Global Secondary Index (GSI) concept in DynamoDB - Step-by-Step Execution

Concept Flow - Global Secondary Index (GSI) concept

Create Table with Primary Key

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Add Global Secondary Index (GSI)

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Write Data to Table

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Data Stored in Table and GSI

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Query Table using Primary Key

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Return Matching Items

This flow shows how a DynamoDB table with a Global Secondary Index stores data and allows queries on both the primary key and the GSI key.

Execution Sample

DynamoDB

Create Table Users (UserID PK)
Add GSI on Email (Email as GSI PK)
Insert Item {UserID: 1, Email: '[e-mail]'}
Query Table by UserID = 1
Query GSI by Email = '[e-mail]'

This example creates a table with a primary key, adds a GSI on Email, inserts a user, then queries by both keys.

Execution Table

Step	Action	Input	Data Stored in Table	Data Stored in GSI	Query Type	Query Input	Query Result
1	Create Table	Users with PK UserID	Empty	Empty	N/A	N/A	N/A
2	Add GSI	GSI on Email	Empty	Empty	N/A	N/A	N/A
3	Insert Item	{UserID:1, Email:'[e-mail]'}	{UserID:1, Email:'[e-mail]'}	{Email:'[e-mail]', UserID:1}	N/A	N/A	N/A
4	Query Table	UserID=1	{UserID:1, Email:'[e-mail]'}	{Email:'[e-mail]', UserID:1}	Primary Key	UserID=1	{UserID:1, Email:'[e-mail]'}
5	Query GSI	Email='[e-mail]'	{UserID:1, Email:'[e-mail]'}	{Email:'[e-mail]', UserID:1}	GSI Key	Email='[e-mail]'	{UserID:1, Email:'[e-mail]'}
6	Query Table	UserID=2	{UserID:1, Email:'[e-mail]'}	{Email:'[e-mail]', UserID:1}	Primary Key	UserID=2	No items found
7	Query GSI	Email='[e-mail]'	{UserID:1, Email:'[e-mail]'}	{Email:'[e-mail]', UserID:1}	GSI Key	Email='[e-mail]'	No items found

💡 Queries stop when no matching items are found or all steps complete.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Table Data	Empty	{UserID:1, Email:'[e-mail]'}	{UserID:1, Email:'[e-mail]'}	{UserID:1, Email:'[e-mail]'}	{UserID:1, Email:'[e-mail]'}
GSI Data	Empty	{Email:'[e-mail]', UserID:1}	{Email:'[e-mail]', UserID:1}	{Email:'[e-mail]', UserID:1}	{Email:'[e-mail]', UserID:1}
Query Result	N/A	N/A	{UserID:1, Email:'[e-mail]'}	{UserID:1, Email:'[e-mail]'}	No items found

Key Moments - 3 Insights

Why does the data appear in both the table and the GSI after insertion?

Because DynamoDB automatically copies the indexed attributes to the GSI, so queries on the GSI can find the data quickly. See execution_table rows 3 and 5.

Can I query the table using the GSI key directly?

No, you must query the GSI itself using its key. The table query only works with the primary key. See execution_table rows 4 and 5.

What happens if I query a key that does not exist in the table or GSI?

The query returns no items found, as shown in execution_table rows 6 and 7.

Visual Quiz - 3 Questions

Look at the execution_table, what data is stored in the GSI after inserting the item?

A {UserID:1, Email:'[e-mail]'}

B Empty

C {Email:'[e-mail]', UserID:1}

D No data stored

💡 Hint

Check the 'Data Stored in GSI' column at Step 3 in the execution_table.

At which step does querying the table by UserID return no items?

A Step 6

B Step 5

C Step 4

D Step 7

💡 Hint

Look at the 'Query Result' column for queries by UserID in execution_table.

If we add another item with Email '[e-mail]', how would the GSI data change after insertion?

A It would remain unchanged

B It would include {Email:'[e-mail]', UserID:2} alongside existing data

C It would replace existing GSI data

D It would delete the old data

💡 Hint

GSIs store indexed attributes for all items, so new items add to GSI data.

Concept Snapshot

Global Secondary Index (GSI) in DynamoDB:
- GSI is an alternate key for querying data.
- It has its own partition and sort keys.
- Data is copied automatically to GSI when inserted.
- Query GSI using its key, not the table's primary key.
- Useful for flexible queries without changing main table keys.

Full Transcript

This visual execution shows how a DynamoDB table with a Global Secondary Index (GSI) works. First, a table is created with a primary key. Then a GSI is added on a different attribute. When an item is inserted, its data is stored in the table and also copied to the GSI. Queries can be made using the table's primary key or the GSI key. Queries return matching items or no items if none match. This helps understand how GSIs enable querying data by alternate keys efficiently.