88 lines
2.1 KiB
Markdown
88 lines
2.1 KiB
Markdown
# 表下推优化器规则
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openLooKeng支持Table下推优化器规则，以改善符合条件的查询延迟。
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用户须执行以下命令为SQL查询中的所有表生成stats，以便根据表的大小对连接源重新排序-
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```sql
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ANALYZE tableName;
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```
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**User Hint Comment-**
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用户还可以添加以下格式的特别注释，通过指定表名及其不重复（主键）列名，来利用表下推规则的好处。
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```sql
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/* #distinct@ table1 = col1, col2, ... #*/
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```
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此提示是可选的，因为如果表有相关统计信息， openLookeng可以标识所有不同的列。
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以下查询（由TPC-H Benchmark的Query 17修改而来）就是一个满足条件的查询，将外层表推入子查询，从而提升整体查询时延。
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**Original Query-**
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```sql
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SELECT
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Sum(lineitem.extendedprice) / 7.0 AS avg_yearly
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FROM
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lineitem,
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part,
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(
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SELECT
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0.2 * Avg(lineitem.quantity) AS s_avg, lineitem.partkey AS s_partkey
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FROM
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lineitem
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GROUP BY
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lineitem.partkey
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)
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WHERE
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part.partkey = lineitem.partkey
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AND
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part.brand = 'Brand#43'
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AND
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part.container = 'LG PACK'
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AND
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part.partkey = s_partkey
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AND
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lineitem.quantity < s_avg /* #distinct@ part = partkey #*/;
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```
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在上述查询中，表`part`是相关的外部查询表，以`partkey`为唯一列，与表`lineitem`连接。下面给出一个等价的重写后的查询语句，将`part`表推入到子查询。
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**Equivalent Rewritten Query-**
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```sql
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SELECT
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Sum(lineitem.extendedprice) / 7.0 AS avg_yearly
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FROM
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lineitem,
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(
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SELECT
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0.2 * Avg(lineitem.quantity) AS s_avg, lineitem.partkey AS s_partkey
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FROM
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lineitem, part
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WHERE
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part.brand = 'Brand#43'
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AND
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part.container = 'LG PACK'
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AND
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part.partkey = lineitem.partkey
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GROUP BY
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lineitem.partkey
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)
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WHERE
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s_partkey = lineitem.partkey
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AND
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lineitem.quantity < s_avg;
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```
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The feature is disabled by default. The user can enable it by executing the following command to set the session parameter-
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该功能默认是关闭状态。用户可以通过以下命令设置session参数来启用该功能-
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```sql
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SET SESSION push_table_through_subquery = true;
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```
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