Query by partition key in DynamoDB - Step-by-Step Execution

Concept Flow - Query by partition key

Start Query

↓

Specify Partition Key

↓

Send Query to DynamoDB

↓

DynamoDB Locates Partition

↓

Retrieve Items with Matching Key

↓

Return Results to User

↓

End

The query starts by specifying the partition key, then DynamoDB finds the matching partition and returns all items with that key.

Execution Sample

DynamoDB

Query(
  TableName='Users',
  KeyConditionExpression='UserID = :uid',
  ExpressionAttributeValues={':uid': {'S': 'user123'}}
)

This query fetches all items from the 'Users' table where the partition key 'UserID' equals 'user123'.

Execution Table

Step	Action	Input	DynamoDB Internal Process	Output
1	Start Query	UserID = 'user123'	Parse KeyConditionExpression	Ready to locate partition
2	Locate Partition	Partition key = 'user123'	Find partition for 'user123'	Partition found
3	Retrieve Items	Partition 'user123'	Query partition for matching items	Items with UserID='user123' found
4	Return Results	Matching items	Send items back to client	List of items with UserID='user123'
5	End	All items returned	Query complete	Query finished successfully

💡 Query ends after all items with the specified partition key are retrieved and returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
PartitionKey	undefined	'user123'	'user123'	'user123'	'user123'
ItemsFound	empty	empty	empty	list of matching items	list of matching items

Key Moments - 2 Insights

Why does the query only need the partition key and not the sort key?

Because DynamoDB organizes data by partition key, it can quickly find all items with that key without needing the sort key. This is shown in execution_table step 2 where only the partition key is used to locate the partition.

What happens if no items match the partition key?

The query returns an empty list of items. This would be reflected in execution_table step 3 where the query finds no matching items, so the output is an empty list.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after Step 3?

A Items with UserID='user123' found

B List of items with UserID='user123'

C Partition found

D Query finished successfully

💡 Hint

Check the 'Output' column for Step 3 in the execution_table.

At which step does DynamoDB locate the partition for the given key?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' and 'DynamoDB Internal Process' columns in the execution_table.

If the partition key was changed to 'user456', how would the variable 'PartitionKey' change after Step 1?

A 'user123'

B 'user456'

C empty

D undefined

💡 Hint

Refer to the variable_tracker for 'PartitionKey' values after Step 1.

Concept Snapshot

Query by partition key in DynamoDB:
- Use KeyConditionExpression with partition key only
- DynamoDB finds partition matching the key
- Returns all items with that partition key
- Fast and efficient retrieval
- Sort key optional for more filtering

Full Transcript

This visual execution shows how a DynamoDB query works when you specify only the partition key. First, the query starts and parses the key condition expression. Then DynamoDB locates the partition that matches the given partition key value. Next, it retrieves all items in that partition with the matching key. Finally, it returns those items to the user and ends the query. Variables like PartitionKey hold the key value throughout, and ItemsFound changes from empty to the list of matching items. Key moments include understanding why only the partition key is needed and what happens if no items match. The quiz questions help reinforce these steps by asking about outputs and variable changes at specific steps.

Practice

1. What does querying by partition key in DynamoDB do?

easy

A. Updates all items with the same partition key value

B. Deletes all items with the same partition key value

C. Fetches all items with the same partition key value

D. Creates a new item with the partition key value

Solution

1. Step 1: Understand partition key role

   The partition key uniquely identifies the partition where items are stored.

2. Step 2: Query by partition key fetches items

   Querying by partition key returns all items that share that key value.

3. Final Answer:

   Fetches all items with the same partition key value -> Option C

4. Quick Check:

   Query by partition key = fetch items [OK]

Hint: Partition key query fetches matching items only [OK]

Common Mistakes:

• Thinking it deletes or updates items
• Confusing partition key with sort key
• Assuming it creates new items

2. Which of the following is the correct syntax to query items by partition key 'UserID' with value '123' in DynamoDB?

easy

A. Table.query(KeyConditionExpression: 'UserID = 123')

B. Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'})

C. Table.query(FilterExpression: 'UserID = 123')

D. Table.query(KeyConditionExpression: 'UserID == 123')

Solution

1. Step 1: Identify correct query syntax

   DynamoDB requires KeyConditionExpression with placeholders and ExpressionAttributeValues.

2. Step 2: Check each option

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) uses correct syntax with placeholder ':val' and value mapping. Others misuse operators or omit placeholders.

3. Final Answer:

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) -> Option B

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Use placeholders (:val) and ExpressionAttributeValues [OK]

Common Mistakes:

• Using '==' instead of '=' in KeyConditionExpression
• Not using ExpressionAttributeValues for values
• Using FilterExpression instead of KeyConditionExpression

3. Given a DynamoDB table with items: {UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}, {UserID: '456', Name: 'Carol'}, what will the query by partition key 'UserID' = '123' return?

medium

A. [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}]

B. [{UserID: '456', Name: 'Carol'}]

C. [{UserID: '123', Name: 'Alice'}]

D. [] (empty list)

Solution

1. Step 1: Identify items with partition key '123'

   Items with UserID '123' are Alice and Bob.

2. Step 2: Query returns all matching items

   Query returns both Alice and Bob items as they share the partition key.

3. Final Answer:

   [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}] -> Option A

4. Quick Check:

   Query by '123' returns Alice and Bob [OK]

Hint: Query returns all items with matching partition key [OK]

Common Mistakes:

• Expecting only one item returned
• Confusing partition key with sort key filtering
• Assuming query returns items with different partition keys

4. You wrote this query to get items with partition key 'OrderID' = '789': Table.query(KeyConditionExpression: 'OrderID = 789'). It returns an error. What is the problem?

medium

A. Partition key name is case-sensitive and should be 'orderid'

B. Using wrong operator '==' instead of '='

C. Query method does not support KeyConditionExpression

D. Missing ExpressionAttributeValues for the value '789'

Solution

1. Step 1: Check query syntax requirements

   DynamoDB requires placeholders and ExpressionAttributeValues for values in KeyConditionExpression.

2. Step 2: Identify missing part

   The query uses 'OrderID = 789' directly without placeholder or ExpressionAttributeValues, causing error.

3. Final Answer:

   Missing ExpressionAttributeValues for the value '789' -> Option D

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Always use placeholders and ExpressionAttributeValues [OK]

Common Mistakes:

• Using raw values instead of placeholders
• Assuming case-insensitivity for partition key
• Using '==' operator instead of '='

5. You want to query a DynamoDB table for all items with partition key 'Category' = 'Books' and sort key 'Price' less than 20. Which approach correctly uses query by partition key with a condition on sort key?

hard

A. Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

B. Use FilterExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

C. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

D. Use FilterExpression: 'Category = :cat' and KeyConditionExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

Solution

1. Step 1: Understand query conditions

   KeyConditionExpression must include partition key equality and supports sort key conditions like <. FilterExpression filters results after the query.

2. Step 2: Analyze options

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} correctly uses KeyConditionExpression: 'Category = :cat AND Price < :price', which efficiently queries the partition with a sort key range condition. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} uses FilterExpression for Price (valid but less efficient). B lacks KeyConditionExpression. D omits partition key from KeyConditionExpression.

3. Final Answer:

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} -> Option A

4. Quick Check:

   KeyConditionExpression: partition = AND sort < [OK]

Hint: KeyConditionExpression: partition = AND sort condition [OK]

Common Mistakes:

• Using FilterExpression for partition key
• Using FilterExpression instead of KeyConditionExpression for sort key range
• Mixing KeyConditionExpression and FilterExpression incorrectly