forked from docs/doc-exports
Lu, Huayi
e6fa411af0
Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com> Co-authored-by: Lu, Huayi <luhuayi@huawei.com> Co-committed-by: Lu, Huayi <luhuayi@huawei.com>
23 lines
4.8 KiB
HTML
23 lines
4.8 KiB
HTML
<a name="EN-US_TOPIC_0000001233563241"></a><a name="EN-US_TOPIC_0000001233563241"></a>
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<h1 class="topictitle1">Related Concepts</h1>
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<div id="body8662426"><div class="section" id="EN-US_TOPIC_0000001233563241__s8eeb92bbb2c9474abec3131c2a3de9be"><h4 class="sectiontitle">Database</h4><p id="EN-US_TOPIC_0000001233563241__a0d56a1d8a499429ab5d27ebc7f8e169b">A database manages data objects and is isolated from other databases. While creating an object, you can specify a tablespace for it. If you do not specify it, the object will be saved to the <strong id="EN-US_TOPIC_0000001233563241__b842352706142447">PG_DEFAULT</strong> space by default. Objects managed by a database can be distributed to multiple tablespaces.</p>
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<div class="section" id="EN-US_TOPIC_0000001233563241__en-us_topic_0093464731_en-us_topic_0059779238_s174b82ec22a94e478b49737f0bced1ac"><h4 class="sectiontitle">Instance</h4><p id="EN-US_TOPIC_0000001233563241__en-us_topic_0093464731_en-us_topic_0059779238_ad9aa75e070294dada1bb1426c733ac00">In <span id="EN-US_TOPIC_0000001233563241__ph165411332142716">GaussDB(DWS)</span>, instances are a group of database processes running in the memory. An instance can manage one or more databases that form a cluster. A cluster is an area in the storage disk. This area is initialized during installation and composed of a directory. The directory, called data directory, stores all data and is created by <strong id="EN-US_TOPIC_0000001233563241__b20745131862814">initdb</strong>. Theoretically, one server can start multiple instances on different ports, but <span id="EN-US_TOPIC_0000001233563241__ph159118115292">GaussDB(DWS)</span> manages only one instance at a time. The start and stop of an instance rely on the specific data directory. For compatibility purposes, the concept of instance name may be introduced.</p>
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<div class="section" id="EN-US_TOPIC_0000001233563241__sd02a43df183c4a5ebb95eb554ab910a9"><h4 class="sectiontitle">Tablespaces</h4><p id="EN-US_TOPIC_0000001233563241__a906223c1bcdb4005a529ff2b5401002d">In <span id="EN-US_TOPIC_0000001233563241__text1998133972">GaussDB(DWS)</span>, a tablespace is a directory storing physical files of the databases the tablespace contains. Multiple tablespaces can coexist. Files are physically isolated using tablespaces and managed by a file system.</p>
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<div class="section" id="EN-US_TOPIC_0000001233563241__s9d3f5dfcecdf4bad99e5f9a2688e21a5"><h4 class="sectiontitle">schema</h4><p id="EN-US_TOPIC_0000001233563241__a2d20c90fc0fe4ac5bc82f0009668ab60"><span id="EN-US_TOPIC_0000001233563241__text169550254">GaussDB(DWS)</span> schemas logically separate databases. All database objects are created under certain schemas. In <span id="EN-US_TOPIC_0000001233563241__text1332219925">GaussDB(DWS)</span>, schemas and users are loosely bound. When you create a user, a schema with the same name as the user will be created automatically. You can also create a schema or specify another schema.</p>
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<div class="section" id="EN-US_TOPIC_0000001233563241__s6b1917ec12c249d59233c11959b6db79"><h4 class="sectiontitle">User and Role</h4><p id="EN-US_TOPIC_0000001233563241__a400f976a3ed34940b5a45614ea2d4d60"><span id="EN-US_TOPIC_0000001233563241__text1828369993">GaussDB(DWS)</span> uses users and roles to control the access to databases. A role can be a database user or a group of database users, depending on role settings. In <span id="EN-US_TOPIC_0000001233563241__text1349326921">GaussDB(DWS)</span>, the difference between roles and users is that a role does not have the LOGIN permission by default. In <span id="EN-US_TOPIC_0000001233563241__text693192024">GaussDB(DWS)</span>, one user can have only one role, but you can put a user's role under a parent role to grant multiple permissions to the user.</p>
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<div class="section" id="EN-US_TOPIC_0000001233563241__s8302eda03d7b4255af235930410f3559"><h4 class="sectiontitle">Transaction Management</h4><p id="EN-US_TOPIC_0000001233563241__a0f7ae2dbc4e64f178f15d7721221bc08">In <span id="EN-US_TOPIC_0000001233563241__text484000412">GaussDB(DWS)</span>, transactions are managed by multi-version concurrency control (MVCC) and two-phase locking (2PL). It enables smooth data reads and writes. In <span id="EN-US_TOPIC_0000001233563241__text674838628">GaussDB(DWS)</span>, MVCC saves historical version data together with the current tuple version. <span id="EN-US_TOPIC_0000001233563241__text796230309">GaussDB(DWS)</span> uses the VACUUM process instead of rollback segments to routinely delete historical version data. Unless in performance optimization, you do not need to pay attention to the VACUUM process. Transactions are automatically submitted in <span id="EN-US_TOPIC_0000001233563241__text1176679149">GaussDB(DWS)</span>.</p>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dws_04_0007.html">System Overview</a></div>
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