Filter expressions in DynamoDB - Step-by-Step Execution

Concept Flow - Filter expressions

Start Scan or Query

↓

Apply Filter Expression

↓

Include Item

↓

Return Filtered Results

↓

End

The database scans or queries items, applies the filter expression to each, includes items that match, excludes others, then returns the filtered results.

Execution Sample

DynamoDB

Scan table 'Books' with FilterExpression: Author = 'Alice'

This scan returns only items where the Author attribute equals 'Alice'.

Execution Table

Step	Item	Author Attribute	Filter Condition (Author = 'Alice')	Include in Result?
1	{BookID: 1, Author: 'Alice', Title: 'A1'}	Alice	True	Yes
2	{BookID: 2, Author: 'Bob', Title: 'B1'}	Bob	False	No
3	{BookID: 3, Author: 'Alice', Title: 'A2'}	Alice	True	Yes
4	{BookID: 4, Author: 'Carol', Title: 'C1'}	Carol	False	No
5	{BookID: 5, Author: 'Alice', Title: 'A3'}	Alice	True	Yes
6	No more items	-	-	-

💡 All items scanned; only those with Author = 'Alice' included in results.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Item	None	{BookID:1,...}	{BookID:2,...}	{BookID:3,...}	{BookID:4,...}	{BookID:5,...}	None
Include in Result	N/A	Yes	No	Yes	No	Yes	N/A

Key Moments - 2 Insights

Why are some items scanned but not included in the results?

Because the filter expression only includes items where Author = 'Alice'. Items with other authors fail the condition and are excluded, as shown in steps 2 and 4 of the execution_table.

Does the filter expression reduce the number of items read from the table?

No, the filter expression applies after reading items. All items are scanned, but only matching items are returned. This is why all items appear in the execution_table steps.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Include in Result value at Step 4?

A No

B True

C Yes

D False

💡 Hint

Check the 'Include in Result?' column at Step 4 in the execution_table.

At which step does the filter condition first evaluate to False?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Filter Condition' column in the execution_table to find the first False.

If the filter expression changed to Author = 'Bob', how would the Include in Result column change?

A All steps would be Yes

B Only Step 2 would be Yes, others No

C All steps would be No

D Steps 1, 3, and 5 would be Yes

💡 Hint

Refer to the 'Author Attribute' column and match with the new filter condition.

Concept Snapshot

Filter expressions in DynamoDB apply after reading items.
They test each item against a condition.
Only items passing the filter are returned.
All items are scanned or queried first.
Filter does not reduce read capacity usage.
Syntax example: FilterExpression = 'Author = :val'.

Full Transcript

In DynamoDB, filter expressions are conditions applied after scanning or querying items. The database reads all items matching the query or scan, then tests each item against the filter expression. Items that meet the condition are included in the results; others are excluded. For example, scanning a 'Books' table with FilterExpression 'Author = Alice' returns only books authored by Alice. The execution table shows each item scanned, the author attribute, whether the filter condition is true or false, and if the item is included. This helps understand that filter expressions do not reduce the number of items read, only the items returned. Key moments include understanding why some items are excluded and that filter expressions apply after reading. The visual quiz tests knowledge of specific steps and how changing the filter affects results.