Hive 表定义主键约束

文章目录

1.建表语句
2.主键约束
3.主键约束的意义
参考文献

1.建表语句

先看一下官方给的完整的见表语句：

sql 复制代码

CREATE [TEMPORARY] [EXTERNAL] TABLE [IF NOT EXISTS] [db_name.]table_name    -- (Note: TEMPORARY available in Hive 0.14.0 and later)
  [(col_name data_type [column_constraint_specification] [COMMENT col_comment], ... [constraint_specification])]
  [COMMENT table_comment]
  [PARTITIONED BY (col_name data_type [COMMENT col_comment], ...)]
  [CLUSTERED BY (col_name, col_name, ...) [SORTED BY (col_name [ASC|DESC], ...)] INTO num_buckets BUCKETS]
  [SKEWED BY (col_name, col_name, ...)                  -- (Note: Available in Hive 0.10.0 and later)]
     ON ((col_value, col_value, ...), (col_value, col_value, ...), ...)
     [STORED AS DIRECTORIES]
  [
   [ROW FORMAT row_format] 
   [STORED AS file_format]
     | STORED BY 'storage.handler.class.name' [WITH SERDEPROPERTIES (...)]  -- (Note: Available in Hive 0.6.0 and later)
  ]
  [LOCATION hdfs_path]
  [TBLPROPERTIES (property_name=property_value, ...)]   -- (Note: Available in Hive 0.6.0 and later)
  [AS select_statement];   -- (Note: Available in Hive 0.5.0 and later; not supported for external tables)
 
CREATE [TEMPORARY] [EXTERNAL] TABLE [IF NOT EXISTS] [db_name.]table_name
  LIKE existing_table_or_view_name
  [LOCATION hdfs_path];
 
data_type
  : primitive_type
  | array_type
  | map_type
  | struct_type
  | union_type  -- (Note: Available in Hive 0.7.0 and later)
 
primitive_type
  : TINYINT
  | SMALLINT
  | INT
  | BIGINT
  | BOOLEAN
  | FLOAT
  | DOUBLE
  | DOUBLE PRECISION -- (Note: Available in Hive 2.2.0 and later)
  | STRING
  | BINARY      -- (Note: Available in Hive 0.8.0 and later)
  | TIMESTAMP   -- (Note: Available in Hive 0.8.0 and later)
  | DECIMAL     -- (Note: Available in Hive 0.11.0 and later)
  | DECIMAL(precision, scale)  -- (Note: Available in Hive 0.13.0 and later)
  | DATE        -- (Note: Available in Hive 0.12.0 and later)
  | VARCHAR     -- (Note: Available in Hive 0.12.0 and later)
  | CHAR        -- (Note: Available in Hive 0.13.0 and later)
 
array_type
  : ARRAY < data_type >
 
map_type
  : MAP < primitive_type, data_type >
 
struct_type
  : STRUCT < col_name : data_type [COMMENT col_comment], ...>
 
union_type
   : UNIONTYPE < data_type, data_type, ... >  -- (Note: Available in Hive 0.7.0 and later)
 
row_format
  : DELIMITED [FIELDS TERMINATED BY char [ESCAPED BY char]] [COLLECTION ITEMS TERMINATED BY char]
        [MAP KEYS TERMINATED BY char] [LINES TERMINATED BY char]
        [NULL DEFINED AS char]   -- (Note: Available in Hive 0.13 and later)
  | SERDE serde_name [WITH SERDEPROPERTIES (property_name=property_value, property_name=property_value, ...)]
 
file_format:
  : SEQUENCEFILE
  | TEXTFILE    -- (Default, depending on hive.default.fileformat configuration)
  | RCFILE      -- (Note: Available in Hive 0.6.0 and later)
  | ORC         -- (Note: Available in Hive 0.11.0 and later)
  | PARQUET     -- (Note: Available in Hive 0.13.0 and later)
  | AVRO        -- (Note: Available in Hive 0.14.0 and later)
  | JSONFILE    -- (Note: Available in Hive 4.0.0 and later)
  | INPUTFORMAT input_format_classname OUTPUTFORMAT output_format_classname
 
column_constraint_specification:
  : [ PRIMARY KEY|UNIQUE|NOT NULL|DEFAULT [default_value]|CHECK  [check_expression] ENABLE|DISABLE NOVALIDATE RELY/NORELY ]
 
default_value:
  : [ LITERAL|CURRENT_USER()|CURRENT_DATE()|CURRENT_TIMESTAMP()|NULL ] 
 
constraint_specification:
  : [, PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
    [, PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
    [, CONSTRAINT constraint_name FOREIGN KEY (col_name, ...) REFERENCES table_name(col_name, ...) DISABLE NOVALIDATE 
    [, CONSTRAINT constraint_name UNIQUE (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
    [, CONSTRAINT constraint_name CHECK [check_expression] ENABLE|DISABLE NOVALIDATE RELY/NORELY ]

2.主键约束

其中关于主键约束的定义语句如下：

PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY

在 Hive 中，可以使用 PRIMARY KEY 子句来定义主键约束，以确保表中的每行数据都具有唯一标识。主键约束要求表中的每个行都必须具有唯一的主键值，这可以通过在表中定义一个或多个列来实现。

当使用 PRIMARY KEY 子句定义主键约束时，可以使用 DISABLE NOVALIDATE 子句来禁用主键约束的验证。

sql 复制代码

CREATE TABLE my_table (
  id INT,
  name STRING,
  PRIMARY KEY (id) DISABLE NOVALIDATE
);

注意： 因为 Hive 目前对主键约束支持的不是很完善，现在暂不支持默认的 ENABLE 和 VALIDATE，所以定义主键约束时，「必须带上 DISABLE NOVALIDATE」，不然会报如下错误：

SemanticException [Error 10326]: Invalid Constraint syntax ENABLE/ENFORCED feature not supported yet. Please use DISABLE/NOT ENFORCED instead. (state=42000,code=1032

既然需要禁用主键约束，那么创建一个无法约束的主键意义何在呢？

3.主键约束的意义

对于唯一约束，目前 Hive 是不支持的，但是其他一些约束是支持的。

进入 HIVE-16575 的版本信息，会看到如下说明：

虽然 Hive 不支持主键与外键约束，但它们将被存储并可以使用 RELY 进行重写/优化。

定义主键约束时，RELY/NORELY 是可选的，缺省为 RELY。

如果一个约束指定 RELY，也就是希望 HIVE 基于代价的优化器 CBO（Cost-Based Optimizer）使用约束信息来获得更好的统计信息，得到更好的执行计划。

具体到主键约束，一般主键常用于多表关联查询，利用主键约束，可以删除不必要的连接（ JOIN ELIMINATION），得到更好的执行计划。

参考文献

Hive LanguageManual - DDL
Hive 建表语句解析 - 阿里云开发者社区