snowflake-known-pitfalls

# Snowflake Known Pitfalls

## Overview

Common mistakes and anti-patterns when using Snowflake, with real SQL examples and fixes.

## Pitfall #1: Leaving Warehouses Running (Cost Killer)

**Anti-Pattern:**
```sql
-- Warehouse with auto_suspend = 0 (never suspends)
CREATE WAREHOUSE ALWAYS_ON_WH
  WAREHOUSE_SIZE = 'XLARGE'
  AUTO_SUSPEND = 0;
-- 16 credits/hour = ~$1,152/day at $3/credit
```

**Fix:**
```sql
ALTER WAREHOUSE ALWAYS_ON_WH SET
  AUTO_SUSPEND = 120,    -- Suspend after 2 min idle
  AUTO_RESUME = TRUE;    -- Resume on next query

-- Audit all warehouses for high auto_suspend
SELECT name, size, auto_suspend, state
FROM INFORMATION_SCHEMA.WAREHOUSES
WHERE auto_suspend > 600 OR auto_suspend = 0;
```

---

## Pitfall #2: Using ACCOUNTADMIN for Everything

**Anti-Pattern:**
```sql
-- Human users with ACCOUNTADMIN default role
ALTER USER analyst SET DEFAULT_ROLE = 'ACCOUNTADMIN';
-- One bad query can drop production databases
```

**Fix:**
```sql
-- Use least-privilege roles
ALTER USER analyst SET DEFAULT_ROLE = 'DATA_ANALYST';

-- Audit ACCOUNTADMIN usage
SELECT grantee_name, role
FROM SNOWFLAKE.ACCOUNT_USAGE.GRANTS_TO_USERS
WHERE role = 'ACCOUNTADMIN' AND deleted_on IS NULL;
-- Should be < 3 users, all named admins
```

---

## Pitfall #3: SELECT * on Wide Tables

**Anti-Pattern:**
```sql
-- Scans ALL columns (Snowflake stores columnar — unused cols waste I/O)
SELECT * FROM events;  -- 200 columns, only need 3
```

**Fix:**
```sql
-- Select only needed columns — dramatically reduces bytes scanned
SELECT event_id, event_type, event_timestamp FROM events;

-- Check column pruning impact
SELECT bytes_scanned FROM TABLE(INFORMATION_SCHEMA.QUERY_HISTORY_BY_SESSION())
ORDER BY start_time DESC LIMIT 1;
```

---

## Pitfall #4: Clustering Keys on Small Tables

**Anti-Pattern:**
```sql
-- Clustering key on a 10,000 row table
ALTER TABLE config_settings CLUSTER BY (category);
-- Costs credits for reclustering with zero performance benefit
```

**Fix:**
```sql
-- Only cluster tables > 1TB with frequent filter queries
-- Check table size before clustering
SELECT table_name, row_count, bytes / 1e9 AS gb
FROM INFORMATION_SCHEMA.TABLES
WHERE table_name = 'CONFIG_SETTINGS';
-- If < 1 GB, clustering is waste

-- Remove unnecessary clustering
ALTER TABLE config_settings DROP CLUSTERING KEY;
```

---

## Pitfall #5: Not Using MERGE for Idempotent Loads

**Anti-Pattern:**
```sql
-- INSERT creates duplicates on retry
INSERT INTO dim_orders SELECT * FROM staging_orders;
-- Network blip → retry → duplicate rows
```

**Fix:**
```sql
-- MERGE is idempotent — safe to retry
MERGE INTO dim_orders AS target
USING staging_orders AS source
ON target.order_id = source.order_id
WHEN MATCHED THEN UPDATE SET
  target.amount = source.amount,
  target.updated_at = CURRENT_TIMESTAMP()
WHEN NOT MATCHED THEN INSERT
  (order_id, amount, created_at)
VALUES (source.order_id, source.amount, CURRENT_TIMESTAMP());
```

---

## Pitfall #6: Ignoring Stale Streams

**Anti-Pattern:**
```sql
-- Stream goes stale when retention period is exceeded
-- (source table changes exceed DATA_RETENTION_TIME_IN_DAYS)
-- Result: DATA LOSS — changes between old and new offset are gone
```

**Fix:**
```sql
-- Monitor stream staleness
SELECT stream_name, stale
FROM INFORMATION_SCHEMA.STREAMS
WHERE stale = TRUE;

-- Increase retention on source tables
ALTER TABLE raw_orders SET DATA_RETENTION_TIME_IN_DAYS = 14;

-- Set up alert for stale streams
CREATE ALERT stale_stream_alert
  WAREHOUSE = ADMIN_WH
  SCHEDULE = '30 MINUTE'
  IF (EXISTS (SELECT 1 FROM INFORMATION_SCHEMA.STREAMS WHERE stale = TRUE))
  THEN CALL SYSTEM$SEND_EMAIL(...);
```

---

## Pitfall #7: Loading Many Small Files

**Anti-Pattern:**
```bash
# 100,000 small files (< 100KB each) in stage
# Each file = separate micro-partition = metadata overhead
```

**Fix:**
```sql
-- Combine small files before loading
-- Or use Snowpipe with recommended file sizes (100-250 MB)

-- Check COPY history for file size issues
SELECT file_name, file_size, row_count
FROM TABLE(INFORMATION_SCHEMA.COPY_HISTORY(
  TABLE_NAME => 'MY_TABLE',
  START_TIME => DATEADD(hours, -24, CURRENT_TIMESTAMP())
))
WHERE file_size < 100000  -- Files under 100KB
ORDER BY file_size;
```

---

## Pitfall #8: No Resource Monitors

**Anti-Pattern:**
```sql
-- No resource monitors = unlimited credit consumption
-- A runaway query or always-on warehouse can burn thousands of credits
```

**Fix:**
```sql
CREATE RESOURCE MONITOR monthly_budget
  WITH CREDIT_QUOTA = 2000
  FREQUENCY = MONTHLY
  START_TIMESTAMP = IMMEDIATELY
  TRIGGERS
    ON 75 PERCENT DO NOTIFY
    ON 100 PERCENT DO SUSPEND
    ON 110 PERCENT DO SUSPEND_IMMEDIATE;

ALTER ACCOUNT SET RESOURCE_MONITOR = monthly_budget;
```

---

## Pitfall #9: Using Transient Tables for Important Data

**Anti-Pattern:**
```sql
-- Transient tables have NO Fail-safe (7 days of extra recovery)
-- and max 1 day of Time Travel
CREATE TRANSIENT TABLE critical_orders (...);
-- Data loss risk if table is accidentally dropped after 1 day
```

**Fix:**
```sql
-- Use permanent tables for important data
CREATE TABLE critical_orders (...);
ALTER TABLE critical_orders SET DATA_RETENTION_TIME_IN_DAYS = 14;

-- Use transient only for truly temporary data
CREATE TRANSIENT TABLE temp_staging_batch (...);
```

---

## Pitfall #10: Wrong Account Identifier Format

**Anti-Pattern:**
```typescript
// Using the full URL instead of account identifier
const conn = snowflake.createConnection({
  account: 'myaccount.us-east-1.snowflakecomputing.com',  // WRONG
});
// Results in: "Could not connect to Snowflake backend"
```

**Fix:**
```typescript
const conn = snowflake.createConnection({
  account: 'myorg-myaccount',  // Correct: orgname-accountname format
});
// For legacy locator format: 'xy12345.us-east-1' (include region)
```

## Quick Audit Script

```sql
-- Run this monthly to catch common pitfalls
SELECT 'Always-on warehouses' AS check,
       COUNT(*) AS issues
FROM INFORMATION_SCHEMA.WAREHOUSES
WHERE auto_suspend = 0 OR auto_suspend > 3600

UNION ALL

SELECT 'ACCOUNTADMIN default role',
       COUNT(*)
FROM SNOWFLAKE.ACCOUNT_USAGE.USERS
WHERE default_role = 'ACCOUNTADMIN' AND disabled = 'false'

UNION ALL

SELECT 'Stale streams',
       COUNT(*)
FROM INFORMATION_SCHEMA.STREAMS
WHERE stale = TRUE

UNION ALL

SELECT 'No resource monitor',
       CASE WHEN COUNT(*) = 0 THEN 1 ELSE 0 END
FROM INFORMATION_SCHEMA.RESOURCE_MONITORS

UNION ALL

SELECT 'Tables without clustering (>1TB)',
       COUNT(*)
FROM INFORMATION_SCHEMA.TABLES
WHERE bytes > 1e12
  AND auto_clustering_on = 'NO';
```

## Quick Reference Card

| Pitfall | Detection | Prevention |
|---------|-----------|------------|
| Always-on warehouse | `auto_suspend = 0` | Set 60-300s |
| ACCOUNTADMIN abuse | `GRANTS_TO_USERS` audit | Enforce least privilege |
| SELECT * | High `bytes_scanned` | Column pruning |
| Unnecessary clustering | Small table < 1TB | Only cluster large tables |
| INSERT duplicates | Row count mismatch | Use MERGE |
| Stale streams | `stale = TRUE` | Increase retention |
| Small files | COPY_HISTORY file_size | Batch files to 100-250MB |
| No resource monitor | Account check | Create immediately |
| Transient for critical data | Table type audit | Use permanent tables |
| Wrong account format | Connection error | Use `orgname-accountname` |

## Resources

- [Warehouse Considerations](https://docs.snowflake.com/en/user-guide/warehouses-considerations)
- [Access Control Best Practices](https://docs.snowflake.com/en/user-guide/security-access-control-considerations)
- [Data Loading Best Practices](https://docs.snowflake.com/en/user-guide/data-load-considerations-prepare)