46 lines
1.7 KiB
Markdown
46 lines
1.7 KiB
Markdown
TABLE LOOKUP
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=================================
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TABLE LOOKUP 算子用于表示全局索引的回表逻辑。
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示例：全局索引回表
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```javascript
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obclient>CREATE TABLE t1(c1 INT PRIMARY KEY, c2 INT, c3 INT) PARTITION BY
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HASH(c1) PARTITIONS 4;
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Query OK, 0 rows affected (0.12 sec)
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obclient>CREATE INDEX i1 ON t1(c2) GLOBAL;
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Query OK, 0 rows affected (0.12 sec)
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obclient>EXPLAIN SELECT * FROM t1 WHERE c2 = 1\G;
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*************************** 1. row ***************************
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Query Plan:
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| ========================================
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|ID|OPERATOR |NAME |EST. ROWS|COST |
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----------------------------------------
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|0 |TABLE LOOKUP|t1 |3960 |31065|
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|1 | TABLE SCAN |t1(i1)|3960 |956 |
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========================================
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Outputs & filters:
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-------------------------------------
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0 - output([t1.c1], [t1.c2], [t1.c3]), filter(nil),
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partitions(p[0-3])
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1 - output([t1.c1]), filter(nil),
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access([t1.c1]), partitions(p0)
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```
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上述示例中，1 号算子是扫描全局索引 i1, 0 号算子表明从主表中获取不在全局索引的列。执行计划展示中的 outputs \& filters 详细展示了 TABLE LOOKUP 算子的输出信息如下：
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| **信息名称** | **含义** |
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|------------|---------------------------------------------------------------------|
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| output | 该算子的输出列。 |
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| filter | 该算子的过滤谓词。 由于示例中 TABLE LOOKUP 算子没有设置 filter，所以为 nil。 |
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| partitions | 查询需要扫描的分区。 |
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