{"id":10356,"date":"2017-07-14T14:52:55","date_gmt":"2017-07-14T12:52:55","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/"},"modified":"2017-07-14T14:52:55","modified_gmt":"2017-07-14T12:52:55","slug":"can-i-do-it-with-postgresql-16-ddl-triggers","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/","title":{"rendered":"Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers"},"content":{"rendered":"<p>A question I received recently from a customer: Oracle gives you the possibility to create <a href=\"http:\/\/docs.oracle.com\/database\/122\/LNPLS\/CREATE-TRIGGER-statement.htm#LNPLS01374\" target=\"_blank\" rel=\"noopener\">DDL triggers<\/a>. DDL triggers fire (as the name implies) when DDL events occur. Can we do that in PostgreSQL? Yes, this feature is called <a href=\"https:\/\/www.postgresql.org\/docs\/current\/static\/event-trigger-definition.html\" target=\"_blank\" rel=\"noopener\">event triggers<\/a>. There is a nice list in the documentation which lists for which DDLs an event trigger can fire: <a href=\"https:\/\/www.postgresql.org\/docs\/current\/static\/event-trigger-matrix.html\" target=\"_blank\" rel=\"noopener\">Event Trigger Firing Matrix<\/a>. Lets go &#8230;<\/p>\n<p><!--more--><\/p>\n<p>As usual lets create a dummy table we can work with:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\npostgres=# create table t1 ( a int );\nCREATE TABLE\npostgres=# d t1\n      Table \"public.t1\"\n Column |  Type   | Modifiers \n--------+---------+-----------\n a      | integer | \n<\/pre>\n<p>What we want to do is to prevent any modifications of the table structure, how do we do this? Obviously we need a way to catch the alter statement against our table and then raise an exception displaying some text. What we need to do is to create a function which returns the pseudo type &#8220;event_trigger&#8221;:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\nCREATE OR REPLACE FUNCTION no_ddl() RETURNS event_trigger AS $$\nDECLARE r RECORD;\nBEGIN\n    FOR r IN SELECT * FROM pg_event_trigger_ddl_commands() LOOP\n        RAISE EXCEPTION 'You are not allowed to change %', r.object_identity;\n    END LOOP;\nEND;\n$$\nLANGUAGE plpgsql;\n<\/pre>\n<p>What this function is doing is to iterate over the result set of <a href=\"https:\/\/www.postgresql.org\/docs\/current\/static\/functions-event-triggers.html\" target=\"_blank\" rel=\"noopener\">pg_event_trigger_ddl_commands<\/a> and then raises an exception. In addition to that we need the event trigger that calls the function:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\nCREATE EVENT TRIGGER no_ddl_allowed\n  ON ddl_command_end WHEN TAG IN ('ALTER TABLE')\n  EXECUTE PROCEDURE no_ddl();\n<\/pre>\n<p>Lets see if it works:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\npostgres=# alter table t1 add column g text;\nERROR:  You are not allowed to change public.t1\nCONTEXT:  PL\/pgSQL function no_ddl() line 5 at RAISE\n<\/pre>\n<p>Cool, but there is an issue with the current implementation:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\npostgres=# create table t2 ( a int );\nCREATE TABLE\npostgres=# alter table t2 add column b text;\nERROR:  You are not allowed to change public.t2\nCONTEXT:  PL\/pgSQL function no_ddl() line 5 at RAISE\n<\/pre>\n<p>What we effectively did is to deny all alter statements for all objects in that database. This is probably not what you want. A better approach is this:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\nCREATE OR REPLACE FUNCTION no_ddl() RETURNS event_trigger AS $$\nDECLARE r RECORD;\nBEGIN\n    FOR r IN SELECT * FROM pg_event_trigger_ddl_commands() LOOP\n      IF ( r.objid::regclass::text = 't1' )\n      THEN\n            RAISE EXCEPTION 'You are not allowed to change %', r.object_identity;\n      END IF;\n    END LOOP;\nEND;\n$$\nLANGUAGE plpgsql;\n<\/pre>\n<p>This way we are only raising the exception when the table &#8220;t1&#8221; is involved and do nothing for all other tables:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">\npostgres=# alter table t2 add column b text;\nALTER TABLE\npostgres=# alter table t1 add column b text;\nERROR:  You are not allowed to change public.t1\n<\/pre>\n<p>Hope this helps &#8230;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A question I received recently from a customer: Oracle gives you the possibility to create DDL triggers. DDL triggers fire (as the name implies) when DDL events occur. Can we do that in PostgreSQL? Yes, this feature is called event triggers. There is a nice list in the documentation which lists for which DDLs an [&hellip;]<\/p>\n","protected":false},"author":29,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[229],"tags":[77],"type_dbi":[],"class_list":["post-10356","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","tag-postgresql"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.2 (Yoast SEO v27.2) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers - dbi Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers\" \/>\n<meta property=\"og:description\" content=\"A question I received recently from a customer: Oracle gives you the possibility to create DDL triggers. DDL triggers fire (as the name implies) when DDL events occur. Can we do that in PostgreSQL? Yes, this feature is called event triggers. There is a nice list in the documentation which lists for which DDLs an [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\" \/>\n<meta property=\"og:site_name\" content=\"dbi Blog\" \/>\n<meta property=\"article:published_time\" content=\"2017-07-14T12:52:55+00:00\" \/>\n<meta name=\"author\" content=\"Daniel Westermann\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@westermanndanie\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Daniel Westermann\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\"},\"author\":{\"name\":\"Daniel Westermann\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/8d08e9bd996a89bd75c0286cbabf3c66\"},\"headline\":\"Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers\",\"datePublished\":\"2017-07-14T12:52:55+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\"},\"wordCount\":244,\"commentCount\":0,\"keywords\":[\"PostgreSQL\"],\"articleSection\":[\"Database Administration &amp; Monitoring\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\",\"name\":\"Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers - dbi Blog\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#website\"},\"datePublished\":\"2017-07-14T12:52:55+00:00\",\"author\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/8d08e9bd996a89bd75c0286cbabf3c66\"},\"breadcrumb\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/can-i-do-it-with-postgresql-16-ddl-triggers\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Accueil\",\"item\":\"https:\/\/www.dbi-services.com\/blog\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"Can I do it with PostgreSQL? \u2013 16 \u2013 DDL triggers\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#website\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/\",\"name\":\"dbi Blog\",\"description\":\"\",\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/www.dbi-services.com\/blog\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"},{\"@type\":\"Person\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/8d08e9bd996a89bd75c0286cbabf3c66\",\"name\":\"Daniel Westermann\",\"image\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/secure.gravatar.com\/avatar\/31350ceeecb1dd8986339a29bf040d4cd3cd087d410deccd8f55234466d6c317?s=96&d=mm&r=g\",\"url\":\"https:\/\/secure.gravatar.com\/avatar\/31350ceeecb1dd8986339a29bf040d4cd3cd087d410deccd8f55234466d6c317?s=96&d=mm&r=g\",\"contentUrl\":\"https:\/\/secure.gravatar.com\/avatar\/31350ceeecb1dd8986339a29bf040d4cd3cd087d410deccd8f55234466d6c317?s=96&d=mm&r=g\",\"caption\":\"Daniel Westermann\"},\"description\":\"Daniel Westermann is Principal Consultant and Technology Leader Open Infrastructure at dbi services. He has more than 15 years of experience in management, engineering and optimization of databases and infrastructures, especially on Oracle and PostgreSQL. Since the beginning of his career, he has specialized in Oracle Technologies and is Oracle Certified Professional 12c and Oracle Certified Expert RAC\/GridInfra. Over time, Daniel has become increasingly interested in open source technologies, becoming \u201cTechnology Leader Open Infrastructure\u201d and PostgreSQL expert. \u00a0Based on community or EnterpriseDB tools, he develops and installs complex high available solutions with PostgreSQL. He is also a certified PostgreSQL Plus 9.0 Professional and a Postgres Advanced Server 9.4 Professional. He is a regular speaker at PostgreSQL conferences in Switzerland and Europe. Today Daniel is also supporting our customers on AWS services such as AWS RDS, database migrations into the cloud, EC2 and automated infrastructure management with AWS SSM (System Manager). 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He has more than 15 years of experience in management, engineering and optimization of databases and infrastructures, especially on Oracle and PostgreSQL. Since the beginning of his career, he has specialized in Oracle Technologies and is Oracle Certified Professional 12c and Oracle Certified Expert RAC\/GridInfra. Over time, Daniel has become increasingly interested in open source technologies, becoming \u201cTechnology Leader Open Infrastructure\u201d and PostgreSQL expert. \u00a0Based on community or EnterpriseDB tools, he develops and installs complex high available solutions with PostgreSQL. He is also a certified PostgreSQL Plus 9.0 Professional and a Postgres Advanced Server 9.4 Professional. He is a regular speaker at PostgreSQL conferences in Switzerland and Europe. Today Daniel is also supporting our customers on AWS services such as AWS RDS, database migrations into the cloud, EC2 and automated infrastructure management with AWS SSM (System Manager). He is a certified AWS Solutions Architect Professional. Prior to dbi services, Daniel was Management System Engineer at LC SYSTEMS-Engineering AG in Basel. Before that, he worked as Oracle Developper &amp;\u00a0Project Manager at Delta Energy Solutions AG in Basel (today Powel AG). Daniel holds a diploma in Business Informatics (DHBW, Germany). 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