Is it possible to stream read_blob results into memory in chunks rather than all at once (read_parquet seems to work already)?

[ms7463]

Hi, I see in this PR that streaming results for the python client was implemented back in 2023 (duckdb/duckdb#6346). However, this doesn't seem to work with read_blob and even though object results are streamed, the memory is loaded for the full query result on first fetch (not sure if this is expected/a bug or not). Given the following query (through the python client):

xx = duckdb.query('''
SELECT *
FROM read_blob('.../**/data.data', hive_partitioning=true)
WHERE ...
''')
xx.fetchone()

When I do xx.fetchone() (or any other xx.fetch* method) it seems to load all the data for the full query into memory first, instead of just that row or chunk. (For the record memory streaming does work properly with read_parquet)

As a work around for now, what I can do is to query with EXCLUDE content and then iterate through the result in whatever chunk size I want and read_blob with that chunks filenames including content. But I'm wondering if there is a first class supported way to stream the results in memory.

(Not sure if this is a python client specific issue, or an underlying duckdb engine issue)

Full repro (ran on google colab):

!pip install --upgrade duckdb
import os
import psutil
import shutil
from pathlib import Path
import duckdb


def display_memory_used(msg):
    proc = psutil.Process(os.getpid())
    mem_info = proc.memory_info()
    print(f'Memory Usage ({msg}): {int(mem_info.rss * 1e-6)}MB')


filepaths = [
    './ddb_stream_issue_repro/a=1/b=1/data.data',
    './ddb_stream_issue_repro/a=1/b=2/data.data',
    './ddb_stream_issue_repro/a=2/b=1/data.data',
]

for filepath in filepaths:
    path = Path(filepath)
    path.parent.mkdir(parents=True, exist_ok=True)
    path.write_bytes(b'1' * 100_000_000)  # ~100 MB per file


display_memory_used('Before Query')

xx = duckdb.query('''
select *
from read_blob('./ddb_stream_issue_repro/**/data.data', hive_partitioning=true)
''')

display_memory_used('After Query, Before Fetchone')

xx.fetchone()

display_memory_used('After Fetchone')

shutil.rmtree('./ddb_stream_issue_repro')

And thank you for all the amazing work on duckdb!