Streams for change data capture in Snowflake - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Stream in Snowflake?

A Stream in Snowflake is a database object that tracks changes (inserts, updates, deletes) made to a table, allowing you to capture and process these changes incrementally.

beginner

How does a Snowflake Stream help with Change Data Capture (CDC)?

It records the changes made to a table since the last time the stream was read, enabling efficient incremental data processing without scanning the entire table.

beginner

What types of changes does a Snowflake Stream capture?

It captures INSERT, UPDATE, and DELETE operations performed on the source table.

intermediate

What happens to the data in a Snowflake Stream after it is consumed?

After the stream data is consumed (read), the stream marks those changes as processed, so the next read only returns new changes.

intermediate

Can a Snowflake Stream be used on a table with no primary key?

Yes, but without a primary key, it may be harder to uniquely identify rows for updates or deletes, so best practice is to have a primary key for accurate CDC.

What does a Snowflake Stream track?

A User login history

B Query execution plans

C Table schema changes

D Changes made to a table since last read

Streams track data changes (inserts, updates, deletes) made to a table since the last time the stream was read.

Which operations are captured by a Snowflake Stream?

A Only INSERTs

B Only DELETEs

C INSERT, UPDATE, DELETE

D Only UPDATEs

Snowflake Streams capture INSERT, UPDATE, and DELETE operations on the source table.

What happens when you read data from a Snowflake Stream?

A The stream resets and loses all data

B The changes are marked as consumed and won't appear again

C The source table is deleted

D The stream duplicates the data

Reading from a stream marks the changes as consumed, so the next read only returns new changes.

Why is having a primary key recommended when using Streams for CDC?

A To uniquely identify rows for accurate change tracking

B To speed up queries unrelated to CDC

C To prevent data from being deleted

D To encrypt the data

A primary key helps uniquely identify rows, which is important for tracking updates and deletes accurately.

Can a Snowflake Stream be created on a view?

A No, streams only work on tables

B Only on external tables

C Only on materialized views

D Yes, streams work on views

Streams can only be created on tables, not on views.

Explain how Snowflake Streams enable change data capture and why they are useful.

Think about how you would keep track of changes in a spreadsheet without copying the whole sheet every time.

Describe the lifecycle of data in a Snowflake Stream from creation to consumption.

Imagine a mailbox that collects letters until you check it, then it empties for new letters.

Practice

1. What is the main purpose of a stream in Snowflake for change data capture?

easy

A. To automatically delete old data from a table

B. To create a full backup of the table data every hour

C. To track only the changes (inserts, updates, deletes) on a table without copying all data

D. To replicate data to another database in real-time

Solution

1. Step 1: Understand the role of streams

   Streams in Snowflake track changes like inserts, updates, and deletes on a table without copying the entire data.

2. Step 2: Compare options with stream purpose

   Options A, B, and D describe deletion, backup, and replication, which are not the main function of streams.

3. Final Answer:

   To track only the changes (inserts, updates, deletes) on a table without copying all data -> Option C

4. Quick Check:

   Streams track changes = C [OK]

Hint: Streams track only changes, not full data copies [OK]

Common Mistakes:

• Thinking streams copy full table data
• Confusing streams with backups
• Assuming streams delete data automatically

2. Which of the following is the correct syntax to create a stream on a table named orders in Snowflake?

easy

A. CREATE STREAM orders_stream ON TABLE orders;

B. CREATE STREAM orders_stream FOR TABLE orders;

C. CREATE STREAM orders_stream ON orders;

D. CREATE STREAM orders_stream FROM orders;

Solution

1. Step 1: Recall Snowflake stream creation syntax

   The correct syntax uses CREATE STREAM <name> ON TABLE <table_name>.

2. Step 2: Match options with syntax

   CREATE STREAM orders_stream ON TABLE orders; matches the correct syntax exactly. Options B, C, and D use incorrect keywords or omit TABLE.

3. Final Answer:

   CREATE STREAM orders_stream ON TABLE orders; -> Option A

4. Quick Check:

   CREATE STREAM ... ON TABLE ... = A [OK]

Hint: Use ON TABLE keyword when creating streams [OK]

Common Mistakes:

• Omitting ON TABLE keywords
• Using FOR TABLE instead of ON TABLE
• Using FROM instead of ON TABLE

3. Given a stream orders_stream on table orders, what will the following query return?

SELECT * FROM orders_stream;

Assume some rows were inserted and updated since the last query.

medium

A. Only the rows that were inserted or updated since the last query

B. An error because streams cannot be queried directly

C. Only the rows that were deleted from the orders table

D. All rows from the orders table

Solution

1. Step 1: Understand what querying a stream returns

   Querying a stream returns the changed rows (inserts, updates, deletes) since the last time it was queried or consumed.

2. Step 2: Analyze the query behavior

   The query selects all changes from orders_stream, so it returns only inserted or updated rows since last read, not all rows or errors.

3. Final Answer:

   Only the rows that were inserted or updated since the last query -> Option A

4. Quick Check:

   Streams return changed rows = D [OK]

Hint: Streams return only changed rows since last read [OK]

Common Mistakes:

• Thinking streams return full table data
• Assuming streams return deleted rows only
• Believing streams cannot be queried

4. You created a stream on a table but notice it does not capture existing rows as inserts initially. What is the likely cause?

medium

A. The stream was created with the wrong table name

B. Streams only capture changes made to the table after the stream was created

C. Streams do not capture inserts, only updates and deletes

D. The table is empty, so no rows to capture

Solution

1. Step 1: Recall stream behavior for existing rows

   Streams in Snowflake track only DML changes made after the stream is created. Existing rows are not captured as inserts.

2. Step 2: Evaluate other options

   The stream was created with the wrong table name would cause errors, not missing initial rows. Streams do not capture inserts, only updates and deletes is false; streams capture inserts too. The table is empty, so no rows to capture is possible but less likely if table has data.

3. Final Answer:

   Streams only capture changes made to the table after the stream was created -> Option B

4. Quick Check:

   Streams capture post-creation changes only [OK]

Hint: Streams track post-creation changes only [OK]

Common Mistakes:

• Not realizing streams only track post-creation changes
• Assuming streams capture only updates and deletes
• Confusing stream creation errors with missing data

5. You want to process only new and changed rows from a large sales table daily. Which approach using streams is best to minimize resource use and ensure no data is missed?

hard

A. Create a stream and query the full sales table daily

B. Create multiple streams on sales for inserts, updates, and deletes separately

C. Create a stream and never consume it, just query the sales table directly

D. Create a stream on sales and consume the stream daily

Solution

1. Step 1: Understand efficient change capture

   Create a stream to track changes since creation, then consume it daily to process only new changes efficiently.

2. Step 2: Evaluate resource efficiency and correctness

   Consuming the stream daily processes only changes, saving resources. A queries full table daily, wasting resources. C never consumes but queries full table, missing stream benefits. D is unnecessary complexity.

3. Final Answer:

   Create a stream on sales and consume the stream daily -> Option D

4. Quick Check:

   Stream + daily consume = efficient change capture [OK]

Hint: Create stream and consume daily for efficiency [OK]

Common Mistakes:

• Querying full table instead of consuming stream
• Not consuming the stream regularly
• Creating multiple unnecessary streams