SUNIONSTORE for storing results in Redis - Step-by-Step Execution

Concept Flow - SUNIONSTORE for storing results

Start with multiple sets

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SUNIONSTORE command runs

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Calculate union of all sets

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Store union result in destination set

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Return number of elements in destination set

SUNIONSTORE takes multiple sets, finds their union, stores it in a new set, and returns the count of elements stored.

Execution Sample

Redis

SADD set1 a b c
SADD set2 b c d
SUNIONSTORE dest set1 set2
SMEMBERS dest

This code creates two sets, unions them into 'dest', and retrieves all members of 'dest'.

Execution Table

Step	Command	Action	Result	Notes
1	SADD set1 a b c	Add elements a,b,c to set1	3	set1 = {a,b,c}
2	SADD set2 b c d	Add elements b,c,d to set2	3	set2 = {b,c,d}
3	SUNIONSTORE dest set1 set2	Union set1 and set2, store in dest	4	dest = {a,b,c,d}
4	SMEMBERS dest	Retrieve all members of dest	[a,b,c,d]	Shows union result
5	End	No more commands		Execution complete

💡 All commands executed; union stored in 'dest' with 4 elements.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
set1	empty	{a,b,c}	{a,b,c}	{a,b,c}	{a,b,c}
set2	empty	empty	{b,c,d}	{b,c,d}	{b,c,d}
dest	empty	empty	empty	{a,b,c,d}	{a,b,c,d}

Key Moments - 2 Insights

Why does SUNIONSTORE overwrite the destination set instead of adding to it?

SUNIONSTORE replaces the destination set with the union result each time, as shown in step 3 of the execution_table where 'dest' is set to the union of 'set1' and 'set2'. It does not append but overwrites.

What happens if one of the source sets does not exist?

If a source set does not exist, SUNIONSTORE treats it as an empty set. The union will be the union of existing sets only, so the destination will contain elements from existing sets as shown in the concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, how many elements does 'dest' contain after SUNIONSTORE runs?

A 4

B 3

C 2

D 5

💡 Hint

Check row 3 in execution_table where SUNIONSTORE stores the union with count 4.

At which step is the 'dest' set created and populated?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table; union and store happen at step 3.

If 'set2' was empty, what would be the size of 'dest' after SUNIONSTORE?

A 0

B 4

C 3

D Depends on 'set1'

💡 Hint

Refer to variable_tracker and concept_flow: union with empty set equals the other set.

Concept Snapshot

SUNIONSTORE destination key sourcekey [sourcekey ...]
- Computes union of all source sets
- Stores result in destination key (overwrites existing)
- Returns number of elements in destination
- If source set missing, treated as empty
- Use SMEMBERS to view stored union

Full Transcript

SUNIONSTORE command in Redis takes multiple sets, finds their union, and stores the result in a destination set. It overwrites any existing data in the destination. The command returns the number of elements stored. For example, if set1 has {a,b,c} and set2 has {b,c,d}, SUNIONSTORE dest set1 set2 will store {a,b,c,d} in dest. If a source set does not exist, it is treated as empty. You can retrieve the union with SMEMBERS dest. This visual trace showed each step: adding elements to sets, running SUNIONSTORE, and checking the result. Key points include understanding that the destination is overwritten and missing sets count as empty.