WINDOW FUNCTION · k8s原理与实践

# WINDOW FUNCTION WINDOW FUNCTION 算子用于实现 SQL 中的分析函数（也叫窗口函数），计算窗口下的相关行的结果。窗口函数与聚集函数不同的是，聚集函数一组只能返回一行，而窗口函数每组可以返回多行，组内每一行都是基于窗口的逻辑计算的结果。因此，在执行含有 WINDOW FUNCTION 的 SQL 时 (格式一般为`OVER(...)`），都会在生成执行计划的时候分配一个 WINDOW FUNCTION 算子。示例：含 WINDOW FUNCTION 算子的执行计划 ~~~ obclient>CREATE TABLE t1(c1 INT, c2 INT); Query OK, 0 rows affected (0.12 sec) obclient>INSERT INTO t1 VALUES(1, 1); Query OK, 1 rows affected (0.12 sec) obclient>INSERT INTO t1 VALUES(2, 2); Query OK, 1 rows affected (0.12 sec) obclient>INSERT INTO t1 VALUES(3, 3); Query OK, 1 rows affected (0.12 sec) Q1: obclient>EXPLAIN SELECT MAX(c1) OVER(PARTITION BY c1 ORDER BY c2) FROM t1\G; *************************** 1. row *************************** Query Plan: | ======================================== |ID|OPERATOR |NAME|EST. ROWS|COST| ---------------------------------------- |0 |WINDOW FUNCTION| |3 |45 | |1 | SORT | |3 |44 | |2 | TABLE SCAN |T1 |3 |37 | ======================================== Outputs & filters: ------------------------------------- 0 - output([T_FUN_MAX(T1.C1)]), filter(nil), win_expr(T_FUN_MAX(T1.C1)), partition_by([T1.C1]), order_by([T1.C2, ASC]), window_type(RANGE), upper(UNBOUNDED PRECEDING), lower(CURRENT ROW) 1 - output([T1.C1], [T1.C2]), filter(nil), sort_keys([T1.C1, ASC], [T1.C2, ASC]) 2 - output([T1.C1], [T1.C2]), filter(nil), access([T1.C1], [T1.C2]), partitions(p0) ~~~ 其中，窗口函数中指定了一个 ORDER BY/PARTITION BY 的时候，会在下层分配一个 SORT 算子，将排序结果返回给窗口函数算子使用。上述示例中，Q1 查询的执行计划展示中的 outputs & filters 详细列出了 WINDOW FUNCTION 算子的输出信息如下： <div 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id="18kRY" data-tag="div" class="div"><table data-tag="table" id="table-5ga-h0g-i98" class="table"><colgroup span="1" width="140" data-tag="col" id="col-rxu-5a6-cjf" colwidth="1*" colnum="1" colname="col1" style="width:50%" class="col"></colgroup><colgroup span="1" width="606" data-tag="col" id="col-1yz-eq1-fct" colwidth="1*" colnum="2" colname="col2" style="width:50%" class="col"></colgroup><thead id="thead-t5d-08w-d0r" class="thead"><tr id="tr-pb8-ytr-3uc"><th id="td-79l-2f4-x6f">信息名称</th><th id="td-4d6-dew-nwy">含义</th></tr></thead><tbody data-tag="tbody" id="tbody-zqr-mca-y11" class="tbody"><tr data-tag="tr" id="tr-0ns-3wh-tey" class="tr"><td data-tag="td" id="td-zht-y4d-lja" class="td">output</td><td data-tag="td" id="td-1de-d2i-y1e" class="td">该算子输出的表达式。</td></tr><tr data-tag="tr" id="tr-124-57x-t57" class="tr"><td data-tag="td" id="td-08r-a3l-nun" class="td">filter</td><td data-tag="td" id="td-uee-3mt-sfd" class="td">该算子上的过滤条件。由于示例中 WINDOW FUNCTION 算子没有设置 filter，所以为 nil。</td></tr><tr data-tag="tr" id="tr-nqr-pxn-fuz" class="tr"><td data-tag="td" id="td-277-qvw-nde" class="td">win_expr</td><td data-tag="td" id="td-4jn-mfq-gv5" class="td">在窗口中使用何种聚合函数。例如，Q1 查询为求 c1 列的最大值，因此为<code data-tag="code" class="code">T_FUN_MAX(t1.c1)</code>。</td></tr><tr data-tag="tr" id="tr-3rj-7mu-kpd" class="tr"><td data-tag="td" id="td-cya-izy-zp6" class="td">partition_by</td><td data-tag="td" id="td-85m-cjl-r9c" class="td">在窗口中按照何种方式分组。例如，Q1 查询为按照 c1 列分组，因此为 <code data-tag="code" class="code">t1.c1</code>。</td></tr><tr data-tag="tr" id="tr-7if-p4x-0mv" class="tr"><td data-tag="td" id="td-uw4-4eq-kqa" class="td">order_by</td><td data-tag="td" id="td-0cn-19z-ut6" class="td">在窗口中按照何种方式排序。例如，Q1 查询为按照 c2 列排序，因此为 <code data-tag="code" class="code">t1.c2</code>。</td></tr><tr data-tag="tr" id="tr-kz3-4ow-5fg" class="tr"><td data-tag="td" id="td-vvq-cn9-heb" class="td">window_type</td><td data-tag="td" id="td-o08-1v8-2w8" class="td">窗口类型，包括 range 和 rows 两种：<ul id="ul-8cd-krc-8de"><li id="li-449-8r3-nwp">range ：按照逻辑位置偏移进行计算窗口上下界限，默认使用 range 方式。</li><li id="li-dbe-mfu-iw2">rows ：按照实际物理位置偏移进行计算窗口上下界限。</li></ul>例如，Q1 查询未设置窗口类型，因此选择了默认方式 range。</td></tr><tr data-tag="tr" id="tr-n5t-bci-oks" class="tr"><td data-tag="td" id="td-ict-0wr-4ut" class="td">upper</td><td data-tag="td" id="td-m16-eru-76d" class="td">设定窗口的上边界：<ul lake-indent="0" data-tag="ul" id="ul-58c-qi9-lbd" class="ul"><li data-tag="li" id="li-mp9-cxr-08m" class="li">UNBOUNDED ：无边界，选择最大的值（默认）。</li><li data-tag="li" id="li-ih2-ns5-9r7" class="li">CURRENT ROW ：从当前行开始，如果出现数字则表示移动的行数。</li><li data-tag="li" id="li-btb-10v-275" class="li">PRECEDING ：向前取边</li><li data-tag="li" id="li-xem-cgl-cyx" class="li">FOLLOWING：向后取边界。</li></ul>例如，Q1 查询设置的上边界为向前无边界。</td></tr><tr data-tag="tr" id="tr-sjx-9wn-nwi" class="tr"><td data-tag="td" id="td-2mq-ns0-hrm" class="td">lower</td><td data-tag="td" id="td-v1y-syf-h8g" class="td">设定窗口的下边界，边界属性设置同 upper。例如，Q1 查询设置的下边界为当前行。</td></tr></tbody></table></div>