Wildcard and prefix queries in Elasticsearch - Step-by-Step Execution

Concept Flow - Wildcard and prefix queries

Start Query

↓

Check Query Type

↓

Prefix?

↓

Build prefix

↓

Search index with pattern

↓

Return matching documents

↓

End

The query starts by checking if it is a prefix or wildcard query, builds the pattern accordingly, searches the index, and returns matching documents.

Execution Sample

Elasticsearch

GET /my_index/_search
{
  "query": {
    "wildcard": { "user": "ki*y" }
  }
}

This query searches for documents where the 'user' field matches the pattern 'ki*y', where '*' matches any characters.

Execution Table

Step	Query Part	Action	Pattern Used	Result
1	Receive query	Parse JSON query	wildcard on user	Ready to process
2	Identify query type	Check if prefix or wildcard	wildcard	Proceed with wildcard logic
3	Build pattern	Use 'ki*y' as pattern	ki*y	Pattern ready
4	Search index	Match documents with user matching 'ki*y'	ki*y	Documents found: user='kitty', user='kiry'
5	Return results	Send matching documents back	N/A	2 documents returned
6	End	Query complete	N/A	Done

💡 Query ends after returning all documents matching the wildcard pattern.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
query_type	undefined	wildcard	wildcard	wildcard	wildcard
pattern	undefined	undefined	ki*y	ki*y	ki*y
matched_docs	empty	empty	empty	['kitty', 'kiry']	['kitty', 'kiry']

Key Moments - 2 Insights

Why does the wildcard query use '*' in 'ki*y'?

The '*' is a special character that matches any sequence of characters, so 'ki*y' matches any word starting with 'ki' and ending with 'y', like 'kitty' or 'kiry' as shown in step 4 of the execution_table.

How is a prefix query different from a wildcard query?

A prefix query only matches documents where the field starts with a given string, without wildcards inside. The execution_table shows the query type check at step 2, where prefix and wildcard queries are handled differently.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pattern used at step 3?

A "*y"

B "ki"

C "ki*y"

D "k*y"

💡 Hint

Check the 'Pattern Used' column at step 3 in the execution_table.

At which step does the query identify it is a wildcard query?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Query Part' and 'Action' columns in the execution_table for step 2.

If the query was a prefix query instead, how would the pattern change?

A It would use a pattern like "ki*" without other wildcards

B It would use the same pattern "ki*y"

C It would use a regex pattern

D It would not use any pattern

💡 Hint

Recall the difference between prefix and wildcard queries explained in key_moments and the concept_flow.

Concept Snapshot

Wildcard and prefix queries in Elasticsearch:
- Wildcard queries use '*' and '?' to match any characters.
- Prefix queries match documents starting with a given string.
- Wildcard queries can match anywhere in the string.
- Prefix queries are faster and simpler.
- Use wildcard for flexible matching, prefix for fixed starts.

Full Transcript

This visual execution trace shows how Elasticsearch processes wildcard and prefix queries. The query starts by parsing the JSON input, then identifies if it is a prefix or wildcard query. For a wildcard query like 'ki*y', it builds the pattern and searches the index for matching documents. The example finds documents with user fields like 'kitty' and 'kiry'. Variables like query_type, pattern, and matched_docs change step by step. Key moments clarify why '*' is used and how prefix queries differ. The quiz tests understanding of pattern usage and query type identification. The snapshot summarizes the main points for quick recall.