# Presto 316 Documentation

## 12.32. Release 0.201

## General Changes

- Change grouped aggregations to use `IS NOT DISTINCT FROM` semantics rather than equality semantics. This fixes incorrect results and degraded performance when grouping on `NaN` floating point values, and adds support for grouping on structural types that contain nulls.
- Fix planning error when column names are reused in `ORDER BY` query.
- System memory pool is now unused by default and it will eventually be removed completely. All memory allocations will now be served from the general/user memory pool. The old behavior can be restored with the `deprecated.legacy-system-pool-enabled` config option.
- Improve performance and memory usage for queries using `row_number()` followed by a filter on the row numbers generated.
- Improve performance and memory usage for queries using `ORDER BY` followed by a `LIMIT`.
- Improve performance of queries that process structural types and contain joins, aggregations, or table writes.
- Add session property `prefer-partial-aggregation` to allow users to disable partial aggregations for queries that do not benefit.
- Add support for `current_user` (see Session Information).

## Security Changes

- Change rules in the Built-in System Access Control for enforcing matches between authentication credentials and a chosen username to allow more fine-grained control and ability to define superuser-like credentials.

## Hive Changes

- Replace ORC writer stripe minimum row configuration `hive.orc.writer.stripe-min-rows` with stripe minimum data size `hive.orc.writer.stripe-min-size`.
- Change ORC writer validation configuration `hive.orc.writer.validate` to switch to a sampling percentage `hive.orc.writer.validation-percentage`.
- Fix optimized ORC writer writing incorrect data of type `map` or `array`.
- Fix `SHOW PARTITIONS` and the `$partitions` table for tables that have null partition values.
- Fix impersonation for the simple HDFS authentication to use login user rather than current user.

## SPI Changes

- Support resource group selection based on resource estimates.