项目中需要采集postgresql的元数据信息，需查询pgsql 的表名以及表中字段的各信息，
通过查找资料，记之如下：
查询表名及备注SQL
select
pc.relname as table_name,
pd.description as comment
from
pg_catalog.pg_class pc
join pg_catalog.pg_description pd on
pc.oid = pd.objoid
where
pc.relname = ‘tableName‘
and pd.objsubid = 0
获取指定表的字段名称、长度、是否为空、是否主键等信息
SELECT
pc.relname AS tableName,
pa.attname AS columnName,
pt.typname AS columnType,
( CASE WHEN pa.attlen > 0 THEN pa.attlen ELSE pa.atttypmod - 4 END ) AS columnLength,
pa.attnotnull AS isNullAble,
( CASE WHEN ( SELECT COUNT(*) FROM pg_constraint WHERE conrelid = pa.attrelid AND conkey[1]= attnum AND contype = ‘p‘ ) > 0 THEN
TRUE ELSE FALSE
END ) AS isPrimary,
pd.description AS columnDescription
FROM
pg_class pc,
pg_attribute pa,
pg_type pt,
pg_description pd
WHERE
pc.oid = pa.attrelid
AND pt.oid = pa.atttypid
AND pd.objoid = pa.attrelid
AND pd.objsubid = pa.attnum
AND pc.relname = ‘table_name‘
ORDER BY
pc.relname DESC,
pa.attnum ASC