Query optimization with EXPLAIN in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the EXPLAIN command in SQL?

EXPLAIN shows how the database plans to execute a query. It helps understand the steps and resources used, so you can find ways to make the query faster.

beginner

What does a sequential scan mean in an EXPLAIN output?

A sequential scan means the database reads the whole table row by row. It can be slow for big tables and might mean missing indexes.

intermediate

How can indexes improve query performance according to EXPLAIN?

Indexes let the database find rows faster without scanning the whole table. EXPLAIN shows index scans when indexes are used, which usually means faster queries.

intermediate

What does the 'cost' value in EXPLAIN output represent?

The cost is a number estimating how much work the database needs to do to run the query. Lower cost means less work and usually faster queries.

beginner

Why is it important to analyze EXPLAIN output before optimizing queries?

Analyzing EXPLAIN helps you see where the query spends time and resources. This guides you to add indexes or rewrite queries to make them faster.

What does EXPLAIN show in Supabase SQL?

A How the database plans to run a query

B The exact time a query took to run

C The number of rows returned by a query

D The syntax errors in a query

EXPLAIN shows the query plan, not execution time or errors.

If EXPLAIN shows a sequential scan on a large table, what might help improve performance?

A Running the query multiple times

B Adding an index on the searched column

C Increasing the database storage size

D Deleting rows from the table

Indexes help avoid full table scans and speed up queries.

What does a lower cost value in EXPLAIN indicate?

A More CPU used

B More rows returned

C Less work needed to run the query

D Longer query time

Lower cost means the database expects less work to execute the query.

Which EXPLAIN output indicates the use of an index?

A Sort

B Seq Scan

C Hash Join

D Index Scan

Index Scan means the query uses an index to find rows.

Why should you use EXPLAIN before changing a query?

A To understand how the query runs and find bottlenecks

B To check spelling mistakes

C To count the number of tables

D To backup the database

EXPLAIN helps identify slow parts of a query to optimize.

Explain how EXPLAIN helps optimize SQL queries in Supabase.

Think about what EXPLAIN reveals about query steps and resource use.

Describe what a sequential scan is and why it might slow down a query.

Imagine reading a whole book to find one word versus using the index.