ORDER BY not using index

Neo4j Graph Platform
1

From what I've read, ORDER BY should use an existing index. This seems not to be the case, at least with Neo4j Community Edition 5.8.0 which I'm currently evaluating.

I have a very simple model with about ~1 million Item nodes that have a name property.

I've created an index using the following command:
CREATE INDEX item_name FOR (i:Item) ON (i.name)

The following query returns the first 100 name properties in ascending order:
MATCH (n:Item) RETURN n.name ORDER BY n.name LIMIT 100

But the query takes ~300 ms. This feels slow. I expected it to use the item_name index and run much faster.

See attachment for the query plan generated via:
PROFILE MATCH (n:Item) RETURN n.name ORDER BY n.name LIMIT 100

plan
plan331×1088 22.7 KB

What am I doing wrong? Or is that a limitation of the Community version?

2

Hello @candritzky and welcome to the Neo4j community :slight_smile:

Can you try this query?

PROFILE MATCH (n:Item) 
WHERE n.name IS NOT NULL 
RETURN n.name 
ORDER BY n.name LIMIT 100

Best regards,
Cobra

3

Thanks @cobra,

indeed, it works with a WHERE clause on n.name. Can be n.name IS NOT NULL as you suggested, or n.name<>'foobar' or whatever. Query performance is now down to 1 or 2 ms.

Here's the query plan (now using NodeIndexScan instead of NodeByLabelScan).

plan
plan331×1064 22.7 KB

So what's the trick here? Do I have to use a (dummy) WHERE clause on each of the (indexed) expressions that are part of ORDER BY?

Thanks again,
Chris

4

Yes, you can find more information here.

Best regards,
Cobra