feat: optimize external link streaming

README.md

@@ -183,7 +183,9 @@ function fetchStream(
 ```
 - Merges `EXTERNAL_LINKS` into a single binary stream.
 - Preserves the original format (`JSON_ARRAY`, `CSV`, `ARROW_STREAM`).
+- Throws if the result is `INLINE`.
 - Ends as an empty stream when no external links exist.
+- `forceMerge: true` forces merge even when there is only a single external link.
 
 ### mergeExternalLinks(statementResult, auth, options)
 ```ts
@@ -196,7 +198,8 @@ function mergeExternalLinks(
 - Creates a merged stream from `EXTERNAL_LINKS`, uploads it via
   `options.mergeStreamToExternalLink`, then returns a `StatementResult`
   with a single external link.
-- Returns the original result unchanged when input is `INLINE`.
+- Returns the original result unchanged when input is `INLINE` or already a
+  single external link (unless `forceMerge: true`).
 
 ### Options (Summary)
 ```ts
@@ -228,10 +231,12 @@ type FetchAllOptions = {
 
 type FetchStreamOptions = {
   signal?: AbortSignal
+  forceMerge?: boolean
 }
 
 type MergeExternalLinksOptions = {
   signal?: AbortSignal
+  forceMerge?: boolean
   mergeStreamToExternalLink: (stream: Readable) => Promise<{
     externalLink: string
     byte_count: number
@@ -243,6 +248,7 @@ type MergeExternalLinksOptions = {
 ## Notes
 - Databricks requires `INLINE` results to use `JSON_ARRAY` format. `INLINE + CSV` is rejected by the API.
 - `EXTERNAL_LINKS` are merged using `@bitofsky/merge-streams`.
+- Requires Node.js >= 20 for global `fetch` and Web streams.
 
 ## Development
 ```bash

src/api/fetchRow.ts

@@ -1,15 +1,15 @@
-import { parser } from 'stream-json'
-import { streamArray } from 'stream-json/streamers/StreamArray'
 import type { Readable } from 'node:stream'
-import { getChunk } from '../databricks-api.js'
-import { DatabricksSqlError, AbortError } from '../errors.js'
 import type {
   AuthInfo,
   FetchRowsOptions,
   RowArray,
   RowObject,
   StatementResult,
 } from '../types.js'
+import { parser } from 'stream-json'
+import { streamArray } from 'stream-json/streamers/StreamArray'
+import { getChunk } from '../databricks-api.js'
+import { DatabricksSqlError, AbortError } from '../errors.js'
 import { validateSucceededResult } from '../util.js'
 import { createRowMapper } from '../createRowMapper.js'
 import { fetchStream } from './fetchStream.js'

src/api/fetchStream.ts

@@ -1,14 +1,15 @@
-import { PassThrough, type Readable } from 'node:stream'
-import { mergeStreamsFromUrls, type MergeFormat } from '@bitofsky/merge-streams'
 import type {
   AuthInfo,
+  ExternalLinkInfo,
   StatementResult,
   FetchStreamOptions,
   StatementManifest,
 } from '../types.js'
+import { PassThrough, Readable } from 'node:stream'
+import { mergeStreamsFromUrls, type MergeFormat } from '@bitofsky/merge-streams'
 import { getChunk } from '../databricks-api.js'
-import { AbortError } from '../errors.js'
-import { validateSucceededResult } from '../util.js'
+import { AbortError, DatabricksSqlError } from '../errors.js'
+import { pipeUrlToOutput, validateSucceededResult } from '../util.js'
 
 /**
  * Create a readable stream from statement result.
@@ -20,31 +21,32 @@ export function fetchStream(
   auth: AuthInfo,
   options: FetchStreamOptions = {}
 ): Readable {
-  const { signal } = options
+  const { signal, forceMerge } = options
   const manifest = validateSucceededResult(statementResult)
   const format = manifest.format as MergeFormat
 
+  if (statementResult.result?.data_array) {
+    throw new DatabricksSqlError(
+      'fetchStream only supports EXTERNAL_LINKS results',
+      'UNSUPPORTED_FORMAT',
+      statementResult.statement_id
+    )
+  }
+
   // Create PassThrough as output (readable by consumer)
   const output = new PassThrough()
 
   // Handle AbortSignal
   if (signal) {
-    const onAbort = () => {
-      output.destroy(new AbortError('Stream aborted'))
-    }
+    const onAbort = () => output.destroy(new AbortError('Stream aborted'))
     signal.addEventListener('abort', onAbort, { once: true })
-    output.once('close', () => {
-      signal.removeEventListener('abort', onAbort)
-    })
+    output.once('close', () => signal.removeEventListener('abort', onAbort))
   }
 
   // Start async merge process
   // Errors are forwarded to the stream consumer via destroy.
-  mergeChunksToStream(statementResult, auth, manifest, format, output, signal).catch(
-    (err) => {
-      output.destroy(err as Error)
-    }
-  )
+  mergeChunksToStream(statementResult, auth, manifest, format, output, signal, forceMerge)
+    .catch((err) => output.destroy(err as Error))
 
   return output
 }
@@ -58,29 +60,59 @@ async function mergeChunksToStream(
   manifest: StatementManifest,
   format: MergeFormat,
   output: PassThrough,
-  signal?: AbortSignal
+  signal?: AbortSignal,
+  forceMerge?: boolean
 ): Promise<void> {
-  const result = statementResult.result
-
-  // Collect all external link URLs
-  let urls = result?.external_links?.map((link) => link.external_link) ?? []
-
-  // If no URLs in initial result, fetch from chunks
-  if (urls.length === 0 && manifest.total_chunk_count > 0) {
-    for (let i = 0; i < manifest.total_chunk_count; i++) {
-      if (signal?.aborted) throw new AbortError('Aborted while collecting URLs')
-
-      // Chunk metadata contains external link URLs when results are chunked.
-      const chunkData = await getChunk(auth, statementResult.statement_id, i, signal)
-      const chunkUrls = chunkData.external_links?.map((link) => link.external_link) ?? []
-      urls.push(...chunkUrls)
-    }
-  }
+  const urls = await collectExternalUrls(statementResult, auth, manifest, signal)
 
   // No external links - close the stream
   if (urls.length === 0)
     return void output.end()
 
+  // Single URL - pipe directly to output unless forcing merge
+  if (urls.length === 1 && !forceMerge)
+    // Avoid merge-streams overhead for a single URL unless forced.
+    return pipeUrlToOutput(urls[0]!, output, signal)
+
   // Merge all URLs using merge-streams
-  await mergeStreamsFromUrls(format, signal ? { urls, output, signal } : { urls, output })
+  return mergeStreamsFromUrls(format, signal ? { urls, output, signal } : { urls, output })
+}
+
+async function collectExternalUrls(
+  statementResult: StatementResult,
+  auth: AuthInfo,
+  manifest: StatementManifest,
+  signal?: AbortSignal
+): Promise<string[]> {
+  const urls = extractExternalLinks(statementResult.result?.external_links)
+  if (urls.length > 0)
+    return urls
+
+  if (!manifest.total_chunk_count)
+    return []
+
+  const chunkUrls: string[] = []
+  for (let i = 0; i < manifest.total_chunk_count; i++) {
+    if (signal?.aborted)
+      throw new AbortError('Aborted while collecting URLs')
+
+    // Chunk metadata contains external link URLs when results are chunked.
+    const chunkData = await getChunk(auth, statementResult.statement_id, i, signal)
+    chunkUrls.push(...extractExternalLinks(chunkData.external_links))
+  }
+
+  return chunkUrls
+}
+
+function extractExternalLinks(externalLinks?: ExternalLinkInfo[]): string[] {
+  if (!externalLinks)
+    return []
+
+  return externalLinks
+    .map((link) => link.external_link)
+    .filter(isNonEmptyString)
+}
+
+function isNonEmptyString(value: unknown): value is string {
+  return typeof value === 'string' && value.length > 0
 }

src/api/mergeExternalLinks.ts

@@ -17,14 +17,27 @@ export async function mergeExternalLinks(
   auth: AuthInfo,
   options: MergeExternalLinksOptions
 ): Promise<StatementResult> {
-  const { signal, mergeStreamToExternalLink } = options
+  const { signal, mergeStreamToExternalLink, forceMerge } = options
 
   // If not external links, return original as-is
   if (!statementResult.result?.external_links)
     return statementResult
 
+  if (!forceMerge) {
+    const totalChunks = statementResult.manifest?.total_chunk_count
+    const externalLinks = statementResult.result.external_links
+    const isSingleChunk = totalChunks === undefined ? externalLinks.length <= 1 : totalChunks <= 1
+
+    // Skip merging when a single external link already exists unless forced.
+    if (isSingleChunk && externalLinks.length <= 1)
+      return statementResult
+  }
+
   // Get merged stream via fetchStream
-  const stream = fetchStream(statementResult, auth, signal ? { signal } : {})
+  const stream = fetchStream(statementResult, auth, {
+    ...signal ? { signal } : {},
+    ...forceMerge !== undefined ? { forceMerge } : {},
+  })
 
   // Upload via callback
   const uploadResult = await mergeStreamToExternalLink(stream)

src/index.ts

@@ -5,4 +5,4 @@ export type * from './types.js'
 export * from './errors.js'
 
 // Core functions
-export * from './api'
+export * from './api/index.js'

src/types.ts

@@ -150,7 +150,10 @@ export type RowObject = Record<string, unknown>
 export type FetchRowFormat = 'JSON_ARRAY' | 'JSON_OBJECT'
 
 /** Options for fetchStream */
-export type FetchStreamOptions = SignalOptions
+export type FetchStreamOptions = SignalOptions & {
+  /** Force merge even when there is only a single external link */
+  forceMerge?: boolean
+}
 
 /** Options for fetchRow */
 export type FetchRowsOptions = SignalOptions & {
@@ -180,6 +183,8 @@ export type MergeExternalLinksResult = {
 export type MergeExternalLinksOptions = SignalOptions & {
   /** Callback to upload merged stream to external link */
   mergeStreamToExternalLink: (stream: Readable) => Promise<MergeExternalLinksResult>
+  /** Force merge even when there is only a single external link chunk */
+  forceMerge?: boolean
 }
 
 /**

src/util.ts

@@ -1,3 +1,6 @@
+import { Readable } from 'node:stream'
+import { pipeline } from 'node:stream/promises'
+import type { ReadableStream as WebReadableStream } from 'node:stream/web'
 import type { StatementResult, StatementManifest } from './types.js'
 import { AbortError, DatabricksSqlError } from './errors.js'
 
@@ -80,3 +83,34 @@ export function validateSucceededResult(
 
   return statementResult.manifest
 }
+
+function isWebReadableStream(body: unknown): body is WebReadableStream {
+  return typeof (body as WebReadableStream).getReader === 'function'
+}
+
+export async function pipeUrlToOutput(
+  url: string,
+  output: NodeJS.WritableStream,
+  signal?: AbortSignal
+): Promise<void> {
+  // Uses Node 20+ global fetch with Web streams.
+  if (signal?.aborted)
+    throw new AbortError('Aborted while streaming')
+
+  const response = await fetch(url, signal ? { signal } : undefined)
+  if (!response.ok) {
+    throw new Error(
+      `Failed to fetch external link: ${response.status} ${response.statusText}`
+    )
+  }
+
+  if (!response.body)
+    return void output.end()
+
+  const body = response.body
+  const input = isWebReadableStream(body)
+    ? Readable.fromWeb(body)
+    : (body as NodeJS.ReadableStream)
+
+  await pipeline(input, output)
+}

test/mergeExternalLinks.spec.ts

@@ -62,6 +62,7 @@ describe('mergeExternalLinks', () => {
 
     const result = await mergeExternalLinks(mockExternalLinksResult, mockAuth, {
       mergeStreamToExternalLink: mockCallback,
+      forceMerge: true,
     })
 
     // Verify callback was called
@@ -113,6 +114,7 @@ describe('mergeExternalLinks', () => {
 
     await mergeExternalLinks(mockExternalLinksResult, mockAuth, {
       mergeStreamToExternalLink: mockCallback,
+      forceMerge: true,
     })
 
     // Verify callback received the merged data
@@ -154,6 +156,7 @@ describe('mergeExternalLinks', () => {
       mergeExternalLinks(mockExternalLinksResult, mockAuth, {
         signal: controller.signal,
         mergeStreamToExternalLink: mockCallback,
+        forceMerge: true,
       })
     ).rejects.toThrow(/abort/i)
   })
@@ -170,6 +173,7 @@ describe('mergeExternalLinks', () => {
     await expect(
       mergeExternalLinks(mockExternalLinksResult, mockAuth, {
         mergeStreamToExternalLink: mockCallback,
+        forceMerge: true,
       })
     ).rejects.toThrow('Upload failed')
   })
@@ -189,6 +193,7 @@ describe('mergeExternalLinks', () => {
 
     const result = await mergeExternalLinks(mockExternalLinksResult, mockAuth, {
       mergeStreamToExternalLink: vi.fn().mockResolvedValue(uploadResult),
+      forceMerge: true,
     })
 
     // Verify original manifest properties are preserved
@@ -198,4 +203,15 @@ describe('mergeExternalLinks', () => {
       mockExternalLinksResult.manifest?.total_row_count
     )
   })
+
+  it('should return original for single external link by default', async () => {
+    const mockCallback = vi.fn()
+
+    const result = await mergeExternalLinks(mockExternalLinksResult, mockAuth, {
+      mergeStreamToExternalLink: mockCallback,
+    })
+
+    expect(result).toBe(mockExternalLinksResult)
+    expect(mockCallback).not.toHaveBeenCalled()
+  })
 })

test/s3.integration.spec.ts

@@ -248,6 +248,7 @@ describe.skipIf(shouldSkip)('S3 Integration Tests', () => {
 
       // Merge and upload to S3
       const mergedResult = await mergeExternalLinks(result, auth, {
+        forceMerge: true,
         mergeStreamToExternalLink: (stream) => uploadToS3(stream, 'CSV'),
       })
 
@@ -296,6 +297,7 @@ describe.skipIf(shouldSkip)('S3 Integration Tests', () => {
       }
 
       const mergedResult = await mergeExternalLinks(result, auth, {
+        forceMerge: true,
         mergeStreamToExternalLink: (stream) => uploadToS3(stream, 'JSON_ARRAY'),
       })
 
@@ -332,6 +334,7 @@ describe.skipIf(shouldSkip)('S3 Integration Tests', () => {
       }
 
       const mergedResult = await mergeExternalLinks(result, auth, {
+        forceMerge: true,
         mergeStreamToExternalLink: (stream) => uploadToS3(stream, 'ARROW_STREAM'),
       })
 
@@ -368,6 +371,7 @@ describe.skipIf(shouldSkip)('S3 Integration Tests', () => {
       }
 
       const mergedResult = await mergeExternalLinks(result, auth, {
+        forceMerge: true,
         mergeStreamToExternalLink: (stream) => uploadToS3(stream, 'CSV'),
       })