oracle10g系统管理之段

<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">1.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">段用于存储特定对象的所有数据，它是由一个或多个区组成的。当建立表，索引，和簇等对象时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">oracle</span></span><span style="">会为这些对象分配存储空间，以存放它们的数据信息。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">2.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">表段，用于存储表的所有数据，其名称与表明相同。建立表时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为该表建立相应的表段，并且表段只能存放在一个表空间中。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">3.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">表分区段，用于存储分区表某个分区的内容，每个分区对应一个表分区段。通过使用分区选项，可以将一张大表的数据部署到多个不同的分区段中，从而降低</span><span lang="EN-US"><span style="font-family: Times New Roman;">I/O</span></span><span style="">次数，提高</span><span lang="EN-US"><span style="font-family: Times New Roman;">IO</span></span><span style="">性能。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">4.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">簇段（</span><span lang="EN-US"><span style="font-family: Times New Roman;">Cluster</span></span><span style="">）簇段用于存储簇表的数据，其名称与簇名相同。簇用于逻辑组织相关表的数据，使用簇的目的是为了降低</span><span lang="EN-US"><span style="font-family: Times New Roman;">IO</span></span><span style="">次数，提高</span><span lang="EN-US"><span style="font-family: Times New Roman;">IO</span></span><span style="">性能。例如，如果经常执行主从表之间的连接查询，可以将这两张表的数据组织到簇中。一个簇可以包含一个或多个表，当将表组织到簇中时，其数据存放在簇段中，而不会为表分配专门的表段。假定在</span><span lang="EN-US"><span style="font-family: Times New Roman;">USERS</span></span><span style="">表空间上建立簇</span><span style="font-family: Times New Roman;"> <span lang="EN-US">ORD_ITEM_CLU</span></span><span style="">，并且将表</span><span lang="EN-US"><span style="font-family: Times New Roman;">ORD</span></span><span style="">和</span><span lang="EN-US"><span style="font-family: Times New Roman;">ITEM</span></span><span style="">组织到该簇中，此时只会分配簇段</span><span lang="EN-US"><span style="font-family: Times New Roman;">ORD_ITEM_CLU</span></span><span style="">。而不会分配表段</span><span lang="EN-US"><span style="font-family: Times New Roman;">ORD</span></span><span style="">和</span><span lang="EN-US"><span style="font-family: Times New Roman;">ITEM</span></span><span style="">。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">5.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">索引段（</span><span lang="EN-US"><span style="font-family: Times New Roman;">Index</span></span><span style="">）</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt 21pt;"><span style="font-size: small;"><span style="">索引段用于存储索引的数据，其名称与索引名完全相同。当建立索引时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为索引建立相应的索引段，并且一个索引段只能存放在一个表空间中。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">6.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">索引段用于存储索引的数据，其名称与索引名完全相同。当建立索引时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为索引建立相应的索引段，并且一个索引段只能存放在一个表空间中。例如，当在</span><span lang="EN-US"><span style="font-family: Times New Roman;">EMP</span></span><span style="">表上建立索引</span><span lang="EN-US"><span style="font-family: Times New Roman;">PK_EMP</span></span><span style="">时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会奖励索引段</span><span lang="EN-US"><span style="font-family: Times New Roman;">PK_EMP</span></span><span style="">。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">7.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">索引分区段</span><span lang="EN-US"><span style="font-family: Times New Roman;">(Index Partition)</span></span><span style="">索引分区段用于存储分区索引某个分区的数据，每个分区对应一个索引分区段。索引分区段类似于表分区，通过分区选项。可以将大索引的数据部署到几个不同的分区段中。索引分区只能建立在分区表上。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">8.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style="font-size: small;"><span style="">索引组织表段（</span><span lang="EN-US"><span style="font-family: Times New Roman;">Index-organized Table</span></span><span style="">）</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt 21pt;"><span style="font-size: small;"><span style="">索引组织表是以</span><span lang="EN-US"><span style="font-family: Times New Roman;">B*-</span></span><span style="">树结构来组织表数据的。如果某表数据经常需要引用主键列来检索，可以考虑建立索引组织表，以加快数据访问速度。一般情况下，表与索引数据分别存放在表段和索引段中。但是，如果建立索引组织表，表和主键索引的数据会存放在索引段中，需要注意，当建立索引组织表时，必须定义主键列。</span></span></p>
<p class="MsoNormal" style=""><span style="font-family: Times New Roman;"><span style="" lang="EN-US"><span style=""><span style="font-size: small;">9.</span><span style='font: 7pt "Times New Roman";'> </span></span></span><span lang="EN-US"><span style="font-size: small;">UNDO</span></span></span><span style=""><span style="font-size: small;">段</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt 21pt;"><span style="font-size: small;"><span lang="EN-US"><span style="font-family: Times New Roman;">UNDO</span></span><span style="">段用于存放事务所修改数据的旧值。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">10.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style=""><span style="font-size: small;">临时段</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt; text-indent: 21pt;"><span style=""><span style="font-size: small;">用于存放临时数据</span></span></p>
<p class="MsoNormal" style=""><span style="font-family: Times New Roman;"><span style="" lang="EN-US"><span style=""><span style="font-size: small;">11.</span><span style='font: 7pt "Times New Roman";'> </span></span></span><span lang="EN-US"><span style="font-size: small;">LOB</span></span></span><span style=""><span style="font-size: small;">段</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt 21pt;"><span style="font-size: small;"><span lang="EN-US"><span style="font-family: Times New Roman;">LOB</span></span><span style="">段用于存储</span><span lang="EN-US"><span style="font-family: Times New Roman;">LOB</span></span><span style="">列的数据。例如文献，图像，音频，视频等数据。建立表时，可以定义</span><span lang="EN-US"><span style="font-family: Times New Roman;">CLOB</span></span><span style="">，</span><span lang="EN-US"><span style="font-family: Times New Roman;">BLOB</span></span><span style="">，</span><span lang="EN-US"><span style="font-family: Times New Roman;">NCLOB</span></span><span style="">类型的列。并且</span><span lang="EN-US"><span style="font-family: Times New Roman;">ORACLE</span></span><span style="">会自动为这些列建立相应的</span><span lang="EN-US"><span style="font-family: Times New Roman;">LOB</span></span><span style="">段。对于</span><span lang="EN-US"><span style="font-family: Times New Roman;">LOB</span></span><span style="">列来说，如果长度低于</span><span lang="EN-US"><span style="font-family: Times New Roman;">4000</span></span><span style="">字节，该列与其他列的数据会一起存放在表段中；如果长度超过</span><span lang="EN-US"><span style="font-family: Times New Roman;">4000</span></span><span style="">，数据将被存放到</span><span lang="EN-US"><span style="font-family: Times New Roman;">LOB</span></span><span style="">段中。</span></span></p>
<p class="MsoNormal" style=""><span style="" lang="EN-US"><span style=""><span style="font-family: Times New Roman;"><span style="font-size: small;">12.</span><span style='font: 7pt "Times New Roman";'> </span></span></span></span><span style=""><span style="font-size: small;">显示段信息</span></span></p>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt 21pt;"><span style="font-size: small;"><span style="">建立簇时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为其分配簇段；建立表时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为其分配表段；建立索引时，</span><span lang="EN-US"><span style="font-family: Times New Roman;">Oracle</span></span><span style="">会为其分配索引段。通过查询</span><span lang="EN-US"><span style="font-family: Times New Roman;">dba_segments</span></span><span style="">。可以查询数据库包含的所有段类型。</span></span></p>