You created an index on email column but queries filtering by email are still slow. What is a likely cause?

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PostgreSQL - Indexing Strategies

You created an index on email column but queries filtering by email are still slow. What is a likely cause?

AThe index is not used because the query applies a function on email (e.g., LOWER(email))

BThe index was created on the wrong table

CPostgreSQL does not support indexes on text columns

DThe email column contains only unique values

Step-by-Step Solution

Solution:

Step 1: Understand index usage with functions
Indexes on columns are not used if queries apply functions unless functional indexes exist.
Step 2: Analyze the query behavior
If the query uses LOWER(email), the normal index on email is ignored, causing slow scans.
Final Answer:
The index is not used because the query applies a function on email (e.g., LOWER(email)) -> Option A
Quick Check:
Functions on columns require functional indexes [OK]

Quick Trick: Use functional indexes when querying with functions on columns [OK]

Common Mistakes:

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