Edb PostgreSQL Basics -- For Oracle Dba

 
As i am  advancing my skillsets and adding  Postgres to  my profile , thought of writing this blog as i am preparing for  edb  postgres exam 

Unfortunately i have  mixed up postgresql and edb-postgresql   but  fundamentals remains same . 

Server Process  , Memory Structure , Physical files :

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Server Process 

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postmaster ( pmon   )  --> 

background process  -->  bgwritter - background writer   ,   logging collector  ,  stats collector  ,  

checkpointer  ,  archiver  , auto-vacuum  , wal writer ,  logical replication  ,  dbms_aq 

files   -->   data files  ,  wal  segments   ,   archived wal ,  error  files  global files 

postmaster ( pmon  )  --> 

Master database control process

Responsible for startup and shutdown

Handling of connection requests

Spawning of other necessary backend processes

 

Background Processes :

Bgwriter  :   background writer , writes  diry block to disk
Wal writer :  flush write ahead  logs to disk
Checkpointer :  performs checkpoint base don config parameters .
Auto-vacuum launcher  :   starts Auto-vacuum works as needed
Auto-vacuum worker :  recover free space  for reuse
Logging collector : routes  log messages to syslog , eventlog or log files
Stats collector :  collects usage statistics  by relation and blocks .
Archiver   :  archives write ahead log files
Local replication  launcher :  starts logical replication apply process for logical replication.
Dbms_aq launcher :   collects information for queuing functionality  of advanced servers .
 

Autovacuum

Dedicated backend for providing vacuum services

Essentially, a garbage collect of data files

Covered later in maintenance section

Writer

Background writer

Writing of pages from memory cache to disk (does not flush)

WAL writer

Responsible for maintaining transaction log (journal)

Only used for asynchronous commits

Checkpointer

A process that performs checkpoints

Flushing of all dirty buffers to disk storage

Archiver

WAL files saved to a specified location for backup

Can also be used to achieve replication

Logger

Responsible for writing information logs

Errors, warnings, slow running queries, etc.

Not used if writing to syslog

Stats collector

Support for collection and reporting of information about server activity

Can count accesses to tables and indexes

Information about vacuum and action analysis

bgworkers

Support for external extensions

Logical replication

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Memory Structure

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Shared Memory :
1) shared buffers
2) wal buffers
3) process aray  ( Pre Session Memory ) 
 

Shared buffers 

• Shared buffers are the primary cache component for the server, storing disk blocks from database files.
• All data sets accessed from the disk are placed in shared buffers, which allow subsequent reads to be memory reads.
• All writes are performed in shared buffers, creating "dirty" pages. The bgwriter and checkpoint processes will write this out to disk.
• Shared buffers contain free buffers, which are never used or freed after using, and dirty buffers, which result from DML.

Wal buffers

• Stores intermediate write-ahead log records.
• Written on commit by wal_writer process (or when full).

Pre Session Memory 

work_mem

• Used for per-backend sort/hash memory.
• Used during certain types of JOINs and for ORDER BY operations.
• Set globally, but can be modified per session.

maintenance_work_mem

• Used for certain types of maintenance operations (vacuum, index creation, re-index).
• Allocated per session that uses it, such as multiple autovacuum workers

Effective_cache_size (integer)

The effective_cache_size parameter estimates how much memory is available for disk caching by the operating system and within the database itself. The PostgreSQL query planner decides whether it’s fixed in RAM or not. Index scans are most likely to be used against higher values; otherwise, sequential scans will be used if the value is low. Recommendations are to set Effective_cache_size at 50% of the machine’s total RAM.

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Physical Storage 

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PGDATA directory

• All on-disk components of a database instance are stored in a data directory, which contains multiple subdirectories.
• Some subdirectories can be moved by configuration and others by symlink (Unix/Linux only).
• Very few files are user readable or modifiable.
• Commonly referred to as PGDATA.

Datafiles : 

• The actual data is stored in files, typically in the PGDATA directory, with one operating system file per page.
• Although PostgreSQL provides some visibility into which files correspond to which tables, you normally don't deal with these files directly.

• Also referred to as relations or base files.
• Stored in subdirectories of the base directory.
• Contain table or index data  

           Tables have a set of segments.
            Indexes have a set of segments.

• Stored on disk as 1 GB segments:

             A 5 GB table will be five 1 GB files.
             Segments are automatically added, no intervention required.

• Filled with data in 8 KB blocks (on disk) or pages (in-memory):

        Blocks and pages are identical, with the name changing based on where data currently resides.                When in-memory, they are referred to as pages.

WAL :

• One transaction may involve writing to several different tables, and thus writing many pages to disk. Rather than doing this, PostgreSQL uses a write-ahead log (WAL), which is a sequential file where transactions are stored. If the server goes down, it can restore the state of its data by replaying these transactions.
• Only one page needs to be written to the WAL for each transaction, which improves performance. Eventually, the transactions get executed in the background, and data pages are saved by utility processes.
• This is the transaction journal.
• Also known as: WAL, xlog, pg_xlog, transaction log, journal, REDO.
• These files contain a persistent record of commits.
• Success is not returned to the client until this is safely persisted to disk.
• During crash recovery, this journal can be used to reconstruct all transactions.
• These files are written by the WAL writer process.

# Perform Switch of  War  files 

select pg_switch_wal() ; 

Archived Logs 

• After WAL entries get executed, the WAL segment eventually gets archived. These archival logs can be used to replay the transactions on a different server, and keep remote servers almost in sync.

To Clean Up Archive after Backups : 

pg_archivecleanup -n <PG_WAL_PATH> <WAL_FILE> | wc -l

wal_keep_segments tells it how many files to keep in the pg_xlog directory, not the archive directory.

It is kind of weird to use both wal_keep_segments and archive_command together, at least for relatively simple setups. If you just want a streaming replica and don't need the ability to do point-in-time recovery, you could turn off archiving altogether.

If you really want to keep using an archive but have it cleaned up when the files are no longer needed, you can use the archive_cleanup_command setting. But now that we have streaming replication and replication slots, that is almost obsolete.

Below related parameters in config files 

# To enable read-only queries on a standby server, wal_level must be set to

# "hot_standby". But you can choose "archive" if you never connect to the

# server in standby mode.

wal_level = hot_standby

min_wal_size=

# Set the maximum number of concurrent connections from the standby servers.

max_wal_senders = 5

# To prevent the primary server from removing the WAL segments required for

# the standby server before shipping them, set the minimum number of segments

# retained in the pg_xlog directory. At least wal_keep_segments should be

# larger than the number of segments generated between the beginning of

# online-backup and the startup of streaming replication. If you enable WAL

# archiving to an archive directory accessible from the standby, this may

# not be necessary.

wal_keep_segments = 32

wal_compressed 

# Enable WAL archiving on the primary to an archive directory accessible from

# the standby. If wal_keep_segments is a high enough number to retain the WAL

# segments required for the standby server, this is not necessary.

archive_mode    = on

archive_command = 'cp %p /path_to/archive/%f'

Stored file examples

PG_VERSION

Version string of the database instance

postmaster.opts

What options were used to start the instance

postmaster.pid

Process ID of the instance

server.crt

Server certificate for SSL

server.key

Server private key

root.crt

Trusted certificate authorities

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PostgreSQL configuration files

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PostgreSQL uses several text-based configuration files. While you can change where you store these files when compiling PostgreSQL using symbolic links on your file system, these configuration files are usually stored in the same folder. This folder is always called PGDATA, even though the actual folder may change.

● PG_VERSION

○ Version String of the Database Cluster

● pg_hba.conf

○ Host-Based access control file (built-in firewall)

● pg_ident.conf

○ ident-based access file for OS User to DB User Mapping

● postgresql.conf

○ Primary Configuration File for the Database

● postmaster.opts

○ Contains the options used to start the PostgreSQL Instance

● postmaster.pid

○ The Parent Process ID or the Postmaster Process ID

pg_hba.conf

• The pg_hba.conf file contains information about who can connect to local databases, through which mechanism (local connection or over the network) from which addresses, and using which authentication method.

pg_ident.conf

• The pg_ident.conf file maps operating system users to PostgreSQL users.

postgresql.conf

• postgresql.conf contains most of the configuration parameters for PostgreSQL.
• postgresql.auto.conf overrides values in postgresql.conf, and allows you to easily track parameters that are different for your local installation.
• The ALTER SYSTEM command writes to this file if you persist its changes.

• postgresql.auto.conf

○ PostgreSQL gives Oracle like compatibility to modify parameters using "ALTER SYSTEM"

○ Any parameter modified using ALTER SYSTEM is written to this file for persistence

○ This is last configuration file read by PostgreSQL, when started. Empty by default

○ Always located in the data directory 

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Software Installation and  Database / User  Creation 

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PostgreSQL Cluster

● After Initializing your PostgreSQL using initdb (similar to mysqld --initialize) and starting it, you can

  create multiple databases in it

● A group of databases running on one Server & One Port - Is called a Cluster in PostgreSQL

● PostgreSQL Cluster may be referred to as a PostgreSQL Instance as well

● A PostgreSQL Cluster or an Instance:

  ○ Serves only one TCP/IP Port

  ○ Has a Dedicated Data Directory

  ○ Contains 3 default databases: postgres, template0 and template1

● When you add a Slave(aka Standby) to your PostgreSQL Cluster(Master), it may be referred to as a

  PostgreSQL High Availability Cluster or a PostgreSQL Replication Cluster

● PostgreSQL Cluster that can accept Writes and ships WALs to Slave(Standby), is called a Master

Software Installation

1) Installation options :  wizard ,  yum , rpm ,
2) while installation os user  enterprisedb is created

3) Default  installation location for Windows is  "C:\program files\edb\as11"  and for Linux   its "/usr/edb/as11"

 
 

1) PGDG Repository : PostgreSQL Global Development Group maintains YUM and APT repository 

For YUM

https://yum.postgresql.org

For APT

https://apt.postgresql.org/pub/repos/apt/

Step 1:

Choose the appropriate rpm that adds pgdg repo to your server

# yum install https://yum.postgresql.org/11/redhat/rhel-7.5-x86_64/pgdg-centos11-11-2.noarch.rpm

Step 2:

Install PostgreSQL using the following step

# yum install postgresql11 postgresql11-contrib postgresql11-libs postgresql11-server

#Optionally Initialize The Database and enable Automatic startup 

/usr/pgsql-11/bin/postgresql-11-setup initdb 

systemctl enable postgresql-11 

systemctl start postgresql-11 

systemctl status postgresql-11 

 

Edb Postgres Server Installation 

-- Downlaod the Edb-repo reposiory  configuration  file from yum.enterprisedb.com 

-- username and password for  yum access  can be  obtained  from  EnterpriseDb 

-- Login as root user and add user enterprisedb using  adduser  or  useradd and  set its password 

-- install  EPEL using  yum 

# yum install  epel-release 

-- Install the downloaded repository configuration  files 

# rpm -ivm edb-repo-latest.noarch.rpm 

-- Update username and password in /etc/yum.repos.d/edb.repo file  and  change enabled parameter to 1 

   for following repositories : 

   edbas11  , enterprisedb-dependencies  ,  enterprisedb-tools  

-- Install Yum 

#yum install edb-as11-server 

-- Configure a package installation using service configuration  file 

#  /usr/lub/system/edb-as11.service 

-- Create a database cluster  and start  cluster using services 

# /usr/edb/as11/bin/edb-as-11-setup  initdb 

# systemctl start edb-as-11 

#Connecting to  default  Edb Instance 

edb-psql     -p 5434 edb entrerprisedb 

edb-psql :

-- edb-psql is name of EnterpriseDB psql executable 

-- EnterpriseDB's  edb-psql provides termonal-based front-end  to advanced server 

-- it enables you to type  in queries interactively 

d, --dbname=, $PGDATABASE

-h, --host=, $PGHOST

-p, --port=, $PGPORT

-U, --username=, $PGUSER

EDB*Plus  : 

--  Edb*plus is command  line user  interface to Edb Postgres Advanced Server 

--  Edb*Plus accepts sql commands 

--  Edb*Plus are compatible with Oracle Sqlplus 

Eg : 

 DB_USER=enterprisedb

 DB_PASSWORD=abc123

 DB_SCHEMA="127.0.0.1:5432/edb"

 edbplus -S $DB_USER/$DB_PASSWORD@$DB_SCHEMA 2>>EDB_ERROR_FILE

Database Creation   : 

Initialize Your First PostgreSQL Cluster :

  initdb is used to Initialize a PostgreSQL cluster

$ echo "PATH=/usr/pgsql-11/bin:$PATH">>~/.bash_profile

$ source .bash_profile

$ echo $PGDATA

/var/lib/pgsql/11/data

$initdb --version

initdb (PostgreSQL) 11.0

$ initdb 

or 

$ initdb -D /usr/local/pgsql/data

pg_ctl -D "Path" start 

pg_ctl  status 

pg_ctl stop 

Default edb database creation :  

 
/usr/edb/as11/bin/edb-as-11-setup initdb

● Base Directory

  ○ Contains Subdirectories for every Database you create

  ○ Every Database Sub-Directory contains files for every Relation/Object created in the Database

● Datafiles

  ○ Datafiles are the files for Relations in the base directory

  ○ Base Directory contains Relations

  ○ Relations stored on Disk as 1GB segments

  ○ Each 1GB Datafile is made up of several 8KB Pages that are allocated as needed

  ○ Segments are automatically added unlike Oracle

Create a database  ( default  tablespace is pg_default ) 

 $ psql -c "CREATE DATABASE sample "

create database sample  owner user1  tablespace table1 

psql -U postgres 

show data_directory 

SHOW PORT 

\q

Get the datid for the database and see if it exists in the base directory

 $ psql -c "select datid, datname from pg_stat_database where datname = 'percona'"

Create a schema named: scott

 $ psql -d percona -c "CREATE SCHEMA scott"

Create a table named: employee in scott schema

 $ psql -d percona -c "CREATE TABLE scott.employee(id int PRIMARY KEY, name varchar(20))"

Locate the file created for the table: scott.employee in the base directory

 $ psql -d percona -c "select pg_relation_filepath('scott.employee')"

Check the size of the table in the OS and value of parameter: block_size

 $ psql -c "show block_size"

INSERT a record in the table and see the size difference

 $ psql -d percona -c "INSERT INTO scott.employee VALUES (1, 'frankfurt')"

INSERT more records and check the size difference

 $ psql -d percona -c "INSERT INTO scott.employee VALUES (generate_series(2,1000), 'junk')"

Connect to service :  

psql –p 5444 –d edb –U enterprisedb

Check Size of Database : 

$  select datname , oid from  pg_database ; 

$  select pg_database_size ('oid or name ') ;

 Tablespaces  :

In  postgres   tablespace are  directory ,

Each user defined tablespace  has a symbolic link under $PGDATA/pg_tblspc

○ Can be used to move Table and Indexes to different disks/locations

○ Helps distributing IO

● Steps to create tablespace in PostgreSQL

● Step 1: Create a directory for the tablespace

▪ $ mkdir -p /tmp/tblspc_1

$ chown postgres:postgres /tmp/tblspc_1

$ chmod 700 /tmp/tblspc_1

● Step 2: Create tablespace using the new directory

▪ $ psql -c "CREATE TABLESPACE tblspc_1 LOCATION '/tmp/tblspc_1'"

● Step 3: Create a table in the new table-space

▪ $ psql -d percona -c "CREATE TABLE scott.foo (id int) TABLESPACE tblspc_1"

SELECT spcname , pg_tablespace_location(oid)  from pg_tablespace  ;

\db+

# to set tablespace 

set default_tablespace= space1 ; 

-- pg_global tablespace corresponds to the PGDATA/global  directory 

-- pg_global  is used to store  cluster-wide tables and shared  system catalog 

-- pg_default  tablespace corresponds to the PGDATA/base directory 

-- pg_defalt  is used  to store  database and  relations 

Enable Archiving 

$ psql

ALTER SYSTEM SET archive_mode TO 'ON';

ALTER SYSTEM SET archive_command TO 'cp %p /var/lib/pgsql/archive/%f';

 $ pg_ctl -D $PGDATA restart -mf

Switch a WAL and see if the WAL is safely archived…

$ psql -c "select pg_switch_wal()"

If archiving has been enabled and the archive_command failed,

● the WAL segment for which the archiving failed will not be removed from pg_wal or pg_xlog

● an empty wal_file_name.ready file is generated in the archive_status directory

● the background process archiver attempts to archive the failed WAL segment until it succeeds

● there is a chance that the pg_wal directory can get filled and doesn't allow any more connections to

database 

Users and Roles : 

-- Public is default schema  .  Be default all  users  has Create and Usage rights on  public schema 

-- pg_catalog contains system tables

-- User and roles are same in postgres 

-- postgres is  super user 

-- Users are global  and schemas are for specific database 

● A read_only Role that only has SELECT, USAGE privileges on Schema: percona

○ CREATE ROLE scott_read_only;

GRANT SELECT ON ALL TABLES IN SCHEMA scott TO scott_read_only;

GRANT USAGE ON SCHEMA scott TO scott_read_only;

● A read_write Role that only has SELECT, INSERT, UPDATE, DELETE privileges on Schema: percona

○ CREATE ROLE scott_read_write;

GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA scott TO

scott_read_write;

GRANT USAGE ON SCHEMA scott TO scott_read_write;

● Create a User and assign either read_only or read_write role

○ CREATE USER pguser WITH LOGIN ENCRYPTED PASSWORD 'pg123pass';

GRANT scott_read_only to pguser;

ALTER USER pguser WITH CONNECTION LIMIT 20;

-- To change  User password 

 ALTER USER postgres PASSWORD '<new-password>'; 

or

ALTER USER username WITH ENCRYPTED PASSWORD 'password';

-- Login to db1 database  from postgres user into public schema 

psql -p 5432 db1 postgres

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Handling Slow Queries

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1) Enable logging_collector 

ALTER DATABASE pgbench SET log_min_duration_statement=0;

or 

It is also possible to enable this globally by adding to PostgreSQL configuration and then reload config:

log_min_duration_statement = 0

SELECT pg_reload_conf();

This enables logging of all queries across all of the databases in your PostgreSQL. If you do not see any logs, you may want to enable logging_collector = on as well. The logs will include all of the traffic coming to PostgreSQL system tables, making it more noisy. For our purposes let’s stick to the database level logging.

2)  Checking Explain Plan : 

EXPLAIN SELECT abalance FROM pgbench_accounts WHERE aid = 3344333;

EXPLAIN ANALYZE SELECT abalance FROM pgbench_accounts WHERE aid = 3344333;

3)  Pg_stat_statements

Pg_stat_statements is the extension that collects execution statistics for different query types.

If you do not have pg_stat_statements enabled, you can do it in a standard way. Either via configuration file and

shared_preload_libraries = 'pg_stat_statements'

Or you can enable it via PostgreSQL command line:

CREATE EXTENSION pg_stat_statements;

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Startup and Shutdown 

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Its not possible to stop individual database . You can disallow connections via ALTER database, or (selectively) disallow new connections via pg_hba.conf. 

$ initdb

Success. 

You can now start the database server using:

postgres -D /u/pg/data

or

pg_ctl -D /u/pg/data -l logfile start

Restart Edb service :    systemctl { start | stop | restart } edb-as-11

Shutdown Modes in PostgreSQL

● -ms (Smart Mode - Default mode)

○ Waits for all connections to exit and does not allow new transactions

○ Committed transactions applied to Disk through a CHECKPOINT before shutdown

○ May take more time on busy systems

$ pg_ctl -D $PGDATA stop -ms

● -mf (Fast Mode - Recommended on Busy Systems)

○ Closes/Kills all the open transactions and does not allow new transactions. SIGTERM is sent to server processes

  to exit promptly

○ Committed transactions applied to Disk through a CHECKPOINT before shutdown

○ Recommended on Busy Systems

$ pg_ctl -D $PGDATA stop -mf

● -mi (Immediate Mode - Forced and Abnormal Shutdown during Emergencies)

  ○ SIGQUIT is sent to all the processes to exit immediately, without properly shutting down

  ○ Requires Crash Recovery after Instance Start

  ○ Recommended in Emergencies

$ pg_ctl -D $PGDATA stop -mi

or 

$ pg_ctl -D $PGDATA stop -m immediate 

Check Status of  Postgres cluster : 

$ pg_ctl   status -D $PGDATA  

Starting and Stopping a PostgreSQL

PostgreSQL can be stopped and started from command line using pg_ctl

  ○ Starting PostgreSQL

▪ pg_ctl -D $PGDATA start

  ○ Stopping PostgreSQL

▪ pg_ctl -D $PGDATA stop

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View/Modify Parameters in postgresql.conf

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• Use show to view a value set to a parameter

  $ psql -c "show work_mem"

• To see all the settings, use show all

  $ psql -c "show all"

• Modifying a parameter value by manually editing the postgresql.conf file

  $ vi $PGDATA/postgresql.conf

• Use ALTER SYSTEM to modify a parameter

 $ psql -c "ALTER SYSTEM SET archive_mode TO ON"

 $ pg_ctl -D $PGDATA restart -mf

• Use reload using the following syntax to get the changes into effect for parameters not needing RESTART

 $ psql -c "select pg_reload_conf()"

 Or

 $ pg_ctl -D $PGDATA reload

pg_controldata 

postgresql.conf   File 

-- Holds Parameters used by a  cluster 

-- Parameters are case sensitive 

-- Normally  stored in a data directory 

-- initdb installs  default copy 

-- some parameters only table  affect  on server restart ( pg_ctl restart ) 

-- # used for comments 

-- one parameter per line 

Ways to  set server parameters 

-- some parameters can only be changed  per session using  SET  command 

-- Some parameter can be Changed   at user level  using "alter user" 

-- some Parameterr can be changed at database level using "alter database "

-- "SHOW" command  show current settings 

-- pg_settings and pg_file_settings catalog table list settings   information 

eg  :  select name , setting   from pg_settings 

Alter system command writes setting to postgresql.auto.conf 

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Setting Environmental Variables :

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PATH :   should point to bin directory

PGDATA : should point to default data cluster directory

PSPORT : should point to port on which database cluster is runing

PGUSER:  specifies the default database  user name

PGDATABASE:  specify the default database

SEARCH_PATH  : 

PostgreSQL has a great feature: schemas. So you have one database with multiple schemas. This is a really great solution for the data separation between different applications. Each of them can use different schema, and they also can share some schemas between them.

You can set the default search_path at the database level:

ALTER DATABASE <database_name> SET search_path TO schema1,schema2;

Or at the user or role level:

ALTER ROLE <role_name> SET search_path TO schema1,schema2;

ALTER ROLE <role_name> IN DATABASE <db_name> SET search_path TO schema1,schema2;

SHOW search_path;

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MVCC: Multi-Version Concurrency Control

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● Data consistency

● Prevents viewing inconsistent data

● Readers and Writers do not block each other

● No Rollback segments for UNDO

● UNDO management is within tables

● A tuple contains the minimum and maximum transaction ids that are permitted to see it

● Just like SELECT statements executing WHERE

xmin <= txid_current() AND (xmax = 0 OR txid_current() < xmax)

●PostgreSQL

○ Maintains UNDO within a table through versions - old and new row versions

○ Transaction ID’s are used to identify a version a query can use

○A background process to delete old row versions explicitly

○ No additional writes to a separate UNDO storage in the event of writes

○ Row locks stored on tuple itself and no separate lock table

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VACUUM / AUTOVACUUM

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●Live Tuples: Tuples that are Inserted or up-to-date or can be read or modified

● Dead Tuples: Tuples that are changed (Updated/Deleted) and unavailable to be used for any future

transactions

●Continuous transactions may lead to a number of dead rows. A lot of space can be rather re-used by

future transactions

●VACUUM in PostgreSQL would cleanup the dead tuples and mark it to free space map

●Transaction ID (xmax) of the deleting transaction must be older than the oldest transaction still active in

PostgreSQL Server for vacuum to delete that tuple ( i.e. xmax < oldest_active_txid )

●If xmax of a tuple is 100 and xact_committed = true and the oldest transaction id that is still active is 99,

then vacuum cannot delete that tuple.

● Autovacuum in PostgreSQL automatically runs VACUUM on tables as a background process

●Autovacuum is also responsible to run ANALYZE that updates the statistics of a Table.

● To start autovacuum, you must have the parameter autovacuum set to ON

● Background Process : Stats Collector tracks the usage and activity information

● PostgreSQL identifies the tables needing vacuum or analyze depending on certain parameters

● Parameters needed to enable autovacuum in PostgreSQL are:

  autovacuum = on # (ON by default)

  track_counts = on # (ON by default)

● An automatic vacuum or analyze runs on a table depending on a certain mathematical equations

Autovacuum VACUUM

○Autovacuum VACUUM threshold for a table =

autovacuum_vacuum_scale_factor * number of tuples + autovacuum_vacuum_threshold

○If the actual number of dead tuples in a table exceeds this effective threshold, due to updates and

deletes, that table becomes a candidate for autovacuum vacuum

Autovacuum ANALYZE

○Autovacuum ANALYZE threshold for a table =

autovacuum_analyze_scale_factor * number of tuples + autovacuum_analyze_threshold

○Any table with a total number of inserts/deletes/updates exceeding this threshold since last

analyze is eligible for an autovacuum analyze 

●autovacuum_vacuum_scale_factor or autovacuum_analyze_scale_factor: Fraction of the table

records that will be added to the formula. For example, a value of 0.2 equals to 20% of the table records

●autovacuum_vacuum_threshold or autovacuum_analyze_threshold: Minimum number of obsolete

records or dml’s needed to trigger an autovacuum

●Let’s consider a table: foo.bar with 1000 records and the following autovacuum parameters

 autovacuum_vacuum_scale_factor = 0.2

 autovacuum_vacuum_threshold = 50

 autovacuum_analyze_scale_factor = 0.1

 autovacuum_analyze_threshold = 50

●Table : foo.bar becomes a candidate for autovacuum VACUUM when,

Total number of Obsolete records = (0.2 * 1000) + 50 = 250

●Table : foo.bar becomes a candidate for autovacuum ANALYZE when,

Total number of Inserts/Deletes/Updates = (0.1 * 1000) + 50 = 150

Manual Vacuuming : 

VACUUM FULL TABLE_NAME 

AutoVaccuming Parameters 

select name , setting  from pg_settings where  name like 'autovacuum%'; 

vacuum threshold = vacuum base threshold + vacuum scale factor + number of tables 

analyze threshold = analyze base threshold + analyze scale factor +  number of tuples 

############################################

Reindexing 

############################################

-- Reindex rebuolds and  Index  using data  stored  index's table 

-- This is avoid index bloating 

-- \h reindex  will show all  reindex options 

-- 'reindex system'  will  

############################################
Backup and Restore 

####################################### 

● PostgreSQL provides native backup tools for both Logical and Physical backups.

● Backups similar to mysqldump and Xtrabackup are automatically included with Community PostgreSQL

● Backups like RMAN in Oracle may be achieved using Open Source tools like pgBackRest and pgBarman

○ Logical Backups

■ pg_dump (Both Custom(Compressed and non human-readable) and Plain Backups)

■ pg_restore (To restore the custom backups taken using pg_dump)

■ pg_dumpall (To backup Globals - Users and Roles)

■ Logical Backups cannot be used to setup Replication and perform a PITR

■ You cannot apply WAL’s after restoring a Backup taken using pg_dump

pg_dump options 

-a : Data only . No data definition 

-s : Data Definition  only 

-n (schema)  : Dump  specified schema 

-t ( table)  : Dump  Specified Table only 

-f ( file name ) : Dump data to specified file 

-Fp : Dump in plan text ( default ) 

-Ft : Dump in tar format  

-Fc : Dump in compressed format 

Eg 

pg_dump -U postgres -d nano > /tmp/backup     ( backup will be created in text format ) 

Let’s use pgbench to create some sample tables

$ pgbench -i percona (Initialize)

$ pgbench -T 10 -c 10 -j 2 percona (load some data)

● Use pg_dump to backup the DDL (schema-only) of database: percona

$ pg_dump -s percona -f /tmp/percona_ddl.sql

● Use pg_dump to backup a table (with data) using custom and plain text format

$ pg_dump -Fc —t public.pgbench_history -d percona -f /tmp/pgbench_history

$ pg_dump -t public.pgbench_branches -d percona -f /tmp/pgbench_branches

● Create an another database and restore both the tables using pg_restore and psql

$ psql -c "CREATE DATABASE testdb"

$ pg_restore -t pgbench_history -d testdb /tmp/pgbench_history

$ psql -d testdb -f /tmp/pgbench_branches

pg_dumpall

○ Can dump all the databases of a cluster into a script file

○ Use psql to restore the backup taken using pg_dumpall

○ Can be used to dump global objects such as ROLES and TABLESPACES

● To dump only Globals using pg_dumpall, use the following syntax

$ pg_dumpall -g > /tmp/globals.sql

● To dump all databases (or entire Cluster), use the following syntax

$ pg_dumpall > /tmp/globals.sql

We can also use unix tools along 

eg :   pg_dump dbname  | gzip  > filename.gz 

pg_dump -U postgres -d nano | gzip > /tmp/backup     ( backup will be created in zipped text format ) 

pg_dump -U postgres -d nano | split -b 1k > /tmp/backup     ( backup will be created in splitted text format ) 

○ Physical Backups

■ pg_basebackup : File System Level & Online Backup, similar to Xtrabackup for MySQL

■ Useful to build Replication and perform PITR

■ This Backup can only use one process and cannot run in parallel

■ Explore Open Source Backup tools like : pgBackRest, pgBarman and WAL-e for more features like Xtrabackup

Command line options for pg_basebackup

$ pg_basebackup --help

-D --> Target Location of Backup

-cfast -—> Issues a fast checkpoint to start the backup earlier

-Ft -—> Tar format. Use -Fp for plain

-v --> Print the Backup statistics/progress.

-U --> A User who has Replication Privilege.

-W --> forcefully ask for password of replication User above. (Not mandatory)

-z --> Compresses the Backup

-R --> Creates a recovery.conf file that can be used to setup replication

-P --> Shows the progress of the backup

-l --> Creates a backup_label file

 pg_basebackup -U postgres -p 5432 -h 127.0.0.1 -D /tmp/backup_11052018 -Ft -z -Xs -P -R -l backup_label

Taking Consistent Backup  using pg_basebackup 

1) Enable Wal archiving   for point in time  recovery 

wal_level = archive

archive_mode = on

psql select * from pg_settings where name ='wal_level';

psql  show wal_level you  

psql select name, setting, sourcefile, sourceline from pg_settings where name = 'wal_level';

psql -c 'show config_file;'

 grep wal_level /data/pgsql/9.5/data/postgresql.conf

2) create a checkpoint using pg_start_backup  and Copy contents of the data directory

 pg_basebackup -U psql -h 10.0.0.110 -x -D /pgbase/

This script will take the backup of postgreSQL cluster  daily basis  and it will  remove the backup files if backup age  reached 7 days. its the 

#Backup Dir

#Make sure this is a dedicated mount point to PostgreSQL Backups

#Don't put traling / in path

#Backup Details

backup_dir=/backup

export PGPASSWORD="Nijam@1234"

export PGPORT="5432"

echo -e "\n\nBackup Status: $(date +"%d-%m-%y")" >> $backup_dir/Status.log

echo -e "-----------------------" >> $backup_dir/Status.log

echo -e "\nStart Time: $(date)\n" >> $backup_dir/Status.log

/usr/lib/postgresql/9.6/bin/pg_basebackup -U psql  -w -D $backup_dir/PostgreSQL_Base_Backup_$(date +"%d-%m-%y") -l "`date`" -P -F tar -z -R &>> $backup_dir/Status.log

echo -e "\nEnd Time: $(date)" >> $backup_dir/Status.log

#Auto Deletion for Backups

#Value 7 for retention_duration will keep 8 days backups

#retention_duration=7

#find $backup_dir/PostgreSQL_Base_Backup* -type d -mtime +$retention_duration -exec rm -rv {} \;

Found  below document relevant to restoration of base backup : 

https://dbsguru.com/restore-backup-using-pg_basebackup-postgresql/

https://www.percona.com/blog/2018/12/21/backup-restore-postgresql-cluster-multiple-tablespaces-using-pg_basebackup/

https://docs.google.com/document/d/1J8feIwMfXmKqOd4oXREFCnO9c7cwFoOT/edit?usp=sharing&ouid=108567388003714488593&rtpof=true&sd=true

####################################### 

High Availability in PostgreSQL

####################################### 

● Streaming Replication for PostgreSQL 9.x and above

○ WAL Segments are streamed to Standby/Slave and replayed on Slave

○ Not a Statement/Row/Mixed Replication like MySQL

○ This can be referred to as a byte-by-byte or Storage Level Replication

○ Slaves are always Open for Read-Only SQLs but not Writes

○ You cannot have different Schema or Data in a Master and a Slave in Streaming Replication

○ Allows Cascading Replication

○ Supports both Synchronous and Asynchronous Replication

○ Supports a Delayed Standby for faster PITR

● Logical Replication and Logical Decoding for PostgreSQL 10 and above

○ Allows for Replication of selected Tables using Publisher and Subscriber Model

○ Similar to binlog_do_db in MySQL, but no DDL Changes are replicated

○ Subscribers are also open for Writes automatically

○ Used in Data Warehouse environments that stores Data fetched from multiple OLTP Databases for Reporting, etc

Streaming Replication: 

● Step 1: Create a user in Master with REPLICATION ROLE

CREATE USER replicator

WITH REPLICATION

ENCRYPTED PASSWORD 'replicator';

● Step 2: Parameters you should know while setting up SR

archive_mode: Must be set to ON to enable Archiving of WALs

wal_level: Must be set to "hot_standy" until 9.5 and "replica" in the later versions.

max_wal_senders: Must be set to 3 if you are starting with 1 Slave. For every Slave, you may add 2 wal senders.

wal_keep_segments: Number of WALs always retained in pg_xlog (Until PostgreSQL 9.6) or pg_wal (From

PostgreSQL 10)

archive_command: This parameter takes a shell command. It can be a simple copy command to copy the WAL

segments to another location or a Script that has the logic to archive the WALs to S3 or a remote Backup Server.

hot_standby: Must be set to ON on Standby/Replica and has no effect on the Master. However, when you setup

your Replication, parameters set on Master are automatically copied. This parameter is important to enable READS

on Slave. Else, you cannot run your SELECTS on Slave. 

● Step 3: Set the parameters that are not set already

ALTER SYSTEM SET wal_keep_segments TO '50';

select pg_reload_conf();

● Step 4: Add an entry to pg_hba.conf of Master to allow Replication connections from Slave

 Default location of pg_hba.conf is the Data Directory

$ vi pg_hba.conf

Add the following line between >>>>> and <<<<<< to the end of the pg_hba.conf file

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

host replication replicator 192.168.0.28/32 md5

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Replace the IP address(192.168.0.28) with your Slave IP address

● Step 5: Give a SIGHUP or RELOAD

$ pg_ctl -D $PGDATA reload

● Step 6: Use pg_basebackup to backup of your Master data directory to the Slave data directory

$ pg_basebackup -U replicator -p 5432 -D /tmp/slave -Fp -Xs -P -R

● Step 7: Change the port number of your slave if you are creating the replication in the same server

for demo

$ echo "port = 5433" >> /tmp/slave/postgresql.auto.conf

● Step 8: Start your Slave

$ pg_ctl -D /tmp/slave start

● Step 9: Check the replication status from Master using the view : pg_stat_replication

select * from pg_stat_replication;

################################################

Applying Minor Version  Patch 

################################################

pg_upgrade has been around for a very long time as a tool for upgrading major versions of PostgreSQL. Using this tool is not required for minor version upgrades, which means that upgrading your current version of 11.9 to 11.13 is not necessary

Follow the below steps to perform a minor version upgrade:

1) Check the current PostgreSQL version

select version();

2) Wait for all replicas to catch up 

3) Take the backup of PostgreSQL cluster: Use pg_dumpall for logical backup and pg_basebackup for physical backup.

pg_dumpall -U postgres -W  -f /u01/backup/dumpall.sql

ls -l /u01/backup/dumpall.sql

 

4) Stop the PostgreSQL cluster

/usr/pgsql-9.5/bin/pg_ctl -D /var/lib/pgsql/9.5/data stop

 systemctl  stop  edb-as-9

There are  3 types of shutdown 

1) Smart  

2) Fast ( Default ) 

3) Immediate 

 

5) Run yum update to update new binaries

[root@test /]#yum -y install https://download.postgresql.org/pub/repos/yum/reporpms/EL-7-x86_64/pgdg-redhat-repo-latest.noarch.rpm 

[root@test /]#yum install postgresql12-server 

Install the below packages on linux server.

[On RHEL/CentOS]

# yum install gcc*

# yum install zlib-devel*

# yum install readline-devel*

[On Debian/Ubuntu]

# apt install gcc*

# apt install zlib1g-dev*

# apt install libreadline6-dev*

[On SUSE Linux ]

# Zypper in gcc*

# zlib1g-dev*

# libreadline6-dev*

# zypper in zlib*

Install the new version of postgres server as root user.

yum check-update <package_name>

yum update <package_name>

yum update edb*

 

[root@test-machine01 ~]# yum check-update postgresql13*

[root@test-machine01 ~]# yum list  postgresql13*

[root@test-machine01 ~]# yum update  postgresql13*

6)  Start PostgreSQL Cluster and check Version

systemctl  start edb-as-9

 

 

################################################

Killing Session 

################################################

Note Down PID for  session that you want to  Kill 

select * from pg_stat_activity;

SELECT pid, state, backend_start, substr(query, 0, 100) q

FROM pg_stat_activity

WHERE backend_type = 'client backend'

ORDER BY backend_start;

 Kill The Session 

select pg_terminate_backend(pid) 

from pg_stat_activity

where pid = '18765';

SELECT pg_cancel_backend(pid);

 

################################################

key  views 

################################################

PG_TABLES

PG_ROLES 

PG_USER

pg_views 

role_table_grants 

PG_TABLESPACE 

pg_stat_activity 

pg_locks 

pg_config 

################################################

#Finding  Blocking Session and Killing  It 

################################################

1) Find  Blocking PID 

select pid, 

       usename, 

       pg_blocking_pids(pid) as blocked_by, 

       query as blocked_query

from pg_stat_activity

where cardinality(pg_blocking_pids(pid)) > 0;

SELECT 'SELECT pg_terminate_backend(' || pid || ');'

FROM pg_stat_activity

WHERE pid = ANY (pg_blocking_pids(blockedpid));

2) Killing Blocking pid 

SELECT 'SELECT pg_terminate_backend(' || pid || ');'

  FROM pg_stat_activity

  WHERE pid = ANY (pg_blocking_pids(613));

################################################

# Checking Long runing  session 

################################################

SELECT 'SELECT pg_terminate_backend(' || pid || ');'

FROM pg_stat_activity

WHERE backend_type = 'client backend'

      AND pid != pg_backend_pid()

      AND backend_start < NOW() - '10 seconds'::interval;

Adjust '10 seconds' as appropriate.

################################################

Monitoring Commands 

################################################

# To View Control information For a database Cluster 

pg_controldata -D datadir 

pg_controldata -D datadir -V

#DN Commands 

\d     ( object information ) 

\dt+  ( size of  table ) 

\dn    ( schema  information ) 

\dn+  ( schema with permission ) 

\l       (  to see databases  )  

\conninfo    (  current connection information ) 

select current_database(); 

select  current_schema  ; 

Show commands : 

show  data_directory 

show work_mem 

# To See Database Parameters 

select name, setting, setting::int * 16 || 'MB' AS setting_in_mb

from pg_settings 

where name in ('min_wal_size', 'max_wal_size');

#Checking Statistics Details 

select relname , reltuples   from  pg_class where  relname='testanalyze'; 

#Check Size of  Database / Tables 

edb-psql dbname username

SELECT pg_size_pretty( pg_database_size('dbname') );

SELECT pg_size_pretty( pg_total_relation_size('tablename') );

#Check File Path for  Table 

select pg_relation_filepath('sales') ; 

select * from pg_database;

select current_database(); 

select current_user;

SELECT version();

select * from pg_tablespace;

################################################

Controlling output of query : 

################################################

 save all query results and output to a file

-o filename

\o filename

\out filename

 output all query results and output to a bash command

\o | command

\out | command

run previous query repeatedly 

\watch