Revert "remove the concept of virtual row groups (#281)" (#283)

This reverts commit f9b7ac0.

Author: severo

src/lib/tableProvider.ts

@@ -12,7 +12,7 @@ type GroupStatus = {
 } | {
   kind: 'fetched'
 }
-interface RowGroup {
+interface VirtualRowGroup {
   groupStart: number
   groupEnd: number
   state: Map<string, GroupStatus>
@@ -28,24 +28,29 @@ export function parquetDataFrame(from: AsyncBufferFrom, metadata: FileMetaData):
 
   const cellCache = new Map<string, ResolvedValue<unknown>[]>(header.map(name => [name, []]))
 
-  const rowGroups: RowGroup[] = []
+  // virtual row groups are up to 1000 rows within row group boundaries
+  const groups: VirtualRowGroup[] = []
   let groupStart = 0
   for (const rg of metadata.row_groups) {
-    const groupEnd = groupStart + Number(rg.num_rows)
-    rowGroups.push({
-      groupStart,
-      groupEnd,
-      state: new Map(header.map(name => [name, { kind: 'unfetched' }])),
-    })
-    groupStart = groupEnd
+    // make virtual row groups of size 1000
+    for (let j = 0; j < rg.num_rows; j += 1000) {
+      const groupSize = Math.min(1000, Number(rg.num_rows) - j)
+      const groupEnd = groupStart + groupSize
+      groups.push({
+        groupStart,
+        groupEnd,
+        state: new Map(header.map(name => [name, { kind: 'unfetched' }])),
+      })
+      groupStart = groupEnd
+    }
   }
 
-  async function fetchRowGroup({ rowGroup, columns }: {
-    rowGroup: RowGroup, columns: string[]
+  async function fetchVirtualRowGroup({ group, columns }: {
+    group: VirtualRowGroup, columns: string[]
   }): Promise<void> {
-    const { groupStart, groupEnd, state } = rowGroup
+    const { groupStart, groupEnd, state } = group
     const columnsToFetch = columns.filter(column => state.get(column)?.kind === 'unfetched')
-    const promises = [...rowGroup.state.values()].filter((status): status is { kind: 'fetching', promise: Promise<void> } => status.kind === 'fetching').map(status => status.promise)
+    const promises = [...group.state.values()].filter((status): status is { kind: 'fetching', promise: Promise<void> } => status.kind === 'fetching').map(status => status.promise)
 
     // TODO(SL): pass AbortSignal to the worker?
     if (columnsToFetch.length > 0) {
@@ -102,12 +107,12 @@ export function parquetDataFrame(from: AsyncBufferFrom, metadata: FileMetaData):
 
       const promises: Promise<void>[] = []
 
-      rowGroups.forEach((rowGroup) => {
-        const { groupStart, groupEnd } = rowGroup
+      groups.forEach((group) => {
+        const { groupStart, groupEnd } = group
         if (groupStart < rowEnd && groupEnd > rowStart) {
           promises.push(
-            fetchRowGroup({
-              rowGroup,
+            fetchVirtualRowGroup({
+              group,
               columns,
             }).then(() => {
               checkSignal(signal)