# Presto 316 Documentation

## 12.164. Release 0.80

## New Hive ORC Reader

We have added a new ORC reader implementation. The new reader supports vectorized reads, lazy loading, and predicate push down, all of which make the reader more efficient and typically reduces wall clock time for a query. Although the new reader has been heavily tested, it is an extensive rewrite of the Apache Hive ORC reader, and may have some latent issues. If you are seeing issues, you can disable the new reader on a per-query basis by setting the `<hive-catalog>.optimized_reader_enabled` session property, or you can disable the reader by default by setting the Hive catalog property `hive.optimized-reader.enabled=false`.

## Hive Changes

- The maximum retry time for the Hive S3 file system can be configured by setting `hive.s3.max-retry-time`.
- Fix Hive partition pruning for null keys (i.e. `__HIVE_DEFAULT_PARTITION__`).

## Cassandra Changes

- Update Cassandra driver to 2.1.0.
- Map Cassandra `TIMESTAMP` type to Presto `TIMESTAMP` type.

## “Big Query” Support

We’ve added experimental support for “big” queries. This provides a separate queue controlled by the following properties:

- `experimental.max-concurrent-big-queries`
- `experimental.max-queued-big-queries`

There are separate configuration options for queries that are submitted with the `experimental_big_query` session property:

- `experimental.big-query-initial-hash-partitions`
- `experimental.big-query-max-task-memory`

Queries submitted with this property will use hash distribution for all joins.

## Metadata-Only Query Optimization

We now support an optimization that rewrites aggregation queries that are insensitive to the cardinality of the input (e.g., `max()`, `min()`, `DISTINCT` aggregates) to execute against table metadata.

For example, if `key`, `key1` and `key2` are partition keys, the following queries will benefit:

    SELECT min(key), max(key) FROM t;

    SELECT DISTINCT key FROM t;

    SELECT count(DISTINCT key) FROM t;

    SELECT count(DISTINCT key + 5) FROM t;

    SELECT count(DISTINCT key) FROM (SELECT key FROM t ORDER BY 1 LIMIT 10);

    SELECT key1, count(DISTINCT key2) FROM t GROUP BY 1;

This optimization is turned off by default. To turn it on, add `optimizer.optimize-metadata-queries=true` to the coordinator config properties.

Warning

This optimization will cause queries to produce incorrect results if the connector allows partitions to contain no data. For example, the Hive connector will produce incorrect results if your Hive warehouse contains partitions without data.

## General Changes

- Add support implicit joins. The following syntax is now allowed:

      SELECT * FROM a, b WHERE a.id = b.id;

- Add property `task.verbose-stats` to enable verbose statistics collection for tasks. The default is `false`.

- Format binary data in the CLI as a hex dump.

- Add approximate numeric histogram function `numeric_histogram()`.

- Add `array_sort()` function.

- Add `map_keys()` and `map_values()` functions.

- Make `row_number()` completely streaming.

- Add property `task.max-partial-aggregation-memory` to configure the memory limit for the partial step of aggregations.

- Fix exception when processing queries with an `UNNEST` operation where the output was not used.

- Only show query progress in UI after the query has been fully scheduled.

- Add query execution visualization to the coordinator UI. It can be accessed via the query details page.