Index capacity and cost in DynamoDB - Step-by-Step Execution

Concept Flow - Index capacity and cost

Create Table

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Add Index (GSI or LSI)

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Write Operation

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Calculate Capacity Units

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Calculate Cost

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Billing Based on Usage

This flow shows how adding an index affects capacity units and cost in DynamoDB, starting from table creation to billing.

Execution Sample

DynamoDB

Create table with 5 WCU and 5 RCU
Add a Global Secondary Index (GSI) with 2 WCU and 2 RCU
Write 10 items to table (propagates to GSI)
Read 5 items from table
Read 5 items from GSI
Calculate total WCU and RCU consumed
Calculate cost based on capacity units

This example shows how writing data to a table with a GSI consumes capacity units on both table and GSI, and reads consume separately, affecting cost.

Execution Table

Step	Action	Table WCU Used	GSI WCU Used	Total WCU	Table RCU Used	GSI RCU Used	Total RCU	Cost Impact
1	Create table with 5 WCU and 5 RCU	0	0	0	0	0	0	No usage cost yet
2	Add GSI with 2 WCU and 2 RCU	0	0	0	0	0	0	No usage cost yet
3	Write 10 items to table (propagates to GSI)	10	10	20	0	0	0	Consumes 10 WCU on table + 10 WCU on GSI
4	Read 5 items from table	10	10	20	5	0	5	Consumes 5 RCU on table
5	Read 5 items from GSI	10	10	20	5	5	10	Consumes 5 RCU on GSI
6	Calculate total capacity	10	10	20	5	5	10	Total: 20 WCU and 10 RCU consumed
7	Billing	-	-	-	-	-	-	Charged for 20 WCU and 10 RCU usage (on-demand mode)

💡 Execution stops after billing calculation based on capacity units used.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Table WCU Used	0	10	10	10	10
GSI WCU Used	0	10	10	10	10
Total WCU	0	20	20	20	20
Table RCU Used	0	0	5	5	5
GSI RCU Used	0	0	0	5	5
Total RCU	0	0	5	10	10

Key Moments - 3 Insights

Why does writing to a table with GSI consume write capacity units on both?

Writes to the table automatically propagate to the GSI, which maintains its own copy of data, consuming WCU on table and GSI separately (see execution_table row 3).

Does reading from a GSI consume read capacity units from the main table?

No, reading from a GSI consumes RCU from the GSI's provisioned/on-demand capacity only (see execution_table row 5).

How is the total cost calculated when using indexes?

In on-demand mode, cost is based on the sum of capacity units actually consumed by the table and all indexes (see execution_table rows 6-7).

Visual Quiz - 3 Questions

Look at the execution_table, what is the total write capacity units used after step 3?

A 10

B 20

C 0

D 5

💡 Hint

Check the 'Total WCU' column at step 3 in the execution_table.

At which step does reading from the GSI first consume read capacity units?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the 'GSI RCU Used' column in the execution_table.

If you remove the GSI, how would the total write capacity units after step 3 change?

A It would stay 20

B It would become 0

C It would become 10

D It would become 5

💡 Hint

Without GSI, writes consume WCU only on the table (10 WCU for 10 items).

Concept Snapshot

DynamoDB indexes (GSI/LSI) have separate capacity units.
Writes to table automatically update GSIs, consuming WCU on table + each GSI.
Reads from table/GSI consume RCU from their respective capacities.
In on-demand mode, billed for consumed units across table + indexes.
Plan indexes carefully to balance query flexibility and cost.

Full Transcript

This visual execution demonstrates DynamoDB index capacity and cost. Create a table, add a GSI—each with their capacity settings. Writing items to the table consumes WCU on both table and GSI since GSIs store data copies. Reads consume RCU separately from table or GSI. Total consumed units sum table + index usage. In on-demand billing, pay for actual consumption. This helps optimize capacity planning and costs.