Detecting Full Table Scans With SQLite

I’m at RubyConf this week, and it’s great!

I recently read that lobste.rs is now running on SQLite. One part from the post caught my attention:

I wish we could say in a test, “Fail if you encounter any full table scans”. Which would have caught the perf issues we experienced during the first deploy.

SQLite collects information about prepared statements and exposes those statistics though an API. The upshot of this is that we can tell whether a statement did a full table scan after executing the statement without using an EXPLAIN.

Here’s an example program that demonstrates detecting a query did a full table scan:

db = SQLite3::Database.new(":memory:")

db.execute("CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT, age INTEGER)")

# Insert a bunch of records
1_000.times do |i|
  db.execute("INSERT INTO users (name, age) VALUES (?, ?)",
    ["user#{i}", i % 100])
end

# Prepare a statement and query it
stmt = db.prepare("SELECT * FROM users WHERE age = ?")
stmt.bind_param(1, 42)
stmt.to_a

# Check the number of full scan steps, we see a bunch because the query
# did a full table scan
fullscan_steps = stmt.stat(:fullscan_steps)
puts "fullscan_steps: #{fullscan_steps}"
if fullscan_steps > 0
  puts "=> query performed a full table scan"
end

# Create an index
db.execute("CREATE INDEX idx_users_age ON users(age)")

# Now the query won't do a full table scan
stmt2 = db.prepare("SELECT * FROM users WHERE age = ?")
stmt2.bind_param(1, 42)
stmt2.to_a

puts "after adding index, fullscan_steps: #{stmt2.stat(:fullscan_steps)}"

Feels like we could integrate this in to Rails and warn or raise in test / development. I’m not sure if we’d want to check this all the time in production, but maybe it would be fine?

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