Moderate SQL Question 89 of 226

When will the planner ignore your index?

Java Specialist · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: DATABASE INDEX

Without indexScan every row

With indexJump to keys

CostWrites slower

Simple meaning

Functions on the column, implicit type conversion, leading wildcards like LIKE '%foo', or low selectivity can force a sequential scan.

WHY — SQL instead of guessing?

Why interviewers care about SQL:

They are checking judgment

on SQL.

A good answer names

the situation, the default choice, and one exception - that reads as experience.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

STEPS — What happens step by step?

Before you speak the answer, walk the interviewer through these steps:

1
Functions on the column,
implicit type conversion, leading wildcards like LIKE '%foo', or low selectivity can force a sequential scan.
2
If the table is
tiny, a scan can be cheaper than random index lookups.
3
Always confirm with EXPLAIN
ANALYZE rather than arguing from theory.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“If the table is tiny, a scan can be cheaper than random index lookups.”

Break into beats

Ifthetableistinya

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

Functions on the column, implicit type conversion, leading wildcards like LIKE '%foo', or low selectivity can force a sequential scan. If the table is tiny, a scan can be cheaper than random index lookups.