{"id":2183,"date":"2011-03-01T12:14:00","date_gmt":"2011-03-01T11:14:00","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/dbmscompression-how-to-determine-a-tables-compression-ratio-without-creating-it\/"},"modified":"2011-03-01T12:14:00","modified_gmt":"2011-03-01T11:14:00","slug":"dbmscompression-how-to-determine-a-tables-compression-ratio-without-creating-it","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/dbmscompression-how-to-determine-a-tables-compression-ratio-without-creating-it\/","title":{"rendered":"DBMS compression: determining a table&#8217;s compression ratio"},"content":{"rendered":"<p>According to the Oracle documentation, the dbms_compression procedure allows to determine the compression ratio for a table without creating it. Let&#8217;s have a look at it.<\/p>\n<p>We\u00a0choose a table with a lot of records and an estimated size of 27 Mo:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">SQL&gt; select count(*) from test1;\u00a0\n161068\n\u00a0\nSQL&gt; select segment_name, bytes\/1024\/1024 from user_segments\n2 where segment_name = 'TEST1';\u00a0\nSEGMENT_NAME BYTES\/1024\/1024\u00a0\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0TEST1\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 27<\/pre>\n<p>Let&#8217;s see if the dbms_compression.get_ratio is correct for the Oracle procedure. We use a script named get_ratio.sql which calculates the compression ratio:<\/p>\n<pre class=\"brush: actionscript3; gutter: true; first-line: 1\">declare\ncomp_ratio number;\ncomptype varchar2(300);\nblockcount_comp number;\nblockcount_nocomp number;\nrow_comp number;\nrow_nocomp number;\nbegin\ndbms_compression.get_compression_ratio(\nscratchtbsname =&gt;'USERS', -- tablespace\nownname =&gt;'PSI', -- owner\ntabname =&gt;'TEST1', -- table name\npartname =&gt;null, -- partition name\ncomptype =&gt;2, -- compression type 2 = OLTP\nblkcnt_cmp =&gt;blockcount_comp, -- number of blocks compressed\nblkcnt_uncmp =&gt;blockcount_nocomp, -- number of blocks non compressed\nrow_cmp\u00a0=&gt;row_comp, -- number of rows compressed\nrow_uncmp\u00a0=&gt;row_nocomp, -- number of rows non compressed\ncmp_ratio\u00a0=&gt;comp_ratio, -- compression ratio\ncomptype_str\u00a0=&gt;comptype); -- compression type\ndbms_output.put_line('Compression Ratio :'||comp_ratio);\ndbms_output.put_line('Block Count :'||blockcount_comp);\ndbms_output.put_line('Compression Type :'||comptype);\ndbms_output.put_line('Blocks Compressed :'||blockcount_comp);\ndbms_output.put_line('BlocksUncompressed:'||blockcount_nocomp);\ndbms_output.put_line('Rows Compressed :'||row_comp);\ndbms_output.put_line('Rowis Uncompressed:'||row_nocomp);end;\n\/<\/pre>\n<p>&nbsp;<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">SQL&gt; @get_ratio\nCompression Ratio :5.3\nBlock Count :361\nCompression Type :\"Compress For OLTP\"\nBlocks Compressed :361\nBlocks Uncompressed:1922\nRows Compressed :263\nRowis Uncompressed:49<\/pre>\n<p>When we analyze the results, this is what we get: Blocks Compressed 361, Blocks Uncompressed 1922. This means we should have a gain of 80 % (361 \/1922 = 0,18).<\/p>\n<p>We create a compressed tablespace:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">SQL&gt; create tablespace users_compress datafile '\/u01\/database\/DB112\/users_compress.dbf'size 100M reuse AUTOEXTEND ON NEXT 1280K MAXSIZE 32767M LOGGING EXTENT MANAGEMENT LOCAL SEGMENT SPACE MANAGEMENT AUTO default compress for all operations;<\/pre>\n<p>We create the TEST1_COMPRESS table in the compressed tablespace:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">SQL&gt; create table test1_comp tablespace users_compress2 as select * from test1;<\/pre>\n<p>We now calculate the effective sizes of the TEST1% tables:<\/p>\n<pre class=\"brush: sql; gutter: true; first-line: 1\">SQL&gt; select segment_name,bytes\/1024\/1024 from user_segments\n2 where segment_name like 'TEST1%';\nSEGMENT_NAME\u00a0\u00a0 \u00a0BYTES\/1024\/1024\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 TEST1\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a027\n\u00a0\u00a0TEST1_COMP\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a05<\/pre>\n<p>Obviously, we have the same ratio, this procedure seems to work perfectly!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>According to the Oracle documentation, the dbms_compression procedure  allows to determine the compression ratio for a table without  creating it. Let&#8217;s have a look at it.<\/p>\n","protected":false},"author":27,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[198],"tags":[246,36],"type_dbi":[],"class_list":["post-2183","post","type-post","status-publish","format-standard","hentry","category-database-management","tag-dbms-compression","tag-tablespaces"],"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>DBMS compression: determining a table&#039;s compression ratio - dbi Blog<\/title>\n<meta name=\"description\" content=\"According to the Oracle documentation, the dbms_compression procedure allows to determine the compression ratio for a table without creating it. 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