Note--SQL

Postgres Tutorials | Crunchy Data

Often times the gap in trying/learning something in Postgres is having a good tangible example. The playground makes that easier by loading a datasets then guiding you step by step through an exercise leveraging that dataset in a practical way.

https://www.crunchydata.com/developers/tutorials

SQL

WITH abc AS( select
             FROM ...)
, XYZ AS(select
         From abc ....) /*This one uses "abc" multiple times*/
  Select 
  From XYZ....   /*using abc, XYZ multiple times*/

Timing

You can have psql run a timer for your queries by setting

SQL

\timing

Index Cache Hit Rate

SQL

SELECT  sum(idx_blks_read) as idx_read,   sum(idx_blks_hit)  as idx_hit,   
(sum(idx_blks_hit) - sum(idx_blks_read)) / sum(idx_blks_hit) as ratio 
FROM pg_statio_user_indexes;

LOGGING

Log slow queries by setting log_min_duration_statement

SQL

ALTER database postgres SET log_min_duration_statement='250ms';

Control which statement types get logged

Control the types of statements that are logged for your database.

SQL

ALTER DATABASE postgres SET log_statement = 'all'

Valid values include all, ddl, none, mod

Log when waiting on a lock

Log when database is waiting on a lock.

SQL

ALTER DATABASE postgres SET log_lock_waits = 'on'

PERFORMANCE

Use statement timeouts to control runaway queries

Setting a statement timeout prevents queries from running longer than the specified time. You can set a statement timeout on the database, user, or session level. We recommend you set a global timeout on the database and then override that one specific users or sessions that need a longer allowed time to run.

SQL

ALTER DATABASE mydatabase SET statement_timeout = '60s';

Use pg_stat_statements to find the queries and processes that use the most resources

SQL

SELECT
	total_exec_time,
	mean_exec_time as avg_ms,
	calls,
	query
FROM pg_stat_statements
ORDER BY mean_exec_time DESC
LIMIT 10;

Monitor connections in Postgres

This query will provide the number of connection based on type.

SQL

SELECT count(*),
	   state
FROM pg_stat_activity
GROUP BY state;

If you see idle is above 20 it's recommended to explore using PgBouncer.

Query size of specific table

Will give you the size of the specific relation you pass in.

SQL

SELECT pg_relation_size('table_name');

# For prettier formatting you can wrap with:
SELECT pg_size_pretty(pg_relation_size('table_name'));

Query all relation sizes

Will report on all table sizes in descending order

SQL

SELECT relname AS relation,
       pg_size_pretty (
         pg_total_relation_size (C .oid)
       ) AS total_size
FROM pg_class C
LEFT JOIN pg_namespace N ON (N.oid = C .relnamespace)
WHERE nspname NOT IN (
        'pg_catalog',
        'information_schema'
      )
  AND C .relkind <> 'i'
  AND nspname !~ '^pg_toast'
  ORDER BY pg_total_relation_size (C .oid) DESC

Check for unused indexes

Will return the unused indexes in descending order of size. Keep in mind you want to also check replicas before dropping indexes.

SQL

SELECT schemaname || '.' || relname AS table,
       indexrelname AS index,
       pg_size_pretty(pg_relation_size(i.indexrelid)) AS "index size",
       idx_scan as "index scans"
FROM pg_stat_user_indexes ui
JOIN pg_index i ON ui.indexrelid = i.indexrelid
WHERE NOT indisunique
  AND idx_scan < 50
  AND pg_relation_size(relid) > 5 * 8192
ORDER BY pg_relation_size(i.indexrelid) / nullif(idx_scan, 0) DESC NULLS FIRST,
pg_relation_size(i.indexrelid) DESC;

Get approximate counts for a table

Will return the approximate count for a table based on PostgreSQL internal statistics. Useful for large tables where performing a `SELECT count(*)` is costly on performance.

SQL

SELECT reltuples::numeric as count
FROM pg_class
WHERE relname='table_name';

Non-blocking index creation

Adding `CONCURRENTLY` during index creation, while not permitted in a transaction, will not hold a lock on the table while creating your index.

SQL

CREATE INDEX CONCURRENTLY foobar ON foo (bar)

PSQL

Automatically log query time in PSQL

Will automatically print the time it took to run a query from within psql.

*Of note this is the round trip time not simply query execution time.*

SQL

\timing

You can save this in your .psqlrc to be a default setting

Autoformat query results in psql

Will automatically reorganize the query output based on your terminal window for better readability.

SQL

\x auto

You can save this in your .psqlrc to be a default setting

Edit your psql queries in editor of your choice

Will automatically open your last run query in your default $EDITOR. When you save and close it will execute that query.

SQL

\e

Set a value for nulls

Will render the nulls as whatever character you specify. Handy for easier parsing of nulls vs. blank text.

SQL


\pset null 👻

You can save this in your .psqlrc to be a default setting

Save your query history per database locally

Will automatically save a history file for each **DBNAME**.

SQL

\set HISTFILE ~/.psql_history- :DBNAME

You can save this in your .psqlrc to be a default setting

Clear your psql screen

Will clear your screen in current psql session

SQL

\! clear

Continually run a query with watch

Will automatically run the last query ever 2 seconds and display the output. You can also specify the query that will run after watch as well.

SQL

\watch

Rollback to previous statement on error when in interactive mode

When you encounter an error when in interactive mode this will automatically rollback to just before the previous command, allowing you to continue working as you would expect.

SQL

\set ON_ERROR_ROLLBACK interactive

Export a CSV from directly in psql

Will run the specific query and return live a SQL for you

SQL

psql --csv -c 'select * from test;'

Run a query from a file in psql

Will execute the specified file when inside psql.

SQL

\i filename'

Provide clean border within Psql

Will give you a border around the output of your queries when in psql

SQL

\pset border 2

You can save this in your .psqlrc to be a default setting

Set linestyle to unicode

Changes the linestyle to unicode, which when combined with above tip leads to much cleaner formatting

SQL

\pset linestyle unicode

You can save this in your .psqlrc to be a default setting

SQL

Replace nulls with other value

Coalesce will use the value and if the value is null display your specified string.

JavaScript

SELECT id,
       coalesce(ip, 'no IP') // or use IfNull
FROM logs;

You can supply two columns as well prior to your replacement value and the function will use first not null value.

Generate data with generate_series

Generates values from the start to the end values supplied based on the interval. Values can be numbers or timestamps. Can be used in a FROM or JOIN clause or CTE. Commonly used when building charts and reports that require all dates to be filled.

SQL

SELECT * FROM generate_series(now() - '3 month'::interval, now(), '1 day');

Round dates with date_trunc

Will truncate the date to the specified level of precision. Some example precision levels include: month, week, day, hour, minute.

SQL

SELECT date_trunc('day', now());

Perform time math with intervals

You can add or subtract specific amounts of time of a timestamp by casting the value you want as an interval.

SQL

SELECT now() - '1 month'::interval;

Explain whether index has been used

JavaScript

explain SELECT * FROM student;

UTILITY

Create read only user

Once already creating a specific user role, you can user the pg_read_all_data to grant read only access to all tables.

SQL

GRANT pg_read_all_data TO username;

LIMIT

Return the results

SQL

LIMIT 1,1 // second and only one record

Function

SQL

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
set N = N-1;
  RETURN (
      # Write your MySQL query statement below.
      SELECT DISTINCT Salary FROM Employee
      ORDER BY Salary DESC LIMIT N,1
  );
END

SQL efficiency

Ref

MySQL Tutorial

MySQL Tutorial website provides a comprehensive MySQL tutorial that helps you master MySQL fast, easy & fun.

https://www.mysqltutorial.org/