{"id":14941,"date":"2022-12-15T17:12:55","date_gmt":"2022-12-15T20:12:55","guid":{"rendered":"https:\/\/dev.dbarj.com.br\/?p=14941"},"modified":"2022-12-15T17:12:55","modified_gmt":"2022-12-15T20:12:55","slug":"log-returning-wrong-results-in-some-cases","status":"publish","type":"post","link":"https:\/\/dev.dbarj.com.br\/en\/2022\/12\/log-returning-wrong-results-in-some-cases\/","title":{"rendered":"LOG(x,y) returning wrong results in some cases"},"content":{"rendered":"<p>I had a requirement in one tool that I was working to find whether a given table column was power of 2.<\/p>\n<p>The easiest way to check would be using the LOG(2,y). If the result was an integer, the number would be power of 2.<\/p>\n<p>2^X = Y \u00a0 \u00a0 -&gt; \u00a0 \u00a0 LOG(2,Y)=X<\/p>\n<p>And to check if X was an integer, I would add a predicate of TRUNC(X) = X.<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"oracledb\">SQL&gt; create table mytab (a number);\r\nTable MYTAB created.\r\n\r\nSQL&gt; insert into mytab select rownum from dual connect by level &lt;= 100;\r\n100 rows inserted.\r\n\r\nSQL&gt; commit;\r\nCommit complete.<\/pre>\n<p>However, when I tried to check the results, I got a weird result:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"oracledb\">SQL&gt; select * from mytab where log(2,a)=0;\r\n         A\r\n----------\r\n         1\r\n\r\nSQL&gt; select * from mytab where log(2,a)=1;\r\n         A\r\n----------\r\n         2\r\n\r\nSQL&gt; select * from mytab where log(2,a)=2;\r\nno rows selected\r\n\r\nSQL&gt; select * from mytab where log(2,a)=3;\r\nno rows selected<\/pre>\n<p>After some research, I found this article (<a href=\"https:\/\/community.oracle.com\/tech\/developers\/discussion\/4198779\/floor-log-x-y-providing-wrong-values-in-some-cases\" target=\"_blank\" rel=\"noopener\">https:\/\/community.oracle.com\/tech\/developers\/discussion\/4198779\/floor-log-x-y-providing-wrong-values-in-some-cases<\/a>) which explains that due to computing limitations, LOG(x,y) is always computed as LN(y)\/LN(x), no matter what x and y. Thus, when I run log(2,4), instead of getting 2, I get a different result:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"oracledb\">SQL&gt; select log(2,4) from dual;\r\n1.99999999999999999999999999999999999998\r\n\r\nSQL&gt; select log(2,8) from dual;\r\n2.99999999999999999999999999999999999999\r\n\r\nSQL&gt; select log(2,16) from dual;\r\n3.99999999999999999999999999999999999998<\/pre>\n<p>So, to solve the problem, I need to do a round on the result number, on the 16+ decimal case just to avoid a false positive:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"oracledb\">SQL&gt; select * from mytab where round(log(2,a),16)=2;\r\n         A\r\n----------\r\n         4\r\n\r\nSQL&gt; select * from mytab where round(log(2,a),16)=3;\r\n         A\r\n----------\r\n         8<\/pre>\n<p>So, in the end, to get only the rows whether a given column was power of 2, I would run:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"oracledb\">SQL&gt; select * from mytab where round(log(2,a),16)=trunc(round(log(2,a),16));\r\n         A\r\n----------\r\n         1\r\n         2\r\n         4\r\n         8\r\n        16\r\n        32\r\n        64\r\n\r\n7 rows selected.<\/pre>\n<b>Have you enjoyed? Please leave a comment or give a ?!<\/b>\n<div class='watch-action'><div class='watch-position align-left'><div class='action-like'><a class='lbg-style2 like-14941 jlk' href='javascript:void(0)' data-task='like' data-post_id='14941' data-nonce='b7aaf4ff99' rel='nofollow'><img class='wti-pixel' src='https:\/\/dev.dbarj.com.br\/wp-content\/plugins\/wti-like-post\/images\/pixel.gif' title='Like' \/><span class='lc-14941 lc'>0<\/span><\/a><\/div><\/div> <div class='status-14941 status align-left'><\/div><\/div><div class='wti-clear'><\/div>","protected":false},"excerpt":{"rendered":"<p>I had a requirement in one tool that I was working to find whether a given table column was power of 2. The easiest way to check would be using the LOG(2,y). If the result was an integer, the number would be power of 2. 2^X = Y \u00a0 \u00a0 -&gt; \u00a0 \u00a0 LOG(2,Y)=X And &hellip; <\/p>\n<p><a class=\"more-link btn\" href=\"https:\/\/dev.dbarj.com.br\/en\/2022\/12\/log-returning-wrong-results-in-some-cases\/\">Continue reading<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[15],"tags":[],"class_list":["post-14941","post","type-post","status-publish","format-standard","hentry","category-database-en","item-wrap"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>LOG(x,y) returning wrong results in some cases - DBA - Rodrigo Jorge - Oracle Tips and Guides<\/title>\n<meta name=\"description\" content=\"How to correct the computing error when LOG(x,y) returns wrong results in some cases on Oracle Database\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/dev.dbarj.com.br\/en\/2022\/12\/log-returning-wrong-results-in-some-cases\/\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"DBA RJ\" \/>\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:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/\"},\"author\":{\"name\":\"DBA RJ\",\"@id\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/#\\\/schema\\\/person\\\/28a44ca3a6633fe4156ad1ea209d40a9\"},\"headline\":\"LOG(x,y) returning wrong results in some cases\",\"datePublished\":\"2022-12-15T20:12:55+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/\"},\"wordCount\":190,\"commentCount\":0,\"publisher\":{\"@id\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/#\\\/schema\\\/person\\\/28a44ca3a6633fe4156ad1ea209d40a9\"},\"articleSection\":[\"Oracle Database General\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/\",\"url\":\"https:\\\/\\\/dev.dbarj.com.br\\\/en\\\/2022\\\/12\\\/log-returning-wrong-results-in-some-cases\\\/\",\"name\":\"LOG(x,y) returning wrong results in some cases - 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