Reading the plan
EXPLAIN on a slow query shows, at the innermost level:
Seq Scan on submissions (rows=4,200,000)
Filter: (student_id = 991)
Rows Removed by Filter: 4,199,987
What does this tell you?
EXPLAIN on a slow query shows, at the innermost level:
Seq Scan on submissions (rows=4,200,000)
Filter: (student_id = 991)
Rows Removed by Filter: 4,199,987
What does this tell you?