TABLE SCAN · k8s原理与实践

# TABLE SCAN TABLE SCAN 算子是存储层和 SQL 层的接口，用于展示优化器选择哪个索引来访问数据。在 OceanBase 数据库中，对于普通索引，索引的回表逻辑是封装在 TABLE SCAN 算子中的；而对于全局索引，索引的回表逻辑由 TABLE LOOKUP 算子完成。示例：含 TABLE SCAN 算子的执行计划 ~~~ obclient>CREATE TABLE t1(c1 INT PRIMARY KEY, c2 INT, c3 INT, c4 INT, INDEX k1(c2,c3)); Query OK, 0 rows affected (0.09 sec) Q1： obclient>EXPLAIN EXTENDED SELECT * FROM t1 WHERE c1 = 1\G; *************************** 1. row *************************** Query Plan: | ================================== |ID|OPERATOR |NAME|EST. ROWS|COST| ---------------------------------- |0 |TABLE GET|t1 |1 |53 | ================================== Outputs & filters: ------------------------------------- 0 - output([t1.c1(0x7f22fbe69340)], [t1.c2(0x7f22fbe695c0)], [t1.c3(0x7f22fbe69840)], [t1.c4(0x7f22fbe69ac0)]), filter(nil), access([t1.c1(0x7f22fbe69340)], [t1.c2(0x7f22fbe695c0)], [t1.c3(0x7f22fbe69840)], [t1.c4(0x7f22fbe69ac0)]), partitions(p0), is_index_back=false, range_key([t1.c1(0x7f22fbe69340)]), range[1 ; 1], range_cond([t1.c1(0x7f22fbe69340) = 1(0x7f22fbe68cf0)]) Q2： obclient>EXPLAIN EXTENDED SELECT * FROM t1 WHERE c2 < 1 AND c3 < 1 AND c4 < 1\G; *************************** 1. row *************************** Query Plan: | ====================================== |ID|OPERATOR |NAME |EST. ROWS|COST | -------------------------------------- |0 |TABLE SCAN|t1(k1)|100 |12422| ====================================== Outputs & filters: ------------------------------------- 0 - output([t1.c1(0x7f22fbd1e220)], [t1.c2(0x7f227decec40)], [t1.c3(0x7f227decf9b0)], [t1.c4(0x7f22fbd1dfa0)]), filter([t1.c3(0x7f227decf9b0) < 1(0x7f227decf360)], [t1.c4(0x7f22fbd1dfa0) < 1(0x7f22fbd1d950)]), access([t1.c2(0x7f227decec40)], [t1.c3(0x7f227decf9b0)], [t1.c4(0x7f22fbd1dfa0)], [t1.c1(0x7f22fbd1e220)]), partitions(p0), is_index_back=true, filter_before_indexback[true,false], range_key([t1.c2(0x7f227decec40)], [t1.c3(0x7f227decf9b0)], [t1.c1(0x7f22fbd1e220)]), range(NULL,MAX,MAX ; 1,MIN,MIN), range_cond([t1.c2(0x7f227decec40) < 1(0x7f227dece5f0)]) ~~~ 上述示例中，执行计划展示中的 outputs & filters 详细展示了 TABLE SCAN 算子的输出信息如下： <div 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id="Odh5c" data-tag="div" class="div"><table data-tag="table" id="table-2q6-tgt-om9" class="table"><colgroup width="240" span="1" data-tag="col" id="col-u9h-91g-y9o" colwidth="1*" colnum="1" colname="col1" style="width:50%" class="col"></colgroup><colgroup width="505" span="1" data-tag="col" id="col-2fm-4nt-3z2" colwidth="1*" colnum="2" colname="col2" style="width:50%" class="col"></colgroup><thead id="thead-rq8-8qb-hcl" class="thead"><tr id="tr-3za-d2e-l0i"><th id="td-gfa-2za-i2c">信息名称</th><th id="td-p7r-85r-6v0">含义</th></tr></thead><tbody data-tag="tbody" id="tbody-uht-qt4-sd3" class="tbody"><tr data-tag="tr" id="tr-ags-03i-n8t" class="tr"><td data-tag="td" id="td-xi8-4ir-h6s" class="td">operator</td><td data-tag="td" id="td-eff-1mg-7gb" class="td">TABLE SCAN 算子的 operator 有两种形式：TABLE SCAN 和 TABLE GET。<ul lake-indent="0" data-tag="ul" id="ul-jp5-f1d-luk" class="ul"><li data-tag="li" id="li-suu-n0x-j5j" class="li">TABLE SCAN：属于范围扫描，会返回 0 行或者多行数据。</li><li data-tag="li" id="li-9uz-rvz-rzk" class="li">TABLE GET：直接用主键定位，返回 0 行或者 1 行数据。</li></ul></td></tr><tr data-tag="tr" id="tr-tbi-rig-fgy" class="tr"><td data-tag="td" id="td-6jz-92n-hvm" class="td">name</td><td data-tag="td" id="td-zrh-dr2-qoh" class="td">选择用哪个索引来访问数据。选择的索引的名字会跟在表名后面，如果没有索引的名字，则说明执行的是主表扫描。这里需要注意，在 OceanBase 数据库中，主表和索引的组织结构是一样的，主表本身也是一个索引。</td></tr><tr data-tag="tr" id="tr-h5k-cqn-fay" class="tr"><td data-tag="td" id="td-jqc-mkr-796" class="td">output</td><td data-tag="td" id="td-uoj-g7s-npx" class="td">该算子的输出列。</td></tr><tr data-tag="tr" id="tr-ffm-qtj-lh0" class="tr"><td id="td-eap-jkg-vhj">filter</td><td id="td-w60-y5x-be8">该算子的过滤谓词。由于示例中 TABLE SCAN 算子没有设置 filter，所以为 nil。</td></tr><tr data-tag="tr" id="tr-8mw-mw6-cqw" class="tr"><td data-tag="td" id="td-d9y-tdd-kd9" class="td">partitions</td><td data-tag="td" id="td-cv3-m2u-zni" class="td">查询需要扫描的分区。</td></tr><tr data-tag="tr" id="tr-1xn-f0o-3do" class="tr"><td data-tag="td" id="td-vgv-x3n-dob" class="td">is_index_back</td><td data-tag="td" id="td-wx5-idi-571" class="td">该算子是否需要回表。例如，在 Q1 查询中，因为选择了主表，所以不需要回表。在 Q2 查询中，索引列是 <code data-tag="code" class="code">(c2,c3,c1)</code>, 由于查询需要返回 c4 列，所以需要回表。</td></tr><tr data-tag="tr" id="tr-prj-h9x-e80" class="tr"><td data-tag="td" id="td-x16-cic-trj" class="td">filter_before_indexback</td><td data-tag="td" id="td-h8y-vns-xnj" class="td">与每个 filter 对应，表明该 filter 是可以直接在索引上进行计算，还是需要索引回表之后才能计算。例如，在 Q2 查询中，filter <code data-tag="code" class="code">c3 < 1</code> 可以直接在索引上计算，能减少回表数量；filter <code data-tag="code" class="code">c4 < 1</code> 需要回表取出 c4 列之后才能计算。</td></tr><tr data-tag="tr" id="tr-gjy-a3o-8sc" class="tr"><td data-tag="td" id="td-ein-hn1-phc" class="td">range_key/range/range_cond</td><td data-tag="td" id="td-att-2j2-wjn" class="td"><ul id="ul-v05-qg7-t33"><li id="li-2v0-zcd-6ov">range_key：索引的 rowkey 列。</li></ul><ul lake-indent="0" data-tag="ul" id="ul-ggh-cef-2aw" class="ul"><li data-tag="li" id="li-qlc-xzd-x08" class="li">range：索引开始扫描和结束扫描的位置。判断是否是全表扫描需要关注 range 的范围。例如，对于一个 rowkey 有三列的场景，<code data-tag="code" class="code">range(MIN,MIN, MIN ; MAX, MAX, MAX)</code>代表的就是真正意义上的全表扫描。</li><li data-tag="li" id="li-c8u-u3f-iwg" class="li">range_cond：决定索引开始扫描和结束扫描位置的相关谓词。</li></ul></td></tr></tbody></table></div>