{"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":28,"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 v28.8 (Yoast SEO v28.8) - 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. 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