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Rhaqim
Rhaqim

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Journal Entry: I Forgot to Clean Up After Myself

#backend #postgres #go #database

TL;DR: Always call defer rows.Close() after querying your database.

Recently, I ran into a silent but critical problem in my Go application backed by PostgreSQL. Everything worked fine on the surface — requests were being handled, responses came back fast… until one day, I hit a wall.

New requests started hanging unexpectedly, and I noticed some routes weren’t returning data at all. But one route continued to work: the one fetching data from the cache instead of hitting the database.

Something wasn’t right.

🔍 The Clue: pg_stat_activity

I ran the following on my Postgres server:

SHOW max_connections; SELECT COUNT(*) FROM pg_stat_activity;
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And here was the result:

  • max_connections was set to 100
  • Active connections in pg_stat_activity? 67 😳

That number seemed way too high. On the client side (Go), I had:

db.SetMaxOpenConns(50)
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So what was holding these connections open?

🧠 Aha Moment: I Forgot defer rows.Close()

After inspecting my codebase, I realized I had this kind of pattern:

rows, err := db.Query("SELECT * FROM rows_fetcher($1)", query_key) if err != nil { return nil, err } for rows.Next() { // scan logic... }
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But no defer rows.Close(). 😱
Without explicitly closing the result set, the connection remains open and eventually floods the pool.

🔬 Digging Deeper: Seeing the Idle State

I ran:

SELECT pid, state, query, backend_start, xact_start, query_start FROM pg_stat_activity WHERE state = 'idle';
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I noticed many of these had timestamps from weeks or even months ago!

🧹 Cleaning Up

I decided to take a few actions:

  1. Update all database queries in my Go code to include: 
defer rows.Close()
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  1. Use pg_terminate_backend(pid) to clean up old, stale processes manually.
  2. Add SetConnMaxIdleTime to my DB setup: 
db.SetConnMaxIdleTime(5 * time.Minute)
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  1. Consider setting PostgreSQL server config options like: 
idle_in_transaction_session_timeout = 60000 # 1 min idle_session_timeout = 300000 # 5 min
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✅ The Result After 24 Hours

I checked back the next day and ran:

SELECT COUNT(*) FROM pg_stat_activity;
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To my surprise and relief, the number was down to 1.

One. From 67.

💡 Takeaways

  • Always close your rows after querying:
  • Use pg_stat_activity to spot stale connections.
  • Monitor timestamps, old queries might still be holding connections open.
  • Use connection pooling wisely (SetMaxOpenConns, SetConnMaxIdleTime).
  • Set database level timeouts to catch idle sessions that your app misses.

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