{"id":37006,"date":"2025-02-10T11:29:41","date_gmt":"2025-02-10T10:29:41","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/?p=37006"},"modified":"2025-02-10T11:29:41","modified_gmt":"2025-02-10T10:29:41","slug":"postgresql-18-virtual-generated-columns","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/postgresql-18-virtual-generated-columns\/","title":{"rendered":"PostgreSQL 18: Virtual generated columns"},"content":{"rendered":"\n<p>This is about another feature which was committed for PostgreSQL 18: Virtual generated columns. Generated columns are available already, but they need to be &#8220;STORED&#8221;. This means the result of an expression is stored to disk and the result is computed while writing, but you cannot write to that column explicitly. Before looking at the virtual generated columns lets do a quick refresh on &#8220;STORED&#8221; generated columns.<\/p>\n\n\n\n<p>Consider a simple a table like this:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; highlight: [1,3]; title: ; notranslate\" title=\"\">\npostgres=# create table t ( a int, b int );\nCREATE TABLE\npostgres=# insert into t values (1,99);\nINSERT 0 1\n<\/pre><\/div>\n\n\n<p>If you want to automatically calculate the sum of &#8220;a&#8221; and &#8220;b&#8221; you could use a generated column for this:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; highlight: [1,3]; title: ; notranslate\" title=\"\">\npostgres=# alter table t add column c int generated always as (a+b) stored; \nALTER TABLE\npostgres=# select * from t;\n a | b  |  c  \n---+----+-----\n 1 | 99 | 100\n(1 row)\n<\/pre><\/div>\n\n\n<p>As mentioned above, you are not allowed to directly write to that column:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; highlight: [1]; title: ; notranslate\" title=\"\">\npostgres=# update t set c = -1 where a = 1;\nERROR:  column &quot;c&quot; can only be updated to DEFAULT\nDETAIL:  Column &quot;c&quot; is a generated column.\n<\/pre><\/div>\n\n\n<p>A downside of this type of generated column is, that the result of the expression is actually stored to disk and obviously consumes space. <\/p>\n\n\n\n<p>With the addition of &#8220;virtual generated columns&#8221; this concept is somehow flipped: The expression of a virtual column is computed while reading, so there is no storage on disk. On the other side you spend the work for computing the expression also while reading, and not while writing as it is done with &#8220;STORED&#8221; virtual columns:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; highlight: [1,3]; title: ; notranslate\" title=\"\">\npostgres=# alter table t add column d int generated always as (a-b) virtual; \nALTER TABLE\npostgres=# select * from t;\n a | b  |  c  |  d  \n---+----+-----+-----\n 1 | 99 | 100 | -98\n(1 row)\n<\/pre><\/div>\n\n\n<p>The default, from PostgreSQL 18 on, will be virtual, which is also reflected here:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; highlight: [1]; title: ; notranslate\" title=\"\">\npostgres=# \\d t\n                               Table &quot;public.t&quot;\n Column |  Type   | Collation | Nullable |              Default               \n--------+---------+-----------+----------+------------------------------------\n a      | integer |           |          | \n b      | integer |           |          | \n c      | integer |           |          | generated always as (a + b) stored\n d      | integer |           |          | generated always as (a - b)\n<\/pre><\/div>\n\n\n<p>Nice, thanks to everyone <a href=\"https:\/\/git.postgresql.org\/gitweb\/?p=postgresql.git;a=commitdiff;h=83ea6c54025bea67bcd4949a6d58d3fc11c3e21b\" target=\"_blank\" rel=\"noreferrer noopener\">involved<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>This is about another feature which was committed for PostgreSQL 18: Virtual generated columns. Generated columns are available already, but they need to be &#8220;STORED&#8221;. This means the result of an expression is stored to disk and the result is computed while writing, but you cannot write to that column explicitly. Before looking at the [&hellip;]<\/p>\n","protected":false},"author":29,"featured_media":0,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[229,198],"tags":[77],"type_dbi":[],"class_list":["post-37006","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","category-database-management","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>PostgreSQL 18: Virtual generated columns - 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\/postgresql-18-virtual-generated-columns\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"PostgreSQL 18: Virtual generated columns\" \/>\n<meta property=\"og:description\" content=\"This is about another feature which was committed for PostgreSQL 18: Virtual generated columns. Generated columns are available already, but they need to be &#8220;STORED&#8221;. This means the result of an expression is stored to disk and the result is computed while writing, but you cannot write to that column explicitly. 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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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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). His branch-related experience mainly covers the pharma industry, the financial sector, energy, lottery and telecommunications.","sameAs":["https:\/\/x.com\/westermanndanie"],"url":"https:\/\/www.dbi-services.com\/blog\/author\/daniel-westermann\/"}]}},"_links":{"self":[{"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/posts\/37006","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/users\/29"}],"replies":[{"embeddable":true,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/comments?post=37006"}],"version-history":[{"count":4,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/posts\/37006\/revisions"}],"predecessor-version":[{"id":37010,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/posts\/37006\/revisions\/37010"}],"wp:attachment":[{"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/media?parent=37006"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/categories?post=37006"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/tags?post=37006"},{"taxonomy":"type","embeddable":true,"href":"https:\/\/www.dbi-services.com\/blog\/wp-json\/wp\/v2\/type_dbi?post=37006"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}