获得mysql、postgresql元数据信息

源于最近新的需求需要一个新功能，需要获得元数据表的信息，包括comment等，展示在页面上。

mysql

-- 查询当前库有多少个表
select 
	count(*) 
from information_schema.tables 
where 
	table_schema = (select database())
	
-- 查询所有的表及其元数据信息
select 
	table_name tableName, 
	engine, 
	table_comment tableComment, 
	create_time createTime 
	from information_schema.tables 
where 
	table_schema = (select database()) 
	
-- 获得表的所有列的元数据信息
select 
	column_name columnName, 
	data_type dataType, 
	column_comment columnComment, 
	column_key columnKey, extra 
from information_schema.columns
where 
	table_name = 'emp' 
	and table_schema = (select database()) 
order by ordinal_position


-- 获得表的引擎和注释
select 
	table_name tableName, 
	engine, 
	table_comment tableComment, 
	create_time createTime 
from information_schema.tables 
where 
	table_schema = (select database()) 
	and table_name = 'emp' 

postgresql

postgresql比较特殊一点，因为postgresql使用了namespace的概念，可能同一个库中不同的namesespace有两个表明一样的表：

-- 查询某个命名空间的所有的表名
select tablename from pg_tables where schemaname = 'public'

-- 查询某个命名空间所有的表的注释信息
select 
 relname as "name",
 obj_description(oid) as "comment"
from pg_class 
where 
  obj_description(relnamespace) like '%public%'
-- 	and relname = 't_yl_hjdm' -- 需要指定某个表的时候给表名的条件

-- 查询某个命名空间的某个表的所有列的元数据信息
-- 两个函数的用法参考官方文档：https://www.postgresql.org/docs/10/functions-info.html
-- 关于存储表和列元数据的信息文档：https://www.postgresql.org/docs/10/catalogs.html
select 
	col_description(pa.attrelid, pa.attnum) as "comment",
	format_type(pa.atttypid, pa.atttypmod) as "type",
	pa.attname as "name"
from pg_class as pc, pg_attribute as pa, pg_namespace as pn
where 
	pa.attrelid = pc.oid
	and pn.nspname = 'public'
	and pc.relname = 't_yl_hjdm'
	and col_description(pa.attrelid, pa.attnum) is not null

参考：
postgresql文档：https://www.postgresql.org/docs/10/index.html