SQL Utils

Summary of commands for working with SQL statements.

SQL Commands

SQL commands for querys. Reference: https://www.w3schools.com/sql/

Lets SELECT !

Some samples when you need to list objects from database:

select id,name,surname from author order by created_timestamp desc limit 5;

Lets UPDATE !

update author set name='Franklin' where id=861;

Lets DELETE !

delete from author where id='57';

Lets ALTER TABLE !

• Editing table structure

ALTER TABLE table_name ADD COLUMN field BOOLEAN DEFAULT true
ALTER TABLE table_name ALTER COLUMN field DROP NOT NULL
ALTER TABLE table_name ALTER COLUMN field SET NOT NULL
ALTER TABLE table_name ALTER COLUMN field TYPE smallint
ALTER TABLE table_name ALTER COLUMN field DROP default
ALTER TABLE table_name ALTER COLUMN field SET DEFAULT 10
ALTER TABLE table_name DROP COLUMN field
ALTER TABLE table_name RENAME COLUMN old_name TO new_name
ALTER TABLE table_name DROP CONSTRAINT constraint_name

• Working with Foreign keys

ALTER TABLE table_name DROP CONSTRAINT nombre_foreign_key_fkey
ALTER TABLE table_name ADD FOREIGN KEY(field) REFERENCES table_name_2(field)

Lets Copy a Table !

With data:

CREATE TABLE new_table AS
TABLE existing_table;

With no data:

CREATE TABLE new_table
AS TABLE existing_table WITH NO DATA;

With some of the data:

CREATE TABLE new_table AS 
SELECT * FROM existing_table WHERE condition;

PSQL Commands

Databases size on disk

Easy ! just do this:

select datname AS db_name,
       pg_size_pretty(pg_database_size(datname)) as db_size
from pg_database 
order by pg_database_size(datname) desc;

Tables disk usage

SELECT 
       relname AS "table_name", 
       pg_size_pretty(pg_table_size(C.oid)) AS "table_size" 
FROM 
       pg_class C 
LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace) 
WHERE nspname NOT IN ('pg_catalog', 'information_schema') AND nspname !~ '^pg_toast' AND relkind IN ('r') 
ORDER BY pg_table_size(C.oid) 
DESC LIMIT 1;

Death rows

select relname, n_dead_tup from pg_stat_user_tables;

Vacuum

Check Autovacuum is on and other settings:

SELECT name, setting FROM pg_settings WHERE name='autovacuum';
SELECT * from pg_settings where category like 'Autovacuum';

Last time (auto)vacuum ran:

SELECT relname, last_vacuum, last_autovacuum FROM pg_stat_user_tables;

Getting psql help

You access psql command line with psql or psql -U <username>.

Then you can get help with the command help

psql commands cheet sheet !

Postgres psql commands.

\! - clear screen

\d <TABLE_NAME> - Describe table

\dt - Show tables

\list - List databases

Backing up database

Sample command: console >pg_dump -U db_username -W -F t database_name > c:\backup_file.tar

Other sample:

timestamp=$(date +%Y%m%d)
pg_dump -U $dbuser $dbname -W -F p -f ${timestamp}_${dbname}.sql

Lets understand the options:

• -U db_username: (--username) User to connect to PostgreSQL database server.
• -W: forces pg_dump to prompt for the password before connecting to server.
• --format : Specifies the output file format that can be one of the following:
  • --format=c: custom-format archive file format
  • --format=d: directory-format archive
  • --format=t:tar
  • --format=p: plain text SQL script file).
• -f: (--file) The file where to save the backup

Restoring database

Restore database from plaintext file...

psql -U db_name < backup.sql