EXPLAIN plan for query analysis in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the EXPLAIN plan in SQL?

The EXPLAIN plan shows how the database will execute a query. It helps understand the steps and order the database uses to get the results.

beginner

Which part of the EXPLAIN plan tells you the order of operations?

The EXPLAIN plan lists operations in the order the database will perform them, usually from bottom to top or left to right, showing the flow of data.

beginner

What does a 'Seq Scan' mean in an EXPLAIN plan?

A 'Seq Scan' means the database reads the whole table row by row to find matching data. It can be slow for big tables.

intermediate

How can EXPLAIN help improve query performance?

By showing how a query runs, EXPLAIN helps find slow parts like full table scans. You can then add indexes or rewrite queries to make them faster.

intermediate

What does the 'cost' in an EXPLAIN plan represent?

The 'cost' is a number estimating how much work the database needs to do for each step. Lower cost usually means faster execution.

What does the EXPLAIN plan show for a SQL query?

A The exact time the query took to run

B The syntax errors in the query

C The number of rows returned by the query

D How the database will execute the query

EXPLAIN shows the execution steps, not runtime or errors.

In an EXPLAIN plan, what does a 'Seq Scan' indicate?

A Sorting the results

B Using an index to find rows quickly

C A full table scan reading all rows

D Joining two tables

'Seq Scan' means scanning the whole table row by row.

Why is the 'cost' value important in an EXPLAIN plan?

A It measures the query's memory usage

B It estimates the work needed to run the query step

C It counts the rows returned

D It shows the number of errors in the query

'Cost' estimates how much work the database does for each step.

How can EXPLAIN help you improve a slow query?

A By showing which parts take the most work

B By fixing syntax errors automatically

C By increasing the database size

D By deleting unnecessary tables

EXPLAIN helps identify slow parts to optimize.

Which of these is NOT shown by an EXPLAIN plan?

A Actual query execution time

B Estimated cost

C Order of operations

D Type of scans used

EXPLAIN shows estimated plans, not actual run times.

Explain what an EXPLAIN plan is and how it helps with SQL queries.

Think about how you would check the steps a recipe follows before cooking.

Describe what a 'Seq Scan' means in an EXPLAIN plan and why it might be slow.

Imagine reading every page of a book to find one word.