PostgreSQL “DESCRIBE TABLE”

技术背景

在数据库操作中，了解表的结构是一项常见需求。在 MySQL 中，可使用 DESCRIBE table_name 语句查看表结构。而在 PostgreSQL 里，没有直接对应的 DESCRIBE TABLE 语句，但有多种方法能实现相同功能。

实现步骤

1. 使用 psql 元命令

在 psql 命令行工具中，可以使用以下元命令：

• \d+ tablename：查看表的详细信息，包括存储、统计目标和描述等。
• \d tablename：查看表的基本信息，如列名、类型和修饰符。
• \dt：简单描述多个表。
• \dt <table>：简单描述单个表。
• \d <table> <table>：详细描述多个表。
• \d+ <table> <table>：更详细地描述多个表。

示例：

\d+ queuerecords

2. 使用 SQL 查询

2.1 查询 INFORMATION_SCHEMA.COLUMNS

SELECT *
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_schema = 'your_schema'
  AND table_name = 'your_table';

若仅需查看特定列的数据类型：

SELECT DATA_TYPE 
FROM INFORMATION_SCHEMA.COLUMNS 
WHERE table_name = 'tbl_name' 
  AND COLUMN_NAME = 'col_name';

2.2 查询系统目录

SELECT
    f.attnum AS number,
    f.attname AS name,
    f.attnum,
    f.attnotnull AS notnull,
    pg_catalog.format_type(f.atttypid,f.atttypmod) AS type,
    CASE
        WHEN p.contype = 'p' THEN 't'
        ELSE 'f'
    END AS primarykey,
    CASE
        WHEN p.contype = 'u' THEN 't'
        ELSE 'f'
    END AS uniquekey,
    CASE
        WHEN p.contype = 'f' THEN g.relname
    END AS foreignkey,
    CASE
        WHEN p.contype = 'f' THEN p.confkey
    END AS foreignkey_fieldnum,
    CASE
        WHEN p.contype = 'f' THEN g.relname
    END AS foreignkey,
    CASE
        WHEN p.contype = 'f' THEN p.conkey
    END AS foreignkey_connnum,
    CASE
        WHEN f.atthasdef = 't' THEN pg_get_expr(d.adbin, d.adrelid)
    END AS default
FROM pg_attribute f
    JOIN pg_class c ON c.oid = f.attrelid
    JOIN pg_type t ON t.oid = f.atttypid
    LEFT JOIN pg_attrdef d ON d.adrelid = c.oid AND d.adnum = f.attnum
    LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
    LEFT JOIN pg_constraint p ON p.conrelid = c.oid AND f.attnum = ANY (p.conkey)
    LEFT JOIN pg_class AS g ON p.confrelid = g.oid
WHERE c.relkind = 'r'::char
  AND n.nspname = '%s'  -- Replace with Schema name
  AND c.relname = '%s'  -- Replace with table name
  AND f.attnum > 0 ORDER BY number;

3. 处理特殊情况

• 若表名以大写字母开头，需将表名放在引号中，例如：\d "Users"。
• 若表不在默认模式中，应使用 \d+ schema_name.table_name，否则会报错“the relation doesn not exist”。

核心代码

查询表的基本列信息

SELECT
    a.attname AS Field,
    t.typname || '(' || a.atttypmod || ')' AS Type,
    CASE WHEN a.attnotnull = 't' THEN 'YES' ELSE 'NO' END AS Null,
    CASE WHEN r.contype = 'p' THEN 'PRI' ELSE '' END AS Key,
    (SELECT substring(pg_catalog.pg_get_expr(d.adbin, d.adrelid), '\'(.*)\'')
            FROM
                    pg_catalog.pg_attrdef d
            WHERE
                    d.adrelid = a.attrelid
                    AND d.adnum = a.attnum
                    AND a.atthasdef) AS Default,
    '' as Extras
FROM
        pg_class c 
        JOIN pg_attribute a ON a.attrelid = c.oid
        JOIN pg_type t ON a.atttypid = t.oid
        LEFT JOIN pg_catalog.pg_constraint r ON c.oid = r.conrelid 
                AND r.conname = a.attname
WHERE
        c.relname = 'tablename'
        AND a.attnum > 0
        
ORDER BY a.attnum;

更详细的表信息查询

SELECT
    n.nspname as schema,
    c.relname as table,
    f.attname as column,  
    f.attnum as column_id,  
    f.attnotnull as not_null,
    f.attislocal not_inherited,
    f.attinhcount inheritance_count,
    pg_catalog.format_type(f.atttypid,f.atttypmod) AS data_type_full,
    t.typname AS data_type_name,
    CASE  
        WHEN f.atttypmod >= 0 AND t.typname <> 'numeric' THEN (f.atttypmod - 4) --first 4 bytes are for storing actual length of data
    END AS data_type_length, 
    CASE  
        WHEN t.typname = 'numeric' THEN (((f.atttypmod - 4) >> 16) & 65535)
    END AS numeric_precision, 	
    CASE  
        WHEN t.typname = 'numeric' THEN ((f.atttypmod - 4)& 65535 )
    END AS numeric_scale, 		
    CASE  
        WHEN p.contype = 'p' THEN 't'  
        ELSE 'f'  
    END AS is_primary_key,  
    CASE
        WHEN p.contype = 'p' THEN p.conname
    END AS primary_key_name,
    CASE  
        WHEN p.contype = 'u' THEN 't'  
        ELSE 'f'
    END AS is_unique_key,
    CASE
        WHEN p.contype = 'u' THEN p.conname
    END AS unique_key_name,
    CASE
        WHEN p.contype = 'f' THEN 't'
        ELSE 'f'
    END AS is_foreign_key,
    CASE
        WHEN p.contype = 'f' THEN p.conname
    END AS foreignkey_name,
    CASE
        WHEN p.contype = 'f' THEN p.confkey
    END AS foreign_key_columnid,
    CASE
        WHEN p.contype = 'f' THEN g.relname
    END AS foreign_key_table,
    CASE
        WHEN p.contype = 'f' THEN p.conkey
    END AS foreign_key_local_column_id,
    CASE
        WHEN f.atthasdef = 't' THEN d.adsrc
    END AS default_value
FROM pg_attribute f  
    JOIN pg_class c ON c.oid = f.attrelid  
    JOIN pg_type t ON t.oid = f.atttypid  
    LEFT JOIN pg_attrdef d ON d.adrelid = c.oid AND d.adnum = f.attnum  
    LEFT JOIN pg_namespace n ON n.oid = c.relnamespace  
    LEFT JOIN pg_constraint p ON p.conrelid = c.oid AND f.attnum = ANY (p.conkey)  
    LEFT JOIN pg_class AS g ON p.confrelid = g.oid  
WHERE c.relkind = 'r'::char  
    AND f.attisdropped = false
    AND n.nspname = '%s'  -- Replace with Schema name  
    AND c.relname = '%s'  -- Replace with table name  
    AND f.attnum > 0 
ORDER BY f.attnum;

最佳实践

• 若仅需快速查看表的基本结构，使用 psql 元命令 \d tablename 或 \d+ tablename 较为便捷。
• 若要在程序中动态获取表结构信息，可使用 SQL 查询 INFORMATION_SCHEMA.COLUMNS 或系统目录。
• 对于复杂的表结构分析，可使用包含约束、继承等信息的详细查询。

常见问题

• 表名大小写问题：PostgreSQL 对表名大小写敏感，若表名以大写字母开头，需用引号括起来。
• 表不在默认模式：使用 \d 或 \d+ 时，若表不在默认模式中，需指定模式名，否则会报错。