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    -- 查询表元数据（唯一性，必填，字段类型）
    select a.attname                             as fieldName,
        d.typname                             as type,
        (case
                when atttypmod - 4 > 0 then atttypmod - 4
                else 0
            end)                                 length,

        (case
                when (select count(*)
                    from pg_constraint
                    where conrelid = a.attrelid and conkey[1] = attnum and contype = 'u') > 0 then 'Y'
                else 'N'
            end)                              as un,
        (case
                when a.attnotnull = true then 'Y'
                else 'N'
            end)                              as nullable,
        col_description(a.attrelid, a.attnum) as comment
    from pg_attribute a
            left join pg_class c on a.attrelid = c.oid
            left join pg_type d on a.atttypid = d.oid
    where attstattarget = -1 and c.relname = 'table_test'