{"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\/it\/blog\/administrirovanie\/postgresql-antipatterns-menyaem-dannye-v-obhod-triggera","title":{"rendered":"Antipatterns di PostgreSQL: modificare i dati aggirando il trigger","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p>Prima o poi, molti si trovano nella necessit\u00e0 di correggere massivamente le registrazioni di una tabella. Ho gi\u00e0 <noindex><a rel=\"nofollow\" href=\"https:\/\/habr.com\/post\/481610\/\">parlato di come farlo nel modo migliore<\/a><\/noindex>, e di come \u2014 non farlo affatto. Oggi parler\u00f2 del secondo aspetto dell'aggiornamento di massa \u2014 <b>l'attivazione dei trigger<\/b>.<\/p>\n<p>Per esempio, su una tabella, dove devi correggere qualcosa, pende un temuto trigger <code>ON UPDATE<\/code>, che trasferisce tutte le modifiche in alcuni aggregati. E tu devi aggiornare tutto (inizializzare un nuovo campo, per esempio) in modo tale che questi aggregati non vengano toccati.<\/p>\n<h2>Facciamo cos\u00ec, disattiviamo i trigger!<\/h2>\n<p><\/p>\n<pre><code class=\"sql\">BEGIN;\n  ALTER TABLE ... DISABLE TRIGGER ...;\n  UPDATE ...; -- qui ci vorr\u00e0 tempo\n  ALTER TABLE ... ENABLE TRIGGER ...;\nCOMMIT;<\/code><\/pre>\n<p>\nIn sostanza, questo \u00e8 tutto \u2014 <b>\u00e8 gi\u00e0 presente<\/b>.<\/p>\n<p>Perch\u00e9 <code>ALTER TABLE<\/code> impone <b>AccessExclusive<\/b>-la blocco, sotto la quale nessuno, nemmeno un'operazione semplice in parallelo, <code>SELECT<\/code>, potr\u00e0 leggere nulla dalla tabella. Cio\u00e8, finch\u00e9 questa transazione non terminer\u00e0, chiunque voglia anche solo \u00ableggere\u00bb dovr\u00e0 aspettare. E noi ricordiamo che <code>UPDATE<\/code> abbiamo un\u2019operazione molto l-unga\u2026<br \/>\n<noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><\/p>\n<h4>Allora velocemente disattiviamo, poi velocemente riattiviamo!<\/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>\nQui la situazione \u00e8 gi\u00e0 migliore, i tempi di attesa sono significativamente pi\u00f9 brevi. Ma ci sono due problemi che rovinano tutto:<\/p>\n<ul>\n<li><code>ALTER TABLE<\/code> sta aspettando tutte le altre operazioni sulla tabella, comprese le lunghe <code>SELECT<\/code><\/li>\n<li>Finch\u00e9 il trigger \u00e8 disattivato, <b>qualsiasi modifica passer\u00e0 inosservata<\/b> nella tabella, anche quelle non nostre. E non entrer\u00e0 negli aggregati, anche se dovrebbe. Un vero peccato!<\/li>\n<\/ul>\n<p><\/p>\n<h2>Gestione delle variabili di sessione<\/h2>\n<p>\nQuindi, nella versione precedente abbiamo affrontato un aspetto fondamentale: dobbiamo in qualche modo insegnare al trigger a distinguere le modifiche \"nostre\" nella tabella dalle \"non nostre\". Dobbiamo passare le \"nostre\" cos\u00ec come sono, mentre per le \"non nostre\" deve attivarsi. Per questo possiamo utilizzare <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client\">le variabili di sessione<\/a><\/noindex>.<\/p>\n<h4>session_replication_role<\/h4>\n<p>\nLeggiamo <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/sql-altertable\">manuale<\/a><\/noindex>:<\/p>\n<blockquote><p>Il meccanismo di attivazione dei trigger \u00e8 influenzato anche da una variabile di configurazione <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-client#GUC-SESSION-REPLICATION-ROLE\">session_replication_role<\/a><\/noindex>. I trigger attivati senza ulteriori indicazioni (per impostazione predefinita) si attiveranno quando il ruolo di replica \u00e8 \"origin\" (predefinito) o \"local\". I trigger attivati specificando <code>ENABLE REPLICA<\/code>, si attiveranno solo se <b>la modalit\u00e0 corrente della sessione<\/b> \u00e8 \"replica\", e i trigger attivati specificando <code>ENABLE ALWAYS<\/code>, si attiveranno indipendentemente dalla modalit\u00e0 attuale di replica.<\/p><\/blockquote>\n<p>Voglio sottolineare che la configurazione non riguarda tutti immediatamente, ma solo il nostro specifico collegamento speciale. In sintesi, per non attivare nessun trigger applicativo: <code>ALTER TABLE<\/code>SET session_replication_role = replica; -- disattiviamo i trigger\nUPDATE ...;\nSET session_replication_role = DEFAULT; -- ripristinato allo stato originale<\/p>\n<pre><code class=\"sql\">Condizione all'interno del trigger<\/code><\/pre>\n<p><\/p>\n<h4>Ma la versione sopra funziona per tutti i trigger contemporaneamente (oppure bisogna \"alterare\" prima i trigger che non si desidera disattivare). E se abbiamo bisogno di<\/h4>\n<p>\n\"disattivare\" un singolo trigger specifico <b>\"variabile\" di sessione personalizzata<\/b>?<\/p>\n<p>In questo ci aiuter\u00e0 <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/runtime-config-custom\">I nomi dei parametri delle estensioni vengono scritti nel seguente modo: nome dell'estensione, punto e poi il nome del parametro, simile ai nomi completi degli oggetti in SQL. Ad esempio: plpgsql.variable_conflict.<\/a><\/noindex>:<\/p>\n<blockquote><p>Poich\u00e9 i parametri non sistemici possono essere impostati nei processi che non caricano il modulo di estensione corrispondente, PostgreSQL accetta<br \/>\nvalori per qualsiasi nome con due componenti <b>Prima raffiniamo il trigger, pi\u00f9 o meno cos\u00ec:<\/b>.<\/p><\/blockquote>\n<p>BEGIN\n    -- il processo di conversione pu\u00f2 fare tutto\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...<\/p>\n<pre><code class=\"sql\">INIZIO\n    -- durante il processo di conversione \u00e8 possibile fare qualsiasi cosa\n    SE current_setting('mycfg.my_table_convert_process') = 'TRUE' ALLORA\n        SE TG_OP IN ('INSERT', 'UPDATE') ALLORA\n            RESTITUISCI NUOVO;\n        ALTRE\n            RESTITUISCI VECCHIO;\n        FINE SE;\n    FINE SE;\n...<\/code><\/pre>\n<p>\nInoltre, \u00e8 possibile farlo \u00abin tempo reale\u00bb, senza bloccaggi, attraverso <code>CREATE OR REPLACE<\/code> per la funzione trigger. Poi, nel connessione speciale, attiviamo la nostra variabile:<\/p>\n<pre><code class=\"sql\">\nSET mycfg.my_table_convert_process = 'TRUE';\nUPDATE ...;\nSET mycfg.my_table_convert_process = ''; -- ripristinato allo stato originale\n<\/code><\/pre>\n<p>\nConoscete altri metodi? Condividete nei commenti.<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 5.0.0.1 - 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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