本節簡單介紹了如何顯示psql命令內置的SQL。
我們在使用psql的時候，可以使用\d,\d+,\df,\sf等命令來查詢相關信息，這些命令背后的SQL是什么呢？可以通過設置ECHO_HIDDEN on來查看相應的SQL。

[local:/data/run/pg12]:5120 pg12@testdb=# \set  ECHO_HIDDEN on
[local:/data/run/pg12]:5120 pg12@testdb=# \d
********* QUERY **********
SELECT n.nspname as "Schema",
  c.relname as "Name",
  CASE c.relkind WHEN 'r' THEN 'table' WHEN 'v' THEN 'view' WHEN 'm' THEN 'materialized view' WHEN 'i' THEN 'index' WHEN 'S' THEN 'sequence' WHEN 's' THEN 'special' WHEN 'f' THEN 'foreign table' WHEN 'p' THEN 'partitioned table' WHEN 'I' THEN 'partitioned index' END as "Type",
  pg_catalog.pg_get_userbyid(c.relowner) as "Owner"
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind IN ('r','p','v','m','S','f','')
      AND n.nspname <> 'pg_catalog'
      AND n.nspname <> 'information_schema'
      AND n.nspname !~ '^pg_toast'
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 1,2;
**************************
              List of relations
 Schema |        Name        | Type  | Owner 
--------+--------------------+-------+-------
 public | a                  | table | pg12
 public | b                  | table | pg12
 public | rel                | table | pg12
 public | t                  | table | pg12
 public | t1                 | table | pg12
 public | t2                 | table | pg12
 public | t_autovacuum_1     | table | pg12
 public | t_autovacuum_db1   | table | pg12
 public | t_big_autovacuum_1 | table | pg12
 public | t_count            | table | pg12
 public | t_fillfactor_100   | table | pg12
 public | t_fillfactor_50    | table | pg12
 public | t_fillfactor_70    | table | pg12
 public | t_mvcc             | table | pg12
 public | t_tx               | table | pg12
 public | tbl                | table | pg12
(16 rows)
[local:/data/run/pg12]:5120 pg12@testdb=# \d+ t_mvcc
********* QUERY **********
SELECT c.oid,
  n.nspname,
  c.relname
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relname OPERATOR(pg_catalog.~) '^(t_mvcc)$' COLLATE pg_catalog.default
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 2, 3;
**************************
********* QUERY **********
SELECT c.relchecks, c.relkind, c.relhasindex, c.relhasrules, c.relhastriggers, c.relrowsecurity, c.relforcerowsecurity, false AS relhasoids, c.relispartition, pg_catalog.array_to_string(c.reloptions || array(select 'toast.' || x from pg_catalog.unnest(tc.reloptions) x), ', ')
, c.reltablespace, CASE WHEN c.reloftype = 0 THEN '' ELSE c.reloftype::pg_catalog.regtype::pg_catalog.text END, c.relpersistence, c.relreplident, am.amname
FROM pg_catalog.pg_class c
 LEFT JOIN pg_catalog.pg_class tc ON (c.reltoastrelid = tc.oid)
LEFT JOIN pg_catalog.pg_am am ON (c.relam = am.oid)
WHERE c.oid = '74886';
**************************
********* QUERY **********
SELECT a.attname,
  pg_catalog.format_type(a.atttypid, a.atttypmod),
  (SELECT substring(pg_catalog.pg_get_expr(d.adbin, d.adrelid, true) for 128)
   FROM pg_catalog.pg_attrdef d
   WHERE d.adrelid = a.attrelid AND d.adnum = a.attnum AND a.atthasdef),
  a.attnotnull,
  (SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type t
   WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation) AS attcollation,
  a.attidentity,
  a.attgenerated,
  a.attstorage,
  CASE WHEN a.attstattarget=-1 THEN NULL ELSE a.attstattarget END AS attstattarget,
  pg_catalog.col_description(a.attrelid, a.attnum)
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = '74886' AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum;
**************************
********* QUERY **********
SELECT pol.polname, pol.polpermissive,
  CASE WHEN pol.polroles = '{0}' THEN NULL ELSE pg_catalog.array_to_string(array(select rolname from pg_catalog.pg_roles where oid = any (pol.polroles) order by 1),',') END,
  pg_catalog.pg_get_expr(pol.polqual, pol.polrelid),
  pg_catalog.pg_get_expr(pol.polwithcheck, pol.polrelid),
  CASE pol.polcmd
    WHEN 'r' THEN 'SELECT'
    WHEN 'a' THEN 'INSERT'
    WHEN 'w' THEN 'UPDATE'
    WHEN 'd' THEN 'DELETE'
    END AS cmd
FROM pg_catalog.pg_policy pol
WHERE pol.polrelid = '74886' ORDER BY 1;
**************************
********* QUERY **********
SELECT oid, stxrelid::pg_catalog.regclass, stxnamespace::pg_catalog.regnamespace AS nsp, stxname,
  (SELECT pg_catalog.string_agg(pg_catalog.quote_ident(attname),', ')
   FROM pg_catalog.unnest(stxkeys) s(attnum)
   JOIN pg_catalog.pg_attribute a ON (stxrelid = a.attrelid AND
        a.attnum = s.attnum AND NOT attisdropped)) AS columns,
  'd' = any(stxkind) AS ndist_enabled,
  'f' = any(stxkind) AS deps_enabled,
  'm' = any(stxkind) AS mcv_enabled
FROM pg_catalog.pg_statistic_ext stat WHERE stxrelid = '74886'
ORDER BY 1;
**************************
********* QUERY **********
SELECT pubname
FROM pg_catalog.pg_publication p
JOIN pg_catalog.pg_publication_rel pr ON p.oid = pr.prpubid
WHERE pr.prrelid = '74886'
UNION ALL
SELECT pubname
FROM pg_catalog.pg_publication p
WHERE p.puballtables AND pg_catalog.pg_relation_is_publishable('74886')
ORDER BY 1;
**************************
********* QUERY **********
SELECT c.oid::pg_catalog.regclass FROM pg_catalog.pg_class c, pg_catalog.pg_inherits i WHERE c.oid=i.inhparent AND i.inhrelid = '74886' AND c.relkind != 'p' ORDER BY inhseqno;
**************************
********* QUERY **********
SELECT c.oid::pg_catalog.regclass,       pg_catalog.pg_get_expr(c.relpartbound, c.oid),       c.relkind FROM pg_catalog.pg_class c, pg_catalog.pg_inherits i WHERE c.oid=i.inhrelid AND i.inhparent = '74886' ORDER BY pg_catalog.pg_get_expr(c.relpartbound, c.oid) = 'DEFAULT',          c.oid::pg_catalog.regclass::pg_catalog.text;
**************************
                                  Table "public.t_mvcc"
 Column |  Type   | Collation | Nullable | Default | Storage | Stats target | Description 
--------+---------+-----------+----------+---------+---------+--------------+-------------
 id     | integer |           |          |         | plain   |              | 
Access method: heap
[local:/data/run/pg12]:5120 pg12@testdb=#