Like any other RDBMS, PostgreSQL generates a Query Execution Plan for each query that it receives.

It is also used Genetic Query Optimizer (GEQO) that does query planning for heuristic search and It reduce query planning time for large and complex queries.

An Optimizer always tries to choose a good Query Execution Plan.

The structure of Query Execution Plan is like a Tree and It contains different information like: type of scan details, join details, sorting details, aggregation details, Estimated rows details, Estimated start-up cost, Estimated total cost and others.

To know about the Query Execution Plan, is very important for all Database Professionals because It helps us to improve the Query Performance.

With the help of Query Execution Plan, We can make our decisions on Index tuning, Join tuning and other.

In PostgreSQL, we have EXPLAIN and EXPLAIN ANALYZE command to know about the Query Execution Plan.

Here, I have used one table tbl_itemtransactions which you can create using below post.

Example of EXPLAIN: It generates the Estimated Query Execution Plan with out executing an actual SQL Query.

EXPLAIN SELECT COUNT(1) FROM tbl_itemtransactions;

/*
Result: 
"Aggregate  (cost=2555525.00..2555525.01 rows=1 width=0)"
"  ->  Seq Scan on tbl_itemtransactions  
(cost=0.00..2216945.00 rows=135432000 width=0)"
*/

Example of EXPLAIN ANALYZE: It first executes the SQL Query and than It generates the actual Query Execution Plan.

EXPLAIN ANALYZE SELECT COUNT(1) FROM tbl_itemtransactions;

/*
Result: 
"Aggregate  (cost=2555525.00..2555525.01 rows=1 width=0) 
(actual time=52660.132..52660.133 rows=1 loops=1)"
"  ->  Seq Scan on tbl_itemtransactions  
(cost=0.00..2216945.00 rows=135432000 width=0) 
(actual time=0.027..46193.484 rows=135432001 loops=1)"
"Planning time: 0.204 ms"
"Execution time: 52660.237 ms"
*/

In this post, I am sharing a new feature of MySQL 5.7 which is EXPLAIN FOR CONNECTION.
As we all aware about the EXPLAIN command of MySQL Server, which we are using to find a query execution information.

EXPLAIN FOR CONNECTION is a new feature and using this we can find query execution information about a running query connection.

When we are diagnosing performance problems, then this is very useful for us.
For example, We are running a statement in one session which is taking a long time, but using EXPLAIN FOR CONNECTION we can check the reason for the delay in another session.

We need to pass connection_id to check the running connection information.
We can check connection_id using different method like:

SHOW PROCESSLIST;

SELECT * FROM INFORMATION_SCHEMA.PROCESSLIST;

SELECT CONNECTION_ID();


You can also visit this article to find and kill the running connections of MySQL.

The syntax :

EXPLAIN [options] FOR CONNECTION connection_id;

Once you execute above command and if result is empty, there are no any explainable statements. If we have SELECT, INSERT, UPDATE and DELETE, we can get execution information using this option.

For example:

mysql> SELECT CONNECTION_ID();
+-----------------+
| CONNECTION_ID() |
+-----------------+
|            8254 |
+-----------------+
1 row in set (0.00 sec)

mysql> EXPLAIN FOR CONNECTION 8254;
ERROR 1889 (HY000): EXPLAIN FOR CONNECTION command is supported
only for SELECT/UPDATE/INSERT/DELETE/REPLACE