sfluor opened a new issue, #16517:
URL: https://github.com/apache/datafusion/issues/16517
### Describe the bug
When aggregating with `array_agg` using a limited memory pool we hit
ResourceExhausted errors in the `array_agg` accumulator.
After investigation this seems to come from over-accounting memory from
shared arrow buffers. We might keep a single value from a 8k rows arrow array
but will account the memory taken by the whole arrow array rather than just the
value we kept.
### To Reproduce
Run `datafusion-cli` with a memory limit `datafusion-cli -m 100m`
Create a table:
```sql
CREATE table logs AS
SELECT
('Chrome-' || (random() * 1000)::int) as user_agent,
(n.value % 40000 )::text as client_id,
(n.value % 21 ==0) as is_internal
FROM
generate_series(1, 300000) n;
```
Try running a simple `array_agg` query:
```
WITH
user_agents AS (
SELECT
DISTINCT user_agent, client_id
FROM
logs
WHERE
NOT is_internal
)
SELECT
client_id,
ARRAY_AGG(user_agent) AS user_agent
FROM
user_agents
GROUP BY
client_id
```
Fails with:
```
Resources exhausted: Additional allocation failed with top memory consumers
(across reservations) as:
GroupedHashAggregateStream[6] (array_agg(user_agents.user_agent))#44(can
spill: true) consumed 2.9 MB,
GroupedHashAggregateStream[2] (array_agg(user_agents.user_agent))#36(can
spill: true) consumed 2.2 MB,
GroupedHashAggregateStream[3] (array_agg(user_agents.user_agent))#38(can
spill: true) consumed 2.2 MB.
Error: Failed to allocate additional 609.0 MB for
GroupedHashAggregateStream[0] (array_agg(user_agents.user_agent)) with 0.0 B
already allocated for this reservation - 75.3 MB remain available
for the total pool
```
Further inspection (adding debug prints) shows that we over-report memory in
the `array_agg` `merge_batch`
[function](https://github.com/apache/datafusion/blob/main/datafusion/functions-aggregate/src/array_agg.rs#L344):
```
Addr:0x418911c18b0 Data type:Utf8 Length: 1, Size of first value: 9, Mem
size: 162362
[X][Utf8] 1 Size before: 56 + mem_size:162362 -> 9
[X][Utf8] 1 Size after: 162482
```
### Expected behavior
Query should succeed within the memory limits
### Additional context
Similar to: https://github.com/apache/datafusion/issues/16055
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