[Solved] You wrote this query:SELECT * FROM orders WHERE customer_id = 123;But EXPLAIN shows a sequential scan instead of an index scan. What could be the reason? | PostgreSQL

PostgreSQL - Performance Tuning

You wrote this query:
SELECT * FROM orders WHERE customer_id = 123;
But EXPLAIN shows a sequential scan instead of an index scan. What could be the reason?

AThere is no index on <code>customer_id</code>

BThe table is empty

CThe query syntax is incorrect

DPostgreSQL always uses sequential scan

Step-by-Step Solution

Solution:

Step 1: Check index presence on filter column
If no index exists on customer_id, PostgreSQL must scan all rows sequentially.
Step 2: Evaluate other options
Empty table would still show scan but no rows; syntax is correct; PostgreSQL chooses scan type based on indexes.
Final Answer:
There is no index on customer_id -> Option A
Quick Check:
No index on filter column = sequential scan [OK]

Quick Trick: No index on filter column causes sequential scan [OK]

Common Mistakes:

Assuming syntax error causes scan type
Thinking PostgreSQL always uses sequential scan
Ignoring missing index as cause

Master "Performance Tuning" in PostgreSQL

9 interactive learning modes - each teaches the same concept differently

Learn Why Deep Visual Try Challenge Project Recall Time

You wrote this query:

Step 1: Check index presence on filter column

Step 2: Evaluate other options

Final Answer:

Quick Check:

Want More Practice?