perf: cache SetOperator sorted keys, pre-allocate null interleave arrays

- SetOperator.rowKey() called Object.keys().sort() per row — now cached on first call
- decodePage null interleave arrays use pre-allocated new Array(n) instead of push()

Author: teamchong

src/decode.ts

@@ -241,10 +241,10 @@ export function decodePage(
           return v2Strings as (string | null)[];
         }
         // Parquet-style: packed non-null values, interleave with nulls
-        const withNulls: (string | null)[] = [];
+        const withNulls = new Array<string | null>(rowCount);
         let vi = 0;
         for (let i = 0; i < rowCount; i++) {
-          withNulls.push(nulls.has(i) ? null : (vi < v2Strings.length ? v2Strings[vi++] : null));
+          withNulls[i] = nulls.has(i) ? null : (vi < v2Strings.length ? v2Strings[vi++] : null);
         }
         return withNulls;
       }
@@ -261,10 +261,10 @@ export function decodePage(
       return values;
     }
     // Parquet-style: packed non-null values, interleave with nulls
-    const withNulls: (number | bigint | string | null)[] = [];
+    const withNulls = new Array<number | bigint | string | null>(rowCount);
     let vi = 0;
     for (let i = 0; i < rowCount; i++) {
-      withNulls.push(nulls.has(i) ? null : (vi < values.length ? values[vi++] : null));
+      withNulls[i] = nulls.has(i) ? null : (vi < values.length ? values[vi++] : null);
     }
     return withNulls;
   }

src/operators.ts

@@ -1142,8 +1142,11 @@ export class SetOperator implements Operator {
     if (mode !== "union_all") this.seen = new Set();
   }
 
+  private cachedKeys: string[] | null = null;
+
   private rowKey(row: Row): string {
-    const keys = Object.keys(row).sort();
+    if (!this.cachedKeys) this.cachedKeys = Object.keys(row).sort();
+    const keys = this.cachedKeys;
     let result = "";
     for (let i = 0; i < keys.length; i++) {
       if (i > 0) result += "\x00";