Incremental strategies (append, merge, delete+insert) in dbt

Introduction

Incremental strategies help update data efficiently by adding or changing only new or changed parts instead of reloading everything.

When you have a large dataset that updates regularly and want to save time by processing only new data.

When you want to keep your data warehouse fresh without reloading all data every time.

When you need to combine new data with existing data without losing old records.

When you want to fix or update some records in your data without deleting everything.

When you want to improve performance by avoiding full table reloads.

Syntax

dbt

incremental_strategy: append | merge | delete+insert

append adds only new rows to the existing table.

merge updates existing rows and inserts new rows based on a key.

delete+insert deletes matching rows and inserts fresh data.

Examples

This adds only new rows to the table without changing existing rows.

dbt

incremental_strategy: append

This updates rows where id matches and inserts new rows if id is new.

dbt

incremental_strategy: merge
unique_key: id

This deletes rows matching id in the new data, then inserts all new rows.

dbt

incremental_strategy: delete+insert
unique_key: id

Sample Program

This dbt model uses the merge strategy to update existing customers and add new ones based on customer_id. It selects only customers updated after the last run.

dbt

version: 2
models:
  - name: customers_incremental
    config:
      materialized: incremental
      incremental_strategy: merge
      unique_key: customer_id

-- SQL to update or insert customers
select
  customer_id,
  customer_name,
  updated_at
from source.customers
where updated_at > (select coalesce(max(updated_at), '1900-01-01') from {{ this }})

Important Notes

Use append when you only add new data and never update old rows.

merge requires a unique key to match rows for update or insert.

delete+insert can be slower but is useful when updates are complex.

Summary

Incremental strategies help update data efficiently without full reloads.

append adds new rows, merge updates and adds rows, delete+insert replaces matching rows.

Choose the strategy based on your data update needs and performance.

Practice

1. Which incremental strategy in dbt only adds new rows without modifying existing ones?

easy

A. delete+insert

B. merge

C. append

D. truncate

Solution

1. Step 1: Understand the append strategy

   The append strategy adds only new rows to the existing table without changing any existing data.

2. Step 2: Compare with other strategies

   Merge updates existing rows and adds new ones; delete+insert replaces matching rows; truncate removes all data first.

3. Final Answer:

   append -> Option C

4. Quick Check:

   append = add new rows only [OK]

Hint: Append means add new rows only, no updates [OK]

Common Mistakes:

• Confusing merge with append
• Thinking delete+insert only adds rows
• Assuming truncate is an incremental strategy

2. Which of the following is the correct syntax to define an incremental model using the merge strategy in dbt?

easy

A. config(materialized='incremental', incremental_strategy='append')

B. config(materialized='incremental', strategy='merge')

C. config(materialized='table', incremental_strategy='merge')

D. config(materialized='incremental', incremental_strategy='merge')

Solution

1. Step 1: Identify correct config keys for incremental merge

   dbt requires materialized='incremental' and incremental_strategy='merge' to use merge strategy.

2. Step 2: Check each option for correctness

   config(materialized='incremental', incremental_strategy='append') uses 'append' strategy, not merge; config(materialized='table', incremental_strategy='merge') uses materialized='table' which is not incremental; config(materialized='incremental', strategy='merge') uses wrong key 'strategy'.

3. Final Answer:

   config(materialized='incremental', incremental_strategy='merge') -> Option D

4. Quick Check:

   incremental_strategy='merge' with materialized='incremental' [OK]

Hint: Use incremental_strategy='merge' inside config() for merge [OK]

Common Mistakes:

• Using wrong materialized type
• Misspelling incremental_strategy key
• Confusing strategy key name

3. Given this dbt incremental model snippet using merge strategy:

select id, value from source_table where updated_at > (select max(updated_at) from {{ this }})

What will happen when this model runs?

medium

A. Only new rows with updated_at greater than max in target are added

B. Rows with matching id are updated or inserted based on updated_at filter

C. All rows from source_table replace the target table

D. No rows are added or updated because filter excludes all

Solution

1. Step 1: Understand the filter condition

   The query selects rows from source_table where updated_at is greater than the max updated_at in the target table ({{ this }}).

2. Step 2: Understand merge behavior

   Merge updates existing rows with matching keys and inserts new rows. The filter ensures only changed rows are processed.

3. Final Answer:

   Rows with matching id are updated or inserted based on updated_at filter -> Option B

4. Quick Check:

   merge updates/inserts filtered rows [OK]

Hint: Merge updates or inserts rows filtered by updated_at [OK]

Common Mistakes:

• Thinking it replaces entire table
• Assuming only new rows are appended
• Ignoring the filter condition effect

4. You wrote this incremental model using delete+insert strategy:

config(materialized='incremental', incremental_strategy='delete+insert')

select * from source_table

But the model fails with an error about missing unique key. What is the likely cause?

medium

A. You did not specify the unique_key config required for delete+insert

B. The source_table is empty

C. delete+insert strategy does not support incremental models

D. You must use append strategy instead

Solution

1. Step 1: Recall delete+insert requirements

   Delete+insert requires a unique_key config to identify rows to delete before insert.

2. Step 2: Analyze error cause

   Missing unique_key causes dbt to error because it cannot identify which rows to delete.

3. Final Answer:

   You did not specify the unique_key config required for delete+insert -> Option A

4. Quick Check:

   delete+insert needs unique_key [OK]

Hint: Always set unique_key for delete+insert strategy [OK]

Common Mistakes:

• Ignoring unique_key requirement
• Assuming delete+insert works without keys
• Confusing append with delete+insert

5. You have a large sales table that updates daily with new sales and corrections to existing sales records. Which incremental strategy should you choose in dbt to efficiently update the table without full reloads?

hard

A. merge, because it updates existing rows and adds new ones

B. delete+insert, because it deletes all rows and inserts fresh data

C. append, because it only adds new rows

D. full refresh, because incremental strategies are not suitable

Solution

1. Step 1: Analyze data update needs

   The table has new sales and corrections, so existing rows need updates and new rows need inserts.

2. Step 2: Match strategy to needs

   Append only adds new rows, so it won't update corrections. Delete+insert replaces matching rows but can be costly. Merge efficiently updates and inserts rows incrementally.

3. Final Answer:

   merge, because it updates existing rows and adds new ones -> Option A

4. Quick Check:

   merge = update + insert for corrections and new data [OK]

Hint: Use merge for updates plus new rows efficiently [OK]

Common Mistakes:

• Choosing append and missing updates
• Thinking delete+insert always best
• Ignoring performance of full refresh