTER TABLE ... DISABLE TRIGGER ...;\n  UPDATE ...; -- aqui leva um bom tempo\n  ALTER TABLE ... ENABLE TRIGGER ...;\nCOMMIT;<\/code><\/pre>\n<p>\nNa verdade, isso \u00e9 tudo \u2014 <b>tudo j\u00e1 est\u00e1 pendurado<\/b>.<\/p>\n<p>Porque <code>ALTER TABLE<\/code> imp\u00f5e <b>AccessExclusive<\/b>- a bloqueio, sob o qual ningu\u00e9m consegue executar em paralelo, nem mesmo uma simples <code>SELECT<\/code>, nada pode ser lido da tabela. Ou seja, enquanto essa transa\u00e7\u00e3o n\u00e3o terminar, todos que quiserem apenas \u201cler\u201d estar\u00e3o esperando. E lembramos que <code>ATUALIZA\u00c7\u00c3O<\/code> temos um longo\u2026<br \/>\n<noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><\/p>\n<h4>Ent\u00e3o vamos desligar rapidamente, depois ligamos rapidamente!<\/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>\nAqui a situa\u00e7\u00e3o j\u00e1 \u00e9 melhor, o tempo de espera \u00e9 significativamente menor. Mas h\u00e1 apenas dois problemas que arru\u00ednam toda a beleza:<\/p>\n<ul>\n<li><code>ALTER TABLE<\/code> espera todas as outras opera\u00e7\u00f5es na tabela, incluindo as longas <code>SELECT<\/code><\/li>\n<li>Enquanto o gatilho estiver desligado, <b>qualquer modifica\u00e7\u00e3o<\/b> na tabela, mesmo que n\u00e3o seja nossa, \u201cpassar\u00e1\u201d despercebida. E n\u00e3o ir\u00e1 para os agregados, apesar de que deveria. Que pena!<\/li>\n<\/ul>\n<p><\/p>\n<h2>Gerenciamento de vari\u00e1veis de sess\u00e3o<\/h2>\n<p>\nEnt\u00e3o, na variante anterior, encontramos um ponto fundamental \u2014 precisamos de algum modo ensinar o gatilho a diferenciar as altera\u00e7\u00f5es \u201cnossas\u201d na tabela das \u201cn\u00e3o nossas\u201d. Aceitar as \u201cnossas\u201d como est\u00e3o, e acionar para as \u201cn\u00e3o nossas\u201d. Para isso, podemos utilizar <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client\">vari\u00e1veis de sess\u00e3o<\/a><\/noindex>.<\/p>\n<h4>session_replication_role<\/h4>\n<p>\nLemos <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/sql-altertable\">manual<\/a><\/noindex>:<\/p>\n<blockquote><p>O mecanismo de acionamento dos gatilhos tamb\u00e9m \u00e9 influenciado pela vari\u00e1vel de configura\u00e7\u00e3o <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client#GUC-SESSION-REPLICATION-ROLE\">session_replication_role<\/a><\/noindex>. Gatilhos habilitados sem indica\u00e7\u00f5es adicionais (por padr\u00e3o) ser\u00e3o acionados quando o papel de replica\u00e7\u00e3o \u00e9 \u201corigin\u201d (por padr\u00e3o) ou \u201clocal\u201d. Gatilhos habilitados com a indica\u00e7\u00e3o <code>ENABLE REPLICA<\/code>, ser\u00e3o acionados apenas se <b>o modo atual da sess\u00e3o<\/b> \u00e9 \u201creplica\u201d, e gatilhos habilitados com a indica\u00e7\u00e3o <code>ENABLE ALWAYS<\/code>, ser\u00e3o acionados independentemente do modo atual de replica\u00e7\u00e3o.<\/p><\/blockquote>\n<p>Ressalto que a configura\u00e7\u00e3o n\u00e3o se refere a todos de uma vez, como <code>ALTER TABLE<\/code>, mas apenas para a nossa conex\u00e3o especial separada. Portanto, para evitar que quaisquer gatilhos de aplicativo sejam acionados:<\/p>\n<pre><code class=\"sql\">SET session_replication_role = replica; -- desativamos os gatilhos\nUPDATE ...;\nSET session_replication_role = DEFAULT; -- retornamos ao estado original<\/code><\/pre>\n<p><\/p>\n<h4>Condi\u00e7\u00e3o dentro do gatilho<\/h4>\n<p>\nMas a op\u00e7\u00e3o acima funciona para todos os gatilhos ao mesmo tempo (ou \u00e9 necess\u00e1rio 'alterar' previamente os gatilhos que n\u00e3o queremos desativar). E se precisarmos <b>desativar um gatilho espec\u00edfico<\/b>?<\/p>\n<p>Para isso, podemos contar com <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-custom\">vari\u00e1vel de sess\u00e3o 'usu\u00e1rio'<\/a><\/noindex>:<\/p>\n<blockquote><p>Os nomes dos par\u00e2metros de extens\u00f5es s\u00e3o escritos da seguinte forma: nome da extens\u00e3o, ponto e, em seguida, o nome do par\u00e2metro propriamente dito, semelhante aos nomes completos de objetos em SQL. Por exemplo: plpgsql.variable_conflict.<br \/>\nComo os par\u00e2metros n\u00e3o sist\u00eamicos podem ser definidos em processos que n\u00e3o carregam o m\u00f3dulo de extens\u00e3o correspondente, o PostgreSQL aceita <b>valores para qualquer nome com duas partes<\/b>.<\/p><\/blockquote>\n<p>Primeiro, aprimoramos o gatilho, mais ou menos assim:<\/p>\n<pre><code class=\"sql\">BEGIN\n    -- O processo de convers\u00e3o pode fazer tudo\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>\nA prop\u00f3sito, isso pode ser feito 'ao vivo', sem bloqueios, atrav\u00e9s de <code>CREATE OR REPLACE<\/code> para a fun\u00e7\u00e3o de gatilho. E ent\u00e3o, na conex\u00e3o especial, ativamos a nossa vari\u00e1vel:<\/p>\n<pre><code class=\"sql\">\nSET mycfg.my_table_convert_process = 'TRUE';\nUPDATE ...;\nSET mycfg.my_table_convert_process = ''; -- retornamos ao estado original\n<\/code><\/pre>\n<p>\nConhece outros m\u00e9todos? Compartilhe nos coment\u00e1rios.<br \/>\n<br \/>Fonte: <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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