PostgreSQL11: psql 新增 \gdesc 显示查询结果的列名和类型
栏目: 数据库 · PostgreSQL · 发布时间: 6年前
内容简介:PostgreSQL 11 的\gdescShows 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 norm
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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