如何使用PostgreSQL从表名中获取列属性？

Question

我有一个项目,我需要一个查询来获取列的所有属性(列名称,位置,数据类型,非空？和注释)所有这些使用表名称.

我实现了获取列名,位置数据类型和非空？使用此查询:

SELECT column_name, data_type, ordinal_position, is_nullable 
FROM information_schema."columns"
WHERE "table_name"='TABLE-NAME'

但是,我需要评论!

Answer 1

这是针对系统目录的查询,该目录应该获取您需要的所有内容(免费提供奖励主键字段).

SELECT DISTINCT
    a.attnum as num,
    a.attname as name,
    format_type(a.atttypid, a.atttypmod) as typ,
    a.attnotnull as notnull, 
    com.description as comment,
    coalesce(i.indisprimary,false) as primary_key,
    def.adsrc as default
FROM pg_attribute a 
JOIN pg_class pgc ON pgc.oid = a.attrelid
LEFT JOIN pg_index i ON 
    (pgc.oid = i.indrelid AND i.indkey[0] = a.attnum)
LEFT JOIN pg_description com on 
    (pgc.oid = com.objoid AND a.attnum = com.objsubid)
LEFT JOIN pg_attrdef def ON 
    (a.attrelid = def.adrelid AND a.attnum = def.adnum)
WHERE a.attnum > 0 AND pgc.oid = a.attrelid
AND pg_table_is_visible(pgc.oid)
AND NOT a.attisdropped
AND pgc.relname = 'TABLE_NAME'  -- Your table name here
ORDER BY a.attnum;

哪个会返回如下结果:

 num |    name     |             typ             | notnull |       comment       | primary_key 
-----+-------------+-----------------------------+---------+---------------------+-------------
   1 | id          | integer                     | t       | a primary key thing | t
   2 | ref         | text                        | f       |                     | f
   3 | created     | timestamp without time zone | t       |                     | f
   4 | modified    | timestamp without time zone | t       |                     | f
   5 | name        | text                        | t       |                     | f

• num:列号
• name:列名
• typ:数据类型
• notnull:列是否定义为 NOT NULL
• 注释:COMMENT为列定义的任何内容
• primary_key:列是否定义为 PRIMARY KEY
• default:用于默认值的命令

Answer 2

建立在@Chris的答案之上:

SELECT a.attnum
      ,a.attname                            AS name
      ,format_type(a.atttypid, a.atttypmod) AS typ
      ,a.attnotnull                         AS notnull
      ,coalesce(p.indisprimary, FALSE)      AS primary_key
      ,f.adsrc                              AS default_val
      ,d.description                        AS col_comment
FROM   pg_attribute    a 
LEFT   JOIN pg_index   p ON p.indrelid = a.attrelid AND a.attnum = ANY(p.indkey)
LEFT   JOIN pg_description d ON d.objoid  = a.attrelid AND d.objsubid = a.attnum
LEFT   JOIN pg_attrdef f ON f.adrelid = a.attrelid  AND f.adnum = a.attnum
WHERE  a.attnum > 0
AND    NOT a.attisdropped
AND    a.attrelid = 'schema.tbl'::regclass  -- table may be schema-qualified
ORDER  BY a.attnum;

但:

表名在数据库中不是唯一的,因此也不在系统目录中.您可能必须对该名称进行架构限定.
使用a.attrelid = 'tbl'::regclass的条件.这样您就可以传递myschema.mytbl名称和消除歧义.pg_class在这种情况下,根本不需要加入.
此外,自动检查可见性,regclass并且不需要pg_table_is_visible().

甲主键可以跨越多个列.我通过加入来pg_index解决这个问题a.attnum = ANY(p.indkey).
indkey属于类型int2vecor,这是一种特殊情况int2[],仅用于目录中.

我觉得psql -E这类问题很有帮助.

兼容性

在主要版本更新后,这样的专用查询可能会中断.Postgres不保证目录表保持稳定.基本元素变化的可能性极小,但查询越复杂和专业化,机会就越大.您可以使用信息模式,这是标准化的,但也相对较慢.