SEQUENCE · k8s原理與實踐

[TOC] # SEQUENCE SEQUENCE 算子用于計算偽列 SEQUENCE 的值。偽列 SEQUENCE 是由`CREATE SEQUENCE`創建的序列，下層算子每輸出一行執行一次計算。示例：計算偽列 SEQUENCE 的當前值與下一個值 ~~~ obclient>CREATE TABLE t1(c1 INT, c2 INT); Query OK, 0 rows affected (0.09 sec) obclient>CREATE TABLE t2(c1 INT, c2 INT); Query OK, 0 rows affected (0.09 sec) obclient>CREATE SEQUENCE seq INCREMENT BY 1 START WITH 1; Query OK, 0 rows affected (0.09 sec) obclient>EXPLAIN SELECT seq.NEXTVAL, seq.CURRVAL FROM t1\G; *************************** 1. row *************************** Query Plan: |===================================== |ID|OPERATOR |NAME|EST. ROWS|COST | ------------------------------------- |0 |SEQUENCE | |100000 |77868| |1 | TABLE SCAN|T1 |100000 |64066| ===================================== Outputs & filters: ------------------------------------- 0 - output([SEQ.NEXTVAL], [SEQ.CURRVAL]), filter(nil) 1 - output([T1.__pk_increment]), filter(nil), access([T1.__pk_increment]), partitions(p0) ~~~ 上述示例中，執行計劃展示中 0 號算子 SEQUENCE 用來計算序列的值，`output([SEQ.NEXTVAL],[SEQ.CURRVAL]`指定了 SEQUENCE 需要計算序列的當前值與下一個值。t1 表每輸出一行數據，SEQUENCE 就會計算一次序列的值。執行計劃展示中 outputs & filters 詳細列出了 SEQUENCE 算子的輸出信息如下： <table data-tag="table" id="table-nr4-bac-a25" class="table"><colgroup span="1" width="139" data-tag="col" id="col-6rq-043-tlc" colwidth="1*" colnum="1" colname="col1" style="width:50%" class="col"></colgroup><colgroup span="1" width="609" data-tag="col" id="col-cdd-rwp-i1x" colwidth="1*" colnum="2" colname="col2" style="width:50%" class="col"></colgroup><thead id="thead-d4m-5jh-r1f" class="thead"><tr id="tr-5mj-1vz-m3a"><th id="td-rcc-qpm-0le"><p id="p-45d-xhb-g20"><b>信息名稱</b></p></th><th id="td-esd-snn-qja"><p id="p-ca2-5m1-rtb"><b>含義</b></p></th></tr></thead><tbody data-tag="tbody" id="tbody-1e1-h5a-2c0" class="tbody"><tr data-tag="tr" id="tr-3c5-alf-mjn" class="tr"><td data-tag="td" id="td-fdz-wzq-3j2" class="td"><p id="p-1vt-829-s5p">output</p></td><td data-tag="td" id="td-3w2-ijr-57t" class="td"><p id="p-dzj-7qx-lym">該算子輸出的列。</p><p id="p-cxt-nra-2m0">SEQUENCE 算子參數信息如下：</p><ul id="ul-x5o-zpr-99k"><li id="li-aqx-ab4-hwl"><p id="p-0nf-xsu-z33">CURRVAl：計算序列的當前值。</p></li><li id="li-983-anr-jlh"><p id="p-0hz-0ip-8t3">NEXTVAL：計算序列的下一個值。</p></li></ul></td></tr><tr data-tag="tr" id="tr-98f-q7q-ssm" class="tr"><td data-tag="td" id="td-fyt-ugi-dqg" class="td"><p id="p-a44-lti-png">filter</p></td><td data-tag="td" id="td-s40-1bw-xjr" class="td"><p id="p-euc-w8l-csi">該算子上的過濾條件。</p><p id="p-vhm-4cz-bo3">由于示例中的 SEQUENCE 算子沒有設置 filter，所以為 nil。</p></td></tr></tbody></table>