Diagnose & Fix a Slow Query with EXPLAIN
Diagnose & Fix a Slow Query with EXPLAIN
MediumDatabases0% accepted
A report query times out. Walk through how you'd diagnose and fix it using EXPLAIN/EXPLAIN ANALYZE. Cover: reading the plan (seq scan vs index scan), spotting a missing index, why a function on an indexed column kills the index (sargability), and the N+1 query problem from the application side.
Approach
Run EXPLAIN ANALYZE: look for sequential scans on big tables, bad row estimates, and expensive joins. Add the right index, keep predicates sargable (no functions on indexed columns), and fix N+1s with a join or batch fetch.
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