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.