# Source: https://docs.lancedb.com/search/optimize-queries.md

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lancedb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Optimize Query Performance

> Analyze and optimize query performance in LanceDB.

LanceDB provides two powerful tools for query analysis and optimization: `explain_plan` and `analyze_plan`. Let's take a better look at how they work:

| Method         | Purpose            | Description                                                                                                                                                                                              |
| :------------- | :----------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `explain_plan` | Query Analysis     | Print the resolved query plan to understand how the query will be executed. Helpful for identifying slow queries or unexpected query results.                                                            |
| `analyze_plan` | Performance Tuning | Execute the query and return a physical execution plan annotated with runtime metrics including execution time, number of rows processed, and I/O stats. Essential for performance tuning and debugging. |

## Query Analysis Tools

### explain\_plan

Reveals the logical query plan before execution, helping you identify potential issues with query structure and index usage. This tool is useful for:

* Verifying query optimization strategies
* Validating index selection
* Understanding query execution order
* Detecting missing indices

### analyze\_plan

Executes the query and provides detailed runtime metrics, including:

* Operation duration (`_elapsed_compute_`)
* Data processing statistics (`_output_rows_`, `_bytes_read_`)
* Index effectiveness (`_index_comparisons_`, `_indices_loaded_`)
* Resource utilization (`_iops_`, `_requests_`)

Together, these tools offer a comprehensive view of query performance, from planning to execution. Use `explain_plan` to verify your query structure and `analyze_plan` to measure and optimize actual performance.

## Reading the Execution Plan

To demonstrate query performance analysis, we'll use a table containing 1.2M rows sampled from the [Wikipedia dataset](https://huggingface.co/datasets/wikimedia/wikipedia). Initially, the table has no indices, allowing us to observe the impact of optimization.

Let's examine a vector search query that:

* Filters rows where `identifier` is between 0 and 1,000,000
* Returns the top 100 matches
* Projects specific columns: `chunk_index`, `title`, and `identifier`

<CodeGroup>
  ```python Python icon="python" theme={"theme":{"light":"vitesse-light","dark":"catppuccin-mocha"}}
  # explain_plan
  query_explain_plan = (
      table.search(query_embed)
      .where("identifier > 0 AND identifier < 1000000")
      .select(["chunk_index", "title", "identifier"])
      .limit(100)
      .explain_plan(True)
  )
  ```

  ```typescript TypeScript icon="square-js" theme={"theme":{"light":"vitesse-light","dark":"catppuccin-mocha"}}
  // explain_plan
  const explainPlan = await table
      .search(queryEmbed)
      .where("identifier > 0 AND identifier < 1000000")
      .select(["chunk_index", "title", "identifier"])
      .limit(100)
      .explainPlan(true);
  ```
</CodeGroup>

### Execution Plan Components

The execution plan reveals the sequence of operations performed to execute your query. Let's examine each component:

```
ProjectionExec: expr=[chunk_index@4 as chunk_index, title@5 as title, identifier@1 as identifier, _distance@3 as _distance]
  RemoteTake: columns="vector, identifier, _rowid, _distance, chunk_index, title"
    CoalesceBatchesExec: target_batch_size=1024
      GlobalLimitExec: skip=0, fetch=100
        FilterExec: _distance@3 IS NOT NULL
          SortExec: TopK(fetch=100), expr=[_distance@3 ASC NULLS LAST], preserve_partitioning=[false]
            KNNVectorDistance: metric=l2
              FilterExec: identifier@1 > 0 AND identifier@1 < 1000000
                LanceScan: uri=***, projection=[vector, identifier], row_id=true, row_addr=false, ordered=false
```

#### 1. Base Layer (LanceScan)

* Initial data scan loading only specified columns to minimize I/O
* Unordered scan enabling parallel processing

```
LanceScan: 
- projection=[vector, identifier]
- row_id=true, row_addr=false, ordered=false
```

#### 2. First Filter

* Apply requested filter on `identifier` column
* Reduces the number of vectors that need KNN computation

```
FilterExec: identifier@1 > 0 AND identifier@1 < 1000000
```

#### 3. Vector Search

* Computes L2 (Euclidean) distances between query vector and all vectors that passed the filter

```
KNNVectorDistance: metric=l2
```

#### 4. Results Processing

* Filters out null distance results
* Sorts by distance and takes top 100 results
* Processes in batches of 1024 for optimal memory usage

```
SortExec: TopK(fetch=100)
- expr=[_distance@3 ASC NULLS LAST]
- preserve_partitioning=[false]
FilterExec: _distance@3 IS NOT NULL
GlobalLimitExec: skip=0, fetch=100
CoalesceBatchesExec: target_batch_size=1024
```

#### 5. Data Retrieval

* `RemoteTake` is a key component of Lance's I/O cache
* Handles efficient data retrieval from remote storage locations
* Fetches specific rows and columns needed for the final output
* Optimizes network bandwidth by only retrieving required data

```
RemoteTake: columns="vector, identifier, _rowid, _distance, chunk_index, title"
```

#### 6. Final Output

* Returns only requested columns and maintains column ordering

```python  theme={"theme":{"light":"vitesse-light","dark":"catppuccin-mocha"}}
ProjectionExec: expr=[chunk_index@4 as chunk_index, title@5 as title, identifier@1 as identifier, _distance@3 as _distance]
```

This plan demonstrates a basic search without index optimizations: it performs a full scan and filter before vector search.

## Performance Analysis

Let's use `analyze_plan` to run the query and analyze the query performance, which will help us identify potential bottlenecks:

<CodeGroup>
  ```python Python icon="python" theme={"theme":{"light":"vitesse-light","dark":"catppuccin-mocha"}}
  # analyze_plan
  query_analyze_plan = (
      table.search(query_embed)
      .where("identifier > 0 AND identifier < 1000000")
      .select(["chunk_index", "title", "identifier"])
      .limit(100)
      .analyze_plan()
  )
  ```

  ```typescript TypeScript icon="square-js" theme={"theme":{"light":"vitesse-light","dark":"catppuccin-mocha"}}
  // analyze_plan
  const analyzePlan = await table
      .search(queryEmbed)
      .where("identifier > 0 AND identifier < 1000000")
      .select(["chunk_index", "title", "identifier"])
      .limit(100)
      .analyzePlan();
  ```
</CodeGroup>

### Performance Metrics Analysis

```
ProjectionExec: expr=[chunk_index@4 as chunk_index, title@5 as title, identifier@1 as identifier, _distance@3 as _distance], metrics=[output_rows=100, elapsed_compute=1.424µs]
    RemoteTake: columns="vector, identifier, _rowid, _distance, chunk_index, title", metrics=[output_rows=100, elapsed_compute=175.53097ms, output_batches=1, remote_takes=100]
      CoalesceBatchesExec: target_batch_size=1024, metrics=[output_rows=100, elapsed_compute=2.748µs]
        GlobalLimitExec: skip=0, fetch=100, metrics=[output_rows=100, elapsed_compute=1.819µs]
          FilterExec: _distance@3 IS NOT NULL, metrics=[output_rows=100, elapsed_compute=10.275µs]
            SortExec: TopK(fetch=100), expr=[_distance@3 ASC NULLS LAST], preserve_partitioning=[false], metrics=[output_rows=100, elapsed_compute=39.259451ms, row_replacements=546]
              KNNVectorDistance: metric=l2, metrics=[output_rows=1099508, elapsed_compute=56.783526ms, output_batches=1076]
                FilterExec: identifier@1 > 0 AND identifier@1 < 1000000, metrics=[output_rows=1099508, elapsed_compute=17.136819ms]
                  LanceScan: uri=***, projection=[vector, identifier], row_id=true, row_addr=false, ordered=false, metrics=[output_rows=1200000, elapsed_compute=21.348178ms, bytes_read=1852931072, iops=78, requests=78]
```

#### 1. Data Loading (LanceScan)

* Scanned 1,200,000 rows from the LanceDB table
* Read 1.86GB of data in 78 I/O operations
* Only loaded necessary columns (`vector` and `identifier`)
* Unordered scan for parallel processing

#### 2. Filtering & Search

* Applied prefilter condition (`identifier > 0 AND identifier < 1000000`)
* Reduced dataset from 1.2M to 1,099,508 rows
* KNN search used L2 (Euclidean) distance metric
* Vector comparisons processed in 1076 batches

#### 3. Results Processing

* KNN results sorted by distance (TopK with fetch=100)
* Null distances filtered out
* Batches coalesced to target size of 1024 rows
* Additional columns fetched for final results
* Remote take operation for 100 results
* Final projection of required columns

### Key Observations

* Vector search is the primary bottleneck (1,099,508 vector comparisons)
* Significant I/O overhead (1.86GB data read)
* Full table scan due to lack of indices
* Substantial optimization potential through proper index implementation

## Optimized Query Execution

After creating vector and scalar indices, the execution plan shows:

```
ProjectionExec: expr=[chunk_index@3 as chunk_index, title@4 as title, identifier@2 as identifier, _distance@0 as _distance]
  RemoteTake: columns="_distance, _rowid, identifier, chunk_index, title"
    CoalesceBatchesExec: target_batch_size=1024
      GlobalLimitExec: skip=0, fetch=100
        SortExec: TopK(fetch=100), expr=[_distance@0 ASC NULLS LAST], preserve_partitioning=[false]
          ANNSubIndex: name=vector_idx, k=100, deltas=1
            ANNIvfPartition: uuid=83916fd5-fc45-4977-bad9-1f0737539bb9, nprobes=20, deltas=1
            ScalarIndexQuery: query=AND(identifier > 0,identifier < 1000000)
```

### Optimized Plan Analysis

#### 1. Scalar Index Query

```
ScalarIndexQuery: query=AND(identifier > 0,identifier < 1000000)
metrics=[
    output_rows=2
    index_comparisons=2,301,824
    indices_loaded=2
    output_batches=1
    parts_loaded=562
    elapsed_compute=86.979354ms
]
```

* Range filter using scalar index
* Only 2 index files and 562 scalar index parts loaded
* 2.3M index comparisons for matches

#### 2. Vector Search

```
ANNSubIndex: name=vector_idx, k=100, deltas=1
metrics=[
    output_rows=2,000
    index_comparisons=25,893
    indices_loaded=0
    output_batches=20
    parts_loaded=20
    elapsed_compute=111.849043ms
]
```

* IVF index with 20 probes
* Only 20 index parts loaded
* 25,893 vector comparisons
* 2,000 matching vectors

#### 3. Results Processing

```
SortExec: TopK(fetch=100), expr=[_distance@0 ASC NULLS LAST], preserve_partitioning=[false]
GlobalLimitExec: skip=0, fetch=100
CoalesceBatchesExec: target_batch_size=1024
```

* Sorts by distance
* Limits to top 100 results
* Batches into groups of 1024

#### 4. Data Fetching

```
RemoteTake: columns="_distance, _rowid, identifier, chunk_index, title"
metrics=[output_rows=100, elapsed_compute=113.491859ms, output_batches=1, remote_takes=100]
```

* Single output batch
* One remote take per row

#### 5. Final Projection

```
ProjectionExec: expr=[chunk_index@3 as chunk_index, title@4 as title, identifier@2 as identifier, _distance@0 as _distance]
```

* Returns specified columns: chunk\_index, title, identifier, and distance

### Performance Improvements

#### 1. Initial Data Access

```
ScalarIndexQuery metrics:
- indices_loaded=2
- parts_loaded=562
- output_batches=1
```

* Before: Full table scan of 1.2M rows, 1.86GB data
* After: Only 2 indices and 562 scalar index parts loaded
* Benefit: Eliminated table scans for prefilter

#### 2. Vector Search Efficiency

```
ANNSubIndex:
- index_comparisons=25,893
- indices_loaded=0
- parts_loaded=20
- output_batches=20
```

* Before: L2 calculations on 1,099,508 vectors
* After:
  * 99.8% reduction in vector comparisons
  * Decreased output batches from 1,076 to 20

#### 3. Data Retrieval Optimization

```
RemoteTake:
- remote_takes=100
- output_batches=1
```

* RemoteTake operation remains consistent

## Performance Optimization Guide

### 1. Index Implementation

#### When to Create Indices

* Columns used in WHERE clauses
* Vector columns for similarity searches
* Join columns used in `merge_insert`

#### Index Type Selection

| Data Type   | Recommended Index     | Use Case                                 |
| ----------- | --------------------- | ---------------------------------------- |
| Vector      | IVF\_PQ/IVF\_HNSW\_SQ | Approximate nearest neighbor search      |
| Scalar      | B-Tree                | Range queries and sorting                |
| Categorical | Bitmap                | Multi-value filters and set operations   |
| `List`      | Label\_list           | Multi-label classification and filtering |

<Card title="Index Coverage Monitoring" icon="lightbulb">
  Use `table.index_stats()` to monitor index coverage.
  A well-optimized table should have `num_unindexed_rows ~ 0`.
</Card>

### 2. Query Plan Optimization

#### Common Patterns and Fixes

| Plan Pattern                                | Optimization                                 |
| ------------------------------------------- | -------------------------------------------- |
| LanceScan with high *bytes\_read* or *iops* | Add missing index                            |
|                                             | Use `select()` to limit returned columns     |
|                                             | Check whether the dataset has been compacted |
| Multiple sequential filters                 | Reorder filter conditions                    |

!!! note "Regular Performance Analysis"
Regularly analyze your query plans to identify and address performance bottlenecks.
The `analyze_plan` output provides detailed metrics to guide optimization efforts.

### 3. Getting Started with Optimization

For vector search performance:

* Create ANN index on your vector column(s) as described in the [index guide](/indexing/vector-index/)
* If you often filter by metadata, create [scalar indices](/indexing/scalar-index/) on those columns