What is vacuuming in PostgreSQL and how do I vacuum a table?

Under the headline "Vacuuming Basics" in the PostgreSQL documentation, you can read amongst other things:

PostgreSQL's VACUUM command has to process each table on a regular basis for several reasons:

* To recover or reuse disk space occupied by updated or deleted rows.
* To update data statistics used by the PostgreSQL query planner.
* To update the visibility map, which speeds up index-only scans.
* To protect against loss of very old data due to transaction ID wraparound or multixact ID wraparound.

Each of these reasons dictates performing VACUUM operations of varying frequency and scope,

Further explaination can be found together with the syntax schema for VACUUM:

VACUUM reclaims storage occupied by dead tuples.
In normal PostgreSQL operation, tuples that are deleted or obsoleted by an update are not physically removed from their table; they remain present until a VACUUM is done. Therefore it's necessary to do VACUUM periodically, especially on frequently-updated tables.

Vacuum a specific table in verbose mode:

psql mydb
You are now connected to database "mydb" as user "postgres".


vacuum (verbose, analyze) myschema.mytable;

How to fetch first n rows only in PostgreSQL

The equivalent of oracles "fetch first n rows only" for postgreSQL is:

select *
from myschema.mytable
limit 10;

If you want to select the coming next 20 rows, but skip the rows you already selected first time, use the keyword OFFSET:

select *
from myschema.mytable
limit 20 offset 10;

The PostgreSQL 11 documentation for this feature can be found here

Solution for no ocijdbc18 in java.library.path when using sqlcl

sqlnet.ora:

NAMES.DIRECTORY_PATH= (TNSNAMES, EZCONNECT)
NAMES.DEFAULT_DOMAIN = oric.no

tnsnames.ora

testdb01,testdb01.oric.no =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = testserver.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = testdb01)
    )
  )
  

Using tnsping:

C:\>tnsping testdb01

TNS Ping Utility for 64-bit Windows: Version 19.0.0.0.0 - Production on 17-AUG-2021 12:42:14

Used parameter files:
C:\app\client\vegard\network\admin\sqlnet.ora
Attempting to contact (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = testserver.oric.no)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = testdb01)))
OK (40 msec)

Using sqlplus:

C:\>sqlplus system/mypassword@testdb01

SQL*Plus: Release 19.0.0.0.0 - Production on Tue Aug 17 12:39:47 2021
Version 19.3.0.0.0

Last Successful login time: Tue Aug 17 2021 12:36:35 +02:00

Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.14.0.0.0

Using sqlcl:

C:\>sql.exe system/mypassword@testdb01

SQLcl: Release 19.2.1 Production on Tue Aug 17 12:44:36 2021

Copyright (c) 1982, 2021, Oracle.  All rights reserved.

  USER          = system
  URL           = jdbc:oracle:oci8:@testdb01
  Error Message = no ocijdbc18 in java.library.path
  USER          = system
  URL           = jdbc:oracle:thin:@testdb01
  Error Message = IO Error: could not resolve the connect identifier  "testdb01
  USER          = system
  URL           = jdbc:oracle:thin:@testdb01:1521/testdb01
  Error Message = IO Error: could not resolve the connect identifier  "testdb01:1521/testdb01"
Username? (RETRYING) ('system/*********@testdb01'?)

Why is sqlcl having trouble connecting, when the listener is accepting requests and connections over sqlplus are indeed accepted?

My experience is that sqlcl doesn't seem to accept multiple tnsnames.ora aliases.

Solution: Split up the tnsnames.ora entry in two different entries.

Change it from

testdb01, testdb01.oric.no =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = testserver.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = testdb01)
    )
  )
  

to

testdb01 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = testserver.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = testdb01)
    )
  )

testdb01.oric.no =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = testserver.oric.no)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = testdb01)
    )
  )
  

How to work around ORA-38323: policy conflicts with policy

You try to enable a tablespace-wide ADO policy:

ORA-38323: policy conflicts with policy 42

If you lookup the error with oerr ora 38323, oracle will give you the following solution:

* Cause: An attempt was made to create a policy with the same action
         type and based on the same statistic as another policy on the
         object.
 *Action: Use a different action and/or statistic for the new policy,
          or delete the old policy.

Verify that there is a policy on the tablespace already:

SELECT * 
FROM DBA_ILMPOLICIES 
WHERE TABLESPACE IS NOT NULL;

Result:

POLICY_NAME	POLICY_TYPE	TABLESPACE	ENABLED	DELETED
P43	DATA MOVEMENT	DATA1	YES	NO

Theres is indeed a policy named P43. What kind of policy is it?

SELECT policy_name,action_type,scope,compression_level,condition_type,condition_days,policy_subtype
FROM DBA_ILMDATAMOVEMENTPOLICIES 
WHERE POLICY_NAME IN (SELECT POLICY_NAME FROM DBA_ILMPOLICIES WHERE TABLESPACE IS NOT NULL); 

Result:

POLICY_NAME	ACTION_TYPE	SCOPE	COMPRESSION_LEVEL	CONDITION_TYPE	CONDITION_DAYS	POLICY_SUBTYPE
P43	COMPRESSION	GROUP	ADVANCED	LAST ACCESS TIME	1	DISK

So the policy is a directive to the oracle server to compress all objects after one day of no access. If you need to alter this policy, the old one must be dropped:

ALTER TABLESPACE DATA1 DEFAULT ILM DELETE POLICY P43;

You are now free to add a new ADO policy to the tablespace:

ALTER TABLESPACE DATA1
DEFAULT ILM ADD POLICY
ROW STORE COMPRESS ADVANCED
GROUP AFTER 30 DAYS OF NO ACCESS;

How to add a default ADO compression policy to a tablespace

Create the tablespace:

CREATE BIGFILE TABLESPACE DATA1 DATAFILE
  '/oradata/mydb/data1.dbf' SIZE 1024M AUTOEXTEND ON NEXT 256M MAXSIZE 2T
DEFAULT
COMPRESS FOR OLTP
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
SEGMENT SPACE MANAGEMENT AUTO
DEFAULT ILM ADD POLICY 
COMPRESS FOR ALL OPERATIONS 
GROUP 
AFTER 1 DAYS OF NO ACCESS;

Notice the "GROUP" keyword. It states the scope of an ADO policy, and can be a group of related objects, segment level or row-level, indicated by the keywords GROUP, ROW, or SEGMENT.

According to the documentation, by default, this will give you advanced compression on heap tables, standard compression for indexes and LOW for LOB segments created in the tablespace.

Any table created in this tablespace from now on will now inherit the ADO policy. Let's see it in action:

create table sh.test_ado_tabspc_compression as select * from sh.sales;

Table created.

Verify:

SELECT policy_name,
       object_owner,
       object_name,
       subobject_name,
       object_type,
       inherited_from,
       enabled,
       deleted
FROM   dba_ilmobjects
WHERE  object_owner='SH'
AND    object_name='SALES'
ORDER BY 1;

Result:

POLICY_NAME	OBJECT_OWNER	OBJECT_NAME	SUBOBJECT_NAME	OBJECT_TYPE	INHERITED_FROM	ENABLED	DELETED
P43	SH	TEST_ADO_TABSPC_COMPRESSION		TABLE	TABLESPACE	YES	NO

The table has inherited the ADO policy from the tablespace it was created in.