debugging-dbt-errors

What this skill does

# dbt Troubleshooting

**Read the full error. Check upstream first. ALWAYS run `dbt build` after fixing.**

## Critical Rules

1. **ALWAYS run `dbt build` after fixing** - compile is NOT enough to verify the fix
2. **If fix fails 3+ times**, stop and reassess your entire approach
3. **Verify data after build** - build passing doesn't mean output is correct

## Workflow

### 1. Get the Full Error

```bash
dbt compile --select <model_name>
# or
dbt build --select <model_name>
```

Read the COMPLETE error message. Note the file, line number, and specific error.

### 2. Inspect Actual Data (For Data Issues)

**Before fixing "wrong output" or "incorrect results", query the actual data:**

```bash
# Preview current output
dbt show --select <model_name> --limit 20

# Check specific values with inline query
dbt show --inline "select * from {{ ref('model_name') }} where <condition>" --limit 10

# Compare with expected - look for patterns
dbt show --inline "select column, count(*) from {{ ref('model_name') }} group by 1 order by 2 desc" --limit 10
```

**Understand what's wrong before attempting to fix it.**

### 3. Read Compiled SQL

```bash
cat target/compiled/<project>/<path>/<model_name>.sql
```

See the actual SQL that will run.

### 4. Analyze Error Type

| Error Type | Look For |
|------------|----------|
| Compilation Error | Jinja syntax, missing refs, YAML issues |
| Database Error | Column not found, type mismatch, SQL syntax |
| Dependency Error | Missing model, circular reference |

### 5. Check Upstream Models

```bash
# Find what this model references
grep -E "ref\(|source\(" models/<path>/<model_name>.sql

# Read upstream model to verify columns
cat models/<path>/<upstream_model>.sql
```

Many errors come from upstream changes, not the current model.

### 6. Apply Fix

Common fixes:

| Error | Fix |
|-------|-----|
| Column not found | Check upstream model's output columns |
| Ambiguous column | Add table alias: `table.column` |
| Type mismatch | Add explicit `CAST()` |
| Division by zero | Use `NULLIF(divisor, 0)` |
| Jinja error | Check matching `{{ }}` and `{% %}` |

### 7. Rebuild (MANDATORY)

```bash
dbt build --select <model_name>
```

**3-Failure Rule**: If build fails 3+ times, STOP. Step back and:
1. Re-read the original error
2. Check if your entire approach is wrong
3. Consider alternative solutions

### 8. Verify Fix

```bash
# Preview the data
dbt show --select <model_name> --limit 10

# Run tests
dbt test --select <model_name>
```

### 9. Re-review Logic Against Requirements

**After fixing, re-read the original request and verify:**
- Does the output match what the user asked for?
- Are the column names exactly as requested?
- Is the calculation logic correct per the requirements?
- Did you solve the actual problem, not just make the error go away?

### 10. Check Downstream Impact

```bash
# Find downstream models
grep -r "ref('<model_name>')" models/ --include="*.sql"

# Rebuild downstream
dbt build --select <model_name>+
```

## Error Categories

### Compilation Errors
- Check Jinja syntax: matching `{{ }}` and `{% %}`
- Verify macro arguments
- Check YAML indentation

### Database Errors
- Read compiled SQL in `target/compiled/`
- Check column names against upstream
- Verify data types

### Test Failures
- Read the test SQL to understand what it checks
- Compare your model output to expected behavior
- Check column names, data types, NULL handling

## Anti-Patterns

- Making random changes without understanding the error
- Assuming the current model is wrong before checking upstream
- Not reading the FULL error message
- Declaring "fixed" without running build
- Getting stuck making small tweaks instead of reassessing