[doc] Add reading blob columns section to daft documentation

docs/docs/pypaimon/daft.md

@@ -225,6 +225,113 @@ Use `pypaimon.daft` when your application is written with Daft DataFrames and
 you want Daft to schedule the execution on Ray. Use `pypaimon.ray` instead when
 your application directly reads or writes Ray Datasets.
 
+## Reading Blob Columns
+
+Tables with BLOB columns (see [Blob Storage](./blob)) can be read with Daft.
+Blob columns are returned as `daft.File` references — the actual bytes are
+**not** loaded until you explicitly read them.
+
+:::caution Daft's built-in connector does not support blob lazy loading yet
+
+`daft.read_paimon()` currently reads blob data eagerly during the scan phase,
+ignoring column pruning and filter pushdown for blob columns.
+Use `from pypaimon.daft import read_paimon` instead for blob lazy loading.
+:::
+
+### Lazy loading
+
+When you read a blob table through `pypaimon.daft`, the connector
+automatically enables descriptor mode internally. Blob columns appear as
+`daft.File` references in the DataFrame — no bytes are fetched from storage
+at this point. The actual I/O only happens when you call `file.open()` inside
+a UDF, and only for the rows that survive earlier filters:
+
+```python
+import daft
+from daft import col
+from pypaimon.daft import read_paimon
+
+df = read_paimon(
+    "my_db.image_table",
+    catalog_options={"warehouse": "/path/to/warehouse"},
+)
+
+# Filter runs BEFORE any blob bytes are read.
+df = df.where(col("id") < 100)
+
+# file.length returns the blob size from the descriptor (zero I/O).
+@daft.func(return_dtype=daft.DataType.int64())
+def file_length(f: daft.File) -> int | None:
+    return None if f is None else f.length
+
+result = df.with_column("size", file_length(col("image")))
+result.show()
+```
+
+To read actual blob content (e.g., decode an image), use `file.open()`.
+Only filtered rows trigger I/O:
+
+```python
+@daft.func
+def decode_image(file: daft.File) -> str:
+    with file.open() as f:
+        data = f.read()
+    return f"decoded {len(data)} bytes"
+
+result = df.with_column("info", decode_image(col("image")))
+result.show()
+```
+
+### Parallel blob reading
+
+By default, `@daft.func` processes rows sequentially. To read blobs
+concurrently within a single worker, use an async UDF with
+`max_concurrency` and `asyncio.to_thread` (because `file.open().read()`
+is synchronous blocking I/O):
+
+```python
+import asyncio
+
+def _read_blob(file: daft.File | None) -> str | None:
+    if file is None:
+        return None
+    with file.open() as f:
+        data = f.read()
+    return f"decoded {len(data)} bytes"
+
+@daft.func(max_concurrency=8)
+async def decode_image(file: daft.File | None) -> str | None:
+    return await asyncio.to_thread(_read_blob, file)
+
+result = df.with_column("info", decode_image(col("image")))
+result.show()
+```
+
+When running on Ray, the UDF calls are distributed across Ray workers
+automatically — each worker processes its partition in parallel, giving you
+batch-level concurrency without any extra code.
+
+### Streaming (chunk) reads
+
+`file.open()` returns a seekable stream that supports `read(size)`, so you can
+process large blobs in chunks without loading everything into memory:
+
+```python
+@daft.func(return_dtype=daft.DataType.binary())
+def first_4k(file: daft.File) -> bytes | None:
+    if file is None:
+        return None
+    with file.open() as f:
+        return f.read(4096)
+
+result = df.with_column("header", first_4k(col("image")))
+result.show()
+```
+
+See [Blob Storage — Streaming for Large Blobs](./blob#streaming-for-large-blobs)
+for more details on the underlying `OffsetInputStream` API (`read(size)` /
+`seek()` / `tell()`).
+
 ## Catalog Abstraction
 
 Paimon catalogs can integrate with Daft's unified `Catalog` / `Table` interfaces: