View a configuration parameter

Oracle

SHOW PARAMETER sga_target;
-- or: SELECT name, value, isdefault FROM v$parameter WHERE name = 'sga_target';

To change it: ALTER SYSTEM SET ... SCOPE=BOTH; (some only with SCOPE=SPFILE + restart).

SHOW shared_buffers;
-- Details: SELECT name, setting, unit, context FROM pg_settings WHERE name = 'shared_buffers';

The context column tells whether the change requires a restart or a reload (SELECT pg_reload_conf()).

SHOW VARIABLES LIKE 'innodb_buffer_pool_size';

Mind the GLOBAL vs SESSION distinction: SHOW GLOBAL VARIABLES shows the server-wide value.

SELECT name, value, value_in_use, is_dynamic
FROM sys.configurations
WHERE name LIKE '%max server memory%';

To change it: sp_configure + RECONFIGURE; options with is_dynamic=0 require a restart.

db.adminCommand({ getParameter: 1, cursorTimeoutMillis: 1 })
// All startup options: db.adminCommand({ getCmdLineOpts: 1 })

Use setParameter for those changeable at runtime; persistent settings live in the mongod.conf file.

CONFIG GET maxmemory
# Wildcard: CONFIG GET max*

Use CONFIG SET to change at runtime; CONFIG REWRITE for persistence (writes to redis.conf).