{"id":3678,"date":"2025-05-29T12:14:20","date_gmt":"2025-05-29T12:14:20","guid":{"rendered":"https:\/\/w3buddy.com\/?post_type=cposts&#038;p=3678"},"modified":"2026-09-26T13:12:10","modified_gmt":"2026-09-26T07:42:10","slug":"system-global-area","status":"publish","type":"post","link":"https:\/\/w3buddy.com\/blog\/system-global-area\/","title":{"rendered":"System Global Area"},"content":{"rendered":"\n<h3 class=\"wp-block-heading\">What is SGA in Oracle?<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">When Oracle says it&#8217;s &#8220;allocating memory&#8221; at startup, it\u2019s setting up the <strong>System Global Area (SGA)<\/strong> \u2014 a <strong>shared memory region<\/strong> used by the Oracle instance to process and manage data efficiently.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It\u2019s like the <strong>central memory workspace<\/strong> for the database.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">\ud83e\udde9 SGA = Shared Memory for the Whole Instance<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The SGA is accessed by all background and server processes. It holds:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Parsed SQL &amp; PL\/SQL<\/li>\n\n\n\n<li>Frequently accessed data blocks<\/li>\n\n\n\n<li>Redo log entries<\/li>\n\n\n\n<li>Control structures<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Without the SGA, Oracle couldn\u2019t cache data or execute queries efficiently.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\ud83d\udee0 When the instance starts (<code>STARTUP<\/code>), Oracle allocates the SGA based on parameters like <code>SGA_TARGET<\/code> or <code>MEMORY_TARGET<\/code>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udd0d Core Components of SGA<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">\ud83d\udd39 1. <strong>Shared Pool<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Stores parsed SQL &amp; PL\/SQL code<\/li>\n\n\n\n<li>Contains data dictionary &amp; system metadata<\/li>\n\n\n\n<li>Crucial for SQL parsing and execution<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">\ud83d\udd0e Related Views: <code>V$SHARED_POOL_ADVICE<\/code>, <code>V$SQLAREA<\/code><\/p>\n\n\n\n<h3 class=\"wp-block-heading\">\ud83d\udd39 2. <strong>Database Buffer Cache<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Holds copies of data blocks read from disk<\/li>\n\n\n\n<li>Reduces physical I\/O<\/li>\n\n\n\n<li>Shared by all sessions<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">\ud83e\udde0 Hot blocks stay in cache longer \u2014 tuning this improves query speed.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">\ud83d\udd39 3. <strong>Redo Log Buffer<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Temporary memory area for redo entries (DML, DDL)<\/li>\n\n\n\n<li>Flushed to redo logs by LGWR<\/li>\n\n\n\n<li>Helps with crash recovery and consistency<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">\ud83d\udd01 Writes are short-lived \u2014 size carefully to avoid bottlenecks.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">\ud83d\udd39 4. <strong>Fixed SGA<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Small, untunable memory region<\/li>\n\n\n\n<li>Stores internal info like process state and system stats<\/li>\n\n\n\n<li>Allocated during instance startup<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">\u2699\ufe0f Optional (Advanced) SGA Components<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">These components are <strong>not always enabled<\/strong> \u2014 you configure them based on features used:<\/p>\n\n\n\n<figure class=\"wp-block-table has-small-font-size\"><table><thead><tr><th>Component<\/th><th>Use Case<\/th><\/tr><\/thead><tbody><tr><td><strong>Large Pool<\/strong><\/td><td>RMAN, Shared Server, Parallel Query, XA<\/td><\/tr><tr><td><strong>Java Pool<\/strong><\/td><td>Running Java inside Oracle DB<\/td><\/tr><tr><td><strong>Streams Pool<\/strong><\/td><td>Oracle Streams, GoldenGate, Data Pump<\/td><\/tr><tr><td><strong>In-Memory Area<\/strong><\/td><td>In-Memory Column Store for analytics<\/td><\/tr><tr><td><strong>Memoptimize Pool<\/strong><\/td><td>Fast key-value lookups<\/td><\/tr><tr><td><strong>Shared I\/O Pool<\/strong><\/td><td>SecureFile LOB I\/O optimization<\/td><\/tr><tr><td><strong>Flashback Buffer<\/strong><\/td><td>Flashback Database recovery<\/td><\/tr><tr><td><strong>Database Smart Flash Cache<\/strong><\/td><td>Extends Buffer Cache to flash disk (Linux\/Solaris)<\/td><\/tr><tr><td><strong>Vector Pool<\/strong><\/td><td>Vector indexes (AI\/ML queries, IVF\/HNSW)<\/td><\/tr><tr><td><strong>True Cache<\/strong><\/td><td>High-speed memory cache for hot tables<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udd0d Useful SGA Queries<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 Show Overall SGA Usage:<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code><code>SHOW SGA;<\/code><\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 Check Dynamic SGA Components:<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code><code>SELECT * FROM v$sga_dynamic_components;<\/code><\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 List All SGA Parameters:<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>SET LINESIZE 200\nSET PAGESIZE 100\n\nCOLUMN name FORMAT A40\nCOLUMN value FORMAT A30\n\nSELECT name, value \nFROM v$parameter \nWHERE name LIKE 'sga_%'\nORDER BY name;<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 Query : View SGA Overview Information (<code>v$sgainfo<\/code>)<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>SET LINESIZE 200\nSET PAGESIZE 100\n\nCOLUMN NAME FORMAT A50\nCOLUMN BYTES FORMAT 999,999,999,999\nCOLUMN RESIZEABLE FORMAT A15\n\nSELECT * \nFROM v$sgainfo;<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 Query : View Detailed SGA Statistics (<code>v$sgastat<\/code>)<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>SET LINESIZE 200\nSET PAGESIZE 200\n\nCOLUMN POOL FORMAT A20\nCOLUMN NAME FORMAT A50\nCOLUMN BYTES FORMAT 999,999,999,999\n\nSELECT * \nFROM v$sgastat \nORDER BY pool, name;<\/code><\/pre>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udee0\ufe0f DBA Tip of the Day<\/h2>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\">&#8220;When troubleshooting performance \u2014 <strong>start with the SGA<\/strong>.&#8221;<\/p>\n<\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\">Most slow queries, memory issues, or I\/O bottlenecks stem from:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>An undersized <strong>Buffer Cache<\/strong><\/li>\n\n\n\n<li>A fragmented <strong>Shared Pool<\/strong><\/li>\n\n\n\n<li>Redo log buffer contention<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">\ud83d\udcca Use views like <code>V$SGAINFO<\/code>, <code>V$SGASTAT<\/code>, and <code>V$MEMORY_TARGET_ADVICE<\/code> to monitor memory and tune the instance proactively.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>What is SGA in Oracle? When Oracle says it&#8217;s &#8220;allocating memory&#8221; at startup, it\u2019s setting up the System Global Area (SGA) \u2014 a shared memory region used by the Oracle instance to process and manage data efficiently. It\u2019s like the central memory workspace for the database. \ud83e\udde9 SGA = Shared Memory for the Whole Instance [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"googlesitekit_rrm_CAowu461DA:productID":"","footnotes":""},"categories":[1225,960],"tags":[],"class_list":["post-3678","post","type-post","status-publish","format-standard","hentry","category-database","category-getting-started-with-oracle-dba"],"_links":{"self":[{"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/posts\/3678","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/comments?post=3678"}],"version-history":[{"count":6,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/posts\/3678\/revisions"}],"predecessor-version":[{"id":3773,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/posts\/3678\/revisions\/3773"}],"wp:attachment":[{"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/media?parent=3678"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/categories?post=3678"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/w3buddy.com\/blog\/wp-json\/wp\/v2\/tags?post=3678"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}