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SHOW TABLES

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列出当前数据库可见的表。只返回业务表，不包含 system schema 与 information_schema 视图：

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\n SQL\n \n
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1> SHOW TABLES;\n2+-------+\n3| table |\n4+-------+\n5| rooms |\n6| temps |\n7+-------+
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information_schema

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标准信息模式视图可查（tables、columns、df_settings 等）。查列定义与类型：

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\n SQL\n \n
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1> SELECT table_name, column_name, data_type FROM information_schema.columns WHERE table_name = 'temps';\n2+------------+-------------+-----------------------------+\n3| table_name | column_name | data_type                   |\n4+------------+-------------+-----------------------------+\n5| temps      | room        | Dictionary(Int32, Utf8)     |\n6| temps      | temp        | Float64                     |\n7| temps      | time        | Timestamp(Nanosecond, None) |\n8+------------+-------------+-----------------------------+
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列类型与写入数据模型的映射见 数据类型与字面量。等价的简写：SHOW COLUMNS FROM temps。

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查询可见范围：information_schema 视图只显示 public schema 下的业务表元数据；system schema 中的系统表不可查询，也不出现在视图结果中。

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EXPLAIN

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EXPLAIN 展示查询的执行计划，不执行查询，用于理解分区裁剪、投影下推等行为。以逻辑计划为例：

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\n SQL\n \n
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1> EXPLAIN SELECT avg(temp) FROM temps;\n2+--------------+---------------------------------------------------+\n3| plan_type    | plan                                              |\n4+--------------+---------------------------------------------------+\n5| logical_plan | Aggregate: groupBy=[[]], aggr=[[avg(temps.temp)]] |\n6|              |   TableScan: temps projection=[temp]              |\n7+--------------+---------------------------------------------------+
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上例只列出逻辑计划；EXPLAIN 的完整输出还包含物理执行计划，给出具体的执行算子与数据扫描信息，随数据布局变化。EXPLAIN ANALYZE 会实际执行查询并附带各算子的运行统计。

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