PostgreSQL 11 的 psql 新增 gdesc 选项，此选项可以返回查询结果的列名和类型，而不实际执行SQL。
Release 说明
psql
Add psql command gdesc to display the column names and types of the query output (Pavel Stehule)
gdesc 选项说明
gdesc
Shows the description (that is, the column names and data types) of the result of the current query buffer. The query is not actually executed; however, if it contains some type of syntax error, that error will be reported in the normal way.
If the current query buffer is empty, the most recently sent query is described instea
gdesc 只是显示查询结果的列名和类型，并不实际执行SQL，下面演示下。
gdesc 选项演示
数据库中存在一张大表big，结构如下:
[pg11@pghost2 ~]$ psql francs francs
psql (11beta3)
Type "help" for help.
francs=> \d big
Table "francs.big"
Column | Type | Collation | Nullable | Default
-----------+--------------------------------+-----------+----------+-------------------
user_id | integer | | |
user_name | text | | |
ctime | timestamp(6) without time zone | | | clock_timestamp()
Indexes:
"idx_big_ctime" btree (ctime)
"idx_big_username" btree (user_name)执行以下查询，如下:
francs=> \timing
Timing is on.
francs=> SELECT count(*),sum(hashtext(user_name)) FROM big;
count | sum
----------+----------------
30000000 | 11924569894736
(1 row)
Time: 1347.527 ms (00:01.348)执行时间为 1347 ms 左右。
使用 gdesc 选项查询，如下:
francs=> SELECT count(*),sum(hashtext(user_name)) FROM big \gdesc
Column | Type
--------+--------
count | bigint
sum | bigint
(2 rows)
Time: 0.634 ms以上返回了查询结果的列和数据类型，执行很快，只需要 0.634 ms，可见没有实际执行SQL。
另一个示例，查询 pg_class 系统表，如下:
francs=> SELECT * FROM pg_class \gdesc
Column | Type
---------------------+--------------
relname | name
relnamespace | oid
reltype | oid
reloftype | oid
relowner | oid
relam | oid
relfilenode | oid
reltablespace | oid
relpages | integer
reltuples | real
relallvisible | integer
reltoastrelid | oid
relhasindex | boolean
relisshared | boolean
relpersistence | "char"
relkind | "char"
relnatts | smallint
relchecks | smallint
relhasoids | boolean
relhasrules | boolean
relhastriggers | boolean
relhassubclass | boolean
relrowsecurity | boolean
relforcerowsecurity | boolean
relispopulated | boolean
relreplident | "char"
relispartition | boolean
relrewrite | oid
relfrozenxid | xid
relminmxid | xid
relacl | aclitem[]
reloptions | text[]
relpartbound | pg_node_tree
(33 rows)这个特性不需要实际执行SQL就能返回查询结果的列和数据类型，在某些特定场景比较有用。
参考
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