Function score query in Elasticsearch - Step-by-Step Execution

Concept Flow - Function score query

Start: Receive Query

↓

Apply Base Query

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Apply Functions to Modify Scores

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Combine Function Scores with Base Score

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Return Documents with Updated Scores

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End

The function score query starts with a base query, applies scoring functions to modify document scores, combines them, and returns the results.

Execution Sample

Elasticsearch

{
  "query": {
    "function_score": {
      "query": { "match": { "field": "value" } },
      "functions": [
        { "weight": 2 },
        { "filter": { "term": { "status": "active" } }, "weight": 3 }
      ],
      "score_mode": "sum"
    }
  }
}

This query matches documents where 'field' contains 'value', then boosts scores by weights, adding extra weight if 'status' is 'active'.

Execution Table

Step	Action	Base Query Result	Function Applied	Score Change	Combined Score
1	Run base query match field:value	Doc1=1.0, Doc2=1.0, Doc3=0.0	None	0	Doc1=1.0, Doc2=1.0, Doc3=0.0
2	Apply function weight=2 to all docs	Same as above	Weight 2	+2	Doc1=3.0, Doc2=3.0, Doc3=2.0
3	Apply function weight=3 filtered by status:active	Same as above	Weight 3 if status=active	+3 to Doc1 only	Doc1=6.0, Doc2=3.0, Doc3=2.0
4	Combine scores using sum mode	Final scores	Sum of weights	N/A	Doc1=6.0, Doc2=3.0, Doc3=2.0

💡 All functions applied, scores combined, final ranked documents returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Doc1 Score	0	1.0	3.0	6.0	6.0
Doc2 Score	0	1.0	3.0	3.0	3.0
Doc3 Score	0	0.0	2.0	2.0	2.0

Key Moments - 3 Insights

Why does Doc3 get a score even though it doesn't match the base query?

Because the weight function applies to all documents by default, Doc3 gets a base weight of 2 even with base score 0, as shown in step 2 of the execution_table.

How does the filter in the second function affect scoring?

The filter limits the weight of 3 to only documents where status is active, so only Doc1 gets this extra weight, as seen in step 3 of the execution_table.

What does score_mode 'sum' do with multiple function scores?

It adds all function weights to the base score, combining them by summing, resulting in the final scores shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is Doc2's score after applying the first weight function?

A 1.0

B 4.0

C 3.0

D 0.0

💡 Hint

Check 'After Step 2' column for Doc2 Score in variable_tracker.

At which step does the filter for status:active apply its weight?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Function Applied' column in execution_table for when the filter is used.

If the score_mode changed from 'sum' to 'multiply', how would Doc1's final score change?

A It would be 0.0

B It would be 1.0 * 2 * 3 = 6.0

C It would be 6.0 multiplied by 3

D It would be 6.0

💡 Hint

Multiplying base score and weights means multiplying 1.0 (base) * 2 * 3 for Doc1.

Concept Snapshot

Function score query syntax:
{
  "query": {
    "function_score": {
      "query": { ... },
      "functions": [ ... ],
      "score_mode": "sum"|"multiply"|...
    }
  }
}

Behavior: Modifies base query scores by applying functions with optional filters.
Key rule: Functions can boost or modify scores; score_mode controls combination.

Full Transcript

The function score query starts by running the base query to find matching documents. Then, it applies one or more functions that adjust the scores of these documents. Each function can have a filter to apply only to certain documents and a weight to boost scores. The scores from all functions combine using a mode like sum or multiply. For example, a base query matches documents with 'field' equal to 'value'. A weight function adds 2 to all documents' scores. Another function adds 3 only to documents where 'status' is 'active'. The final scores are the sum of base scores and these weights. This process helps rank documents better based on custom criteria.