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DynamoDBquery~5 mins

Creating GSI in DynamoDB - Performance & Efficiency

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Time Complexity: Creating GSI
O(n)
Understanding Time Complexity

When we create a Global Secondary Index (GSI) in DynamoDB, we want to know how the time to build and use it changes as data grows.

We ask: How does the work increase when the table gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following DynamoDB GSI creation snippet.


    UpdateTable {
      TableName: "Orders",
      AttributeDefinitions: [
        { AttributeName: "OrderId", AttributeType: "S" },
        { AttributeName: "CustomerId", AttributeType: "S" }
      ],
      GlobalSecondaryIndexes: [{
        IndexName: "CustomerIndex",
        KeySchema: [{ AttributeName: "CustomerId", KeyType: "HASH" }],
        Projection: { ProjectionType: "ALL" }
      }]
    }

This code adds a GSI on CustomerId to the Orders table to allow fast queries by customer.

Identify Repeating Operations

Look for repeated work during GSI creation and usage.

  • Primary operation: Scanning or writing each item to the GSI.
  • How many times: Once per item in the main table, as each item is copied or indexed.
How Execution Grows With Input

As the number of items grows, the work to build and maintain the GSI grows too.

Input Size (n)Approx. Operations
1010 writes to GSI
100100 writes to GSI
10001000 writes to GSI

Pattern observation: The work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to create and update the GSI grows linearly with the number of items.

Common Mistake

[X] Wrong: "Creating a GSI is instant and does not depend on table size."

[OK] Correct: Each item must be processed to build the GSI, so more items mean more work and time.

Interview Connect

Understanding how GSI creation scales helps you explain database design choices clearly and confidently.

Self-Check

"What if we added multiple GSIs? How would the time complexity change?"

Practice

(1/5)
1. What is the main purpose of creating a Global Secondary Index (GSI) in DynamoDB?
easy
A. To automatically backup the table data
B. To increase the storage capacity of the table
C. To allow querying the table using alternative key attributes
D. To encrypt the table data at rest

Solution

  1. Step 1: Understand what a GSI does

    A GSI lets you query a DynamoDB table using different key attributes than the primary key.

  2. Step 2: Compare options to GSI purpose

    Options A, B, and D describe backup, storage, and encryption, which are unrelated to GSI functionality.

  3. Final Answer:

    To allow querying the table using alternative key attributes -> Option C

  4. Quick Check:

    GSI = alternative query keys [OK]
Hint: GSIs let you query by new keys without changing the table [OK]
Common Mistakes:
  • Thinking GSI increases storage size
  • Confusing GSI with backup or encryption features
  • Assuming GSI changes the primary key
2. Which of the following is the correct syntax snippet to define a GSI named OrderDateIndex with OrderDate as the partition key in a DynamoDB table creation JSON?
easy
A. "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "HASH"}], "Projection": {"ProjectionType": "ALL"}}]
B. "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}]
C. "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "PRIMARY"}], "Projection": {"ProjectionType": "ALL"}}]
D. "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "PARTITION"}], "Projection": {"ProjectionType": "ALL"}}]

Solution

  1. Step 1: Identify correct KeyType for partition key in GSI

    The partition key in DynamoDB GSI uses KeyType "HASH".

  2. Step 2: Check options for correct KeyType and syntax

    "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "HASH"}], "Projection": {"ProjectionType": "ALL"}}] uses "HASH" correctly; B uses "RANGE" (wrong for partition key), C uses invalid "PRIMARY", D uses invalid "PARTITION".

  3. Final Answer:

    "GlobalSecondaryIndexes": [{"IndexName": "OrderDateIndex", "KeySchema": [{"AttributeName": "OrderDate", "KeyType": "HASH"}], "Projection": {"ProjectionType": "ALL"}}] -> Option A

  4. Quick Check:

    Partition key = HASH [OK]
Hint: Partition key in GSI always uses KeyType 'HASH' [OK]
Common Mistakes:
  • Using 'RANGE' instead of 'HASH' for partition key
  • Using invalid KeyType values like 'PRIMARY' or 'PARTITION'
  • Forgetting to define ProjectionType
3. Given a DynamoDB table with a GSI named UserEmailIndex where Email is the partition key, what will be the result of this query?
QueryInput = {
  TableName: "Users",
  IndexName: "UserEmailIndex",
  KeyConditionExpression: "Email = :email",
  ExpressionAttributeValues: {":email": "alice@example.com"}
}
medium
A. Returns all items where Email attribute equals 'alice@example.com' using the GSI
B. Returns all items where Email attribute equals 'alice@example.com' using the main table primary key
C. Returns an error because IndexName cannot be used in QueryInput
D. Returns all items regardless of Email attribute

Solution

  1. Step 1: Understand QueryInput with IndexName

    Specifying IndexName in QueryInput tells DynamoDB to query the GSI instead of the main table.

  2. Step 2: Analyze KeyConditionExpression

    The query filters items where Email equals 'alice@example.com' on the GSI partition key.

  3. Final Answer:

    Returns all items where Email attribute equals 'alice@example.com' using the GSI -> Option A

  4. Quick Check:

    Query with IndexName uses GSI keys [OK]
Hint: Use IndexName in query to search by GSI keys [OK]
Common Mistakes:
  • Assuming query uses main table keys when IndexName is set
  • Thinking IndexName causes error in QueryInput
  • Ignoring KeyConditionExpression filtering
4. You tried to create a GSI with this snippet but got an error:
"GlobalSecondaryIndexes": [{
  "IndexName": "StatusIndex",
  "KeySchema": [{"AttributeName": "Status", "KeyType": "HASH"}],
  "Projection": {"ProjectionType": "INCLUDE", "NonKeyAttributes": []}
}]

What is the likely cause of the error?
medium
A. IndexName cannot contain the word 'Index'
B. NonKeyAttributes must be a non-empty array when ProjectionType is INCLUDE
C. KeyType must be RANGE for GSIs
D. ProjectionType cannot be INCLUDE for GSIs

Solution

  1. Step 1: Check ProjectionType and NonKeyAttributes

    When ProjectionType is INCLUDE, NonKeyAttributes must be a non-empty array listing attributes to project.

  2. Step 2: Verify given snippet

    NonKeyAttributes is [], which is empty and invalid when ProjectionType is INCLUDE.

  3. Step 3: Eliminate other options

    ProjectionType INCLUDE is valid; KeyType HASH is valid for partition key; IndexName can contain 'Index'.

  4. Final Answer:

    NonKeyAttributes must be a non-empty array when ProjectionType is INCLUDE -> Option B

  5. Quick Check:

    INCLUDE projection requires NonKeyAttributes [OK]
Hint: INCLUDE projection needs NonKeyAttributes array [OK]
Common Mistakes:
  • Using empty or missing NonKeyAttributes with INCLUDE
  • Confusing KeyType HASH and RANGE for partition key
  • Thinking IndexName naming causes errors
5. You have a DynamoDB table with primary key UserID. You want to query users by their City and SignupDate. Which GSI definition allows efficient queries for all users in a city sorted by signup date?
hard
A. "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "SignupDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}]
B. "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "SignupDate", "KeyType": "HASH"}, {"AttributeName": "City", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}]
C. "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "City", "KeyType": "RANGE"}, {"AttributeName": "SignupDate", "KeyType": "HASH"}], "Projection": {"ProjectionType": "ALL"}}]
D. "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "City", "KeyType": "HASH"}, {"AttributeName": "SignupDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}]

Solution

  1. Step 1: Identify partition and sort keys for query pattern

    To query all users in a city sorted by signup date, City should be the partition key (HASH) and SignupDate the sort key (RANGE).

  2. Step 2: Check options for correct KeySchema order

    "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "City", "KeyType": "HASH"}, {"AttributeName": "SignupDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}] correctly sets City as HASH and SignupDate as RANGE. Options B and C swap keys incorrectly. "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "SignupDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}] lacks partition key.

  3. Final Answer:

    "GlobalSecondaryIndexes": [{"IndexName": "CitySignupIndex", "KeySchema": [{"AttributeName": "City", "KeyType": "HASH"}, {"AttributeName": "SignupDate", "KeyType": "RANGE"}], "Projection": {"ProjectionType": "ALL"}}] -> Option D

  4. Quick Check:

    Partition key = City, Sort key = SignupDate [OK]
Hint: Partition key first (HASH), sort key second (RANGE) in KeySchema [OK]
Common Mistakes:
  • Swapping HASH and RANGE keys
  • Missing partition key in GSI
  • Using only one key when two are needed for sorting
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