Scan with filter expressions in DynamoDB - Step-by-Step Execution

Concept Flow - Scan with filter expressions

Start Scan

↓

Read all items

↓

Apply Filter Expression

Yes No↓

Keep

↓

Return filtered items

↓

End

Scan reads all items, then filter expression keeps only matching items before returning.

Execution Sample

DynamoDB

Scan table 'Books' with filter: Author = 'Alice'
Return only items where Author is 'Alice'

This scan reads all books but returns only those written by Alice.

Execution Table

Step	Item Read	Filter Condition (Author='Alice')	Result	Action
1	{"Title":"Book1","Author":"Alice"}	True	Keep	Add to result
2	{"Title":"Book2","Author":"Bob"}	False	Discard	Skip
3	{"Title":"Book3","Author":"Alice"}	True	Keep	Add to result
4	{"Title":"Book4","Author":"Carol"}	False	Discard	Skip
5	{"Title":"Book5","Author":"Alice"}	True	Keep	Add to result
End	-	-	-	All items scanned, filtered results returned

💡 All items scanned; only items with Author='Alice' kept

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Item	None	{"Title":"Book1","Author":"Alice"}	{"Title":"Book2","Author":"Bob"}	{"Title":"Book3","Author":"Alice"}	{"Title":"Book4","Author":"Carol"}	{"Title":"Book5","Author":"Alice"}	None
Result Set	[]	[Book1]	[Book1]	[Book1, Book3]	[Book1, Book3]	[Book1, Book3, Book5]	[Book1, Book3, Book5]

Key Moments - 3 Insights

Why does the scan read all items even if we only want some?

Scan reads every item first because filter expressions are applied after reading all items, as shown in execution_table rows 1-5.

Does the filter expression reduce read capacity usage?

No, filter expressions do not reduce read capacity because all items are read first; filtering happens after, as seen in the 'Action' column.

What happens to items that do not match the filter?

Items not matching the filter are discarded and not added to the result set, as shown in rows 2 and 4 of execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A Item is kept because Author is Alice

B Item is discarded because Author is not Alice

C Scan stops at this step

D Filter expression is not applied

💡 Hint

Check the 'Filter Condition' and 'Result' columns at step 3 in execution_table

At which step does the filter expression first discard an item?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the first 'Discard' in the 'Result' column in execution_table

If the filter was changed to Author='Bob', how would the result set change after step 5?

A Result set would include Book1, Book3, and Book5

B Result set would include only Book2

C Result set would be empty

D Result set would include all books

💡 Hint

Refer to variable_tracker 'Result Set' and which items have Author='Bob'

Concept Snapshot

Scan reads all items in a table.
Filter expressions apply after reading.
Only items matching filter are returned.
Filter does not reduce read cost.
Useful for simple filtering without indexes.

Full Transcript

In DynamoDB, a scan operation reads every item in a table. After reading, it applies a filter expression to decide which items to keep. Items that match the filter are included in the results; others are discarded. This means the scan reads all data first, so filter expressions do not reduce the amount of data read or the read capacity used. For example, scanning a 'Books' table with filter Author='Alice' reads all books but returns only those by Alice. This process is shown step-by-step in the execution table and variable tracker, helping beginners see how each item is processed and filtered.