Parallel scan in DynamoDB - Step-by-Step Execution

Concept Flow - Parallel scan

Start Scan Request

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Divide Table into Segments

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Each Segment Scanned in Parallel

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Collect Results from All Segments

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Combine Results

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Return Full Scan Output

Parallel scan splits the table into segments and scans each segment at the same time, then combines all results.

Execution Sample

DynamoDB

Scan with Segment=0, TotalSegments=2
Scan with Segment=1, TotalSegments=2

This example scans a table in 2 parallel segments, each scanning half the table.

Execution Table

Step	Segment	Scan Range	Items Scanned	Items Returned	Action
1	0	First half of table	50	45	Scan segment 0 starts
2	1	Second half of table	50	48	Scan segment 1 starts
3	0	First half of table	50	45	Segment 0 scan completes
4	1	Second half of table	50	48	Segment 1 scan completes
5	-	-	100	93	Combine results from both segments
6	-	-	-	-	Return combined scan results

💡 All segments scanned and results combined, scan completes.

Variable Tracker

Variable	Start	After Segment 0	After Segment 1	Final
ItemsScanned	0	50	100	100
ItemsReturned	0	45	93	93
SegmentsCompleted	0	1	2	2

Key Moments - 2 Insights

Why do we need to specify Segment and TotalSegments in a parallel scan?

Each scan worker must know which part of the table to scan (Segment) and how many total parts exist (TotalSegments) to avoid overlapping scans, as shown in execution_table rows 1 and 2.

Does parallel scan return results in order?

No, each segment scans independently and results are combined after all segments finish, so order is not guaranteed. See execution_table rows 3-6.

Visual Quiz - 3 Questions

Look at the execution_table, how many items were scanned in total after both segments finished?

A 93

B 100

C 50

D 45

💡 Hint

Check the ItemsScanned column in rows 3 and 4 and the combined total in row 5.

At which step do both segments complete their scans?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the Action column for segment scan completions in rows 3 and 4.

If TotalSegments was increased to 4, how would the execution_table change?

A There would be 4 segment scan rows instead of 2

B ItemsScanned would double

C SegmentsCompleted would stay at 2

D Scan results would be combined earlier

💡 Hint

Parallel scan divides the table into TotalSegments parts, so more segments mean more scan rows.

Concept Snapshot

Parallel scan splits a DynamoDB table into multiple segments.
Each segment is scanned independently and simultaneously.
Specify Segment and TotalSegments to control scanning.
Results from all segments are combined after all finish.
Improves scan speed by using parallel workers.

Full Transcript

Parallel scan in DynamoDB divides the table into segments. Each segment is scanned in parallel by separate workers. You specify which segment to scan and how many total segments exist. Each segment scans only its part of the table. After all segments finish scanning, their results are combined and returned as one complete scan output. This method speeds up scanning large tables by using multiple workers at once. The execution table shows two segments scanning halves of the table, then combining results. Variables track items scanned and returned per segment and overall. Key points include specifying segments correctly and understanding that results are combined after all segments finish. The visual quiz tests understanding of total items scanned, completion steps, and effect of changing total segments.