* Added the `system.macros` table and auto updating of macros when the config file is changed.
* Added the `SYSTEM RELOAD CONFIG` query.
* Added the `maxIntersections(left_col, right_col)` aggregate function, which returns the maximum number of simultaneously intersecting intervals `[left; right]`. The `maxIntersectionsPosition(left, right)` function returns the beginning of the "maximum" interval. ([Michael Furmur](https://github.com/yandex/ClickHouse/pull/2012)).
## Improvements:
* When inserting data in a `Replicated` table, fewer requests are made to `ZooKeeper` (and most of the user-level errors have disappeared from the `ZooKeeper` log).
* Added the ability to create aliases for sets. Example: `WITH (1, 2, 3) AS set SELECT number IN set FROM system.numbers LIMIT 10`.
## Bug fixes:
* Fixed the `Illegal PREWHERE` error when reading from `Merge` tables over `Distributed` tables.
* Added fixes that allow you to run `clickhouse-server` in IPv4-only Docker containers.
* Fixed a race condition when reading from system `system.parts_columns` tables.
* Removed double buffering during a synchronous insert to a `Distributed` table, which could have caused the connection to timeout.
* Fixed a bug that caused excessively long waits for an unavailable replica before beginning a `SELECT` query.
* Fixed incorrect dates in the `system.parts` table.
* Fixed a bug that made it impossible to insert data in a `Replicated` table if `chroot` was non-empty in the configuration of the `ZooKeeper` cluster.
* Fixed the vertical merging algorithm for an empty `ORDER BY` table.
* Restored the ability to use dictionaries in queries to remote tables, even if these dictionaries are not present on the requestor server. This functionality was lost in release 1.1.54362.
* Restored the behavior for queries like `SELECT * FROM remote('server2', default.table) WHERE col IN (SELECT col2 FROM default.table)` when the right side argument of the `IN` should use a remote `default.table` instead of a local one. This behavior was broken in version 1.1.54358.
* Removed extraneous error-level logging of `Not found column ... in block`.
* Aggregation without `GROUP BY` for an empty set (such as `SELECT count(*) FROM table WHERE 0`) now returns a result with one row with null values for aggregate functions, in compliance with the SQL standard. To restore the old behavior (return an empty result), set `empty_result_for_aggregation_by_empty_set` to 1.
* Added type conversion for `UNION ALL`. Different alias names are allowed in `SELECT` positions in `UNION ALL`, in compliance with the SQL standard.
* Arbitrary expressions are supported in `LIMIT BY` sections. Previously, it was only possible to use columns resulting from `SELECT`.
* An index of `MergeTree` tables is used when `IN` is applied to a tuple of expressions from the columns of the primary key. Example: `WHERE (UserID, EventDate) IN ((123, '2000-01-01'), ...)` (Anastasiya Tsarkova).
* Added the `clickhouse-copier` tool for copying between clusters and resharding data (beta).
* Added consistent hashing functions: `yandexConsistentHash`, `jumpConsistentHash`, `sumburConsistentHash`. They can be used as a sharding key in order to reduce the amount of network traffic during subsequent reshardings.
* Added the `arrayCumSum` function (Javi Santana).
* Added the `parseDateTimeBestEffort`, `parseDateTimeBestEffortOrZero`, and `parseDateTimeBestEffortOrNull`functions to read the DateTime from a string containing text in a wide variety of possible formats.
* It is now possible to change the logging settings without restarting the server.
* Added the `cluster` table function. Example: `cluster(cluster_name, db, table)`. The `remote` table function can accept the cluster name as the first argument, if it is specified as an identifier.
* Added the `create_table_query` and `engine_full` virtual columns to the `system.tables`table . The `metadata_modification_time` column is virtual.
* Added the `data_path` and `metadata_path` columns to `system.tables` and` system.databases` tables, and added the `path` column to the `system.parts` and `system.parts_columns` tables.
* Added additional information about merges in the `system.part_log` table.
* An arbitrary partitioning key can be used for the `system.query_log` table (Kirill Shvakov).
* The `SHOW TABLES` query now also shows temporary tables. Added temporary tables and the `is_temporary` column to `system.tables` (zhang2014).
* Added the `DROP TEMPORARY TABLE` query (zhang2014).
* Support for `SHOW CREATE TABLE` for temporary tables (zhang2014).
* Added the `system_profile` configuration parameter for the settings used by internal processes.
* Support for loading `object_id` as an attribute in `MongoDB` dictionaries (Pavel Litvinenko).
* Reading `null` as the default value when loading data for an external dictionary with the `MongoDB` source (Pavel Litvinenko).
* Reading `DateTime` values in the `Values` format from a Unix timestamp without single quotes.
* Failover is supported in `remote` table functions for cases when some of the replicas are missing the requested table.
* Configuration settings can be overridden in the command line when you run `clickhouse-server`. Example: `clickhouse-server -- --logger.level=information`.
* Implemented the `empty` function from a `FixedString` argument: the function returns 1 if the string consists entirely of null bytes (zhang2014).
* Added the `listen_try`configuration parameter for listening to at least one of the listen addresses without quitting, if some of the addresses can't be listened to (useful for systems with disabled support for IPv4 or IPv6).
* Added the `VersionedCollapsingMergeTree` table engine.
* Support for rows and arbitrary numeric types for the `library` dictionary source.
*`MergeTree` tables can be used without a primary key (you need to specify `ORDER BY tuple()`).
* A `Nullable` type can be `CAST` to a non-`Nullable` type if the argument is not `NULL`.
*`RENAME TABLE` can be performed for `VIEW`.
* Added the `odbc_default_field_size` option, which allows you to extend the maximum size of the value loaded from an ODBC source (by default, it is 1024).
## Improvements:
* Limits and quotas on the result are no longer applied to intermediate data for `INSERT SELECT` queries or for `SELECT` subqueries.
* Fewer false triggers of `force_restore_data` when checking the status of `Replicated` tables when the server starts.
* Added the `allow_distributed_ddl` option.
* Nondeterministic functions are not allowed in expressions for `MergeTree` table keys.
* Files with substitutions from `config.d` directories are loaded in alphabetical order.
* Improved performance of the `arrayElement` function in the case of a constant multidimensional array with an empty array as one of the elements. Example: `[[1], []][x]`.
* The server starts faster now when using configuration files with very large substitutions (for instance, very large lists of IP networks).
* When running a query, table valued functions run once. Previously, `remote` and `mysql` table valued functions performed the same query twice to retrieve the table structure from a remote server.
* The `MkDocs` documentation generator is used.
* When you try to delete a table column that `DEFAULT`/`MATERIALIZED` expressions of other columns depend on, an exception is thrown (zhang2014).
* Added the ability to parse an empty line in text formats as the number 0 for `Float` data types. This feature was previously available but was lost in release 1.1.54342.
*`Enum` values can be used in `min`, `max`, `sum` and some other functions. In these cases, it uses the corresponding numeric values. This feature was previously available but was lost in the release 1.1.54337.
* Added `max_expanded_ast_elements` to restrict the size of the AST after recursively expanding aliases.
## Bug fixes:
* Fixed cases when unnecessary columns were removed from subqueries in error, or not removed from subqueries containing `UNION ALL`.
* Fixed a bug in merges for `ReplacingMergeTree` tables.
* Fixed synchronous insertions in `Distributed` tables (`insert_distributed_sync = 1`).
* Fixed segfault for certain uses of `FULL` and `RIGHT JOIN` with duplicate columns in subqueries.
* Fixed the order of the `source` and `last_exception` columns in the `system.dictionaries` table.
* Fixed a bug when the `DROP DATABASE` query did not delete the file with metadata.
* Fixed the `DROP DATABASE` query for `Dictionary` databases.
* Fixed the low precision of `uniqHLL12` and `uniqCombined` functions for cardinalities greater than 100 million items (Alex Bocharov).
* Fixed the calculation of implicit default values when necessary to simultaneously calculate default explicit expressions in `INSERT` queries (zhang2014).
* Fixed a rare case when a query to a `MergeTree` table couldn't finish (chenxing-xc).
* Fixed a crash that occurred when running a `CHECK` query for `Distributed` tables if all shards are local (chenxing.xc).
* Fixed a slight performance regression with functions that use regular expressions.
* Fixed a performance regression when creating multidimensional arrays from complex expressions.
* Fixed a bug that could cause an extra `FORMAT` section to appear in an `.sql` file with metadata.
* Fixed a bug that caused the `max_table_size_to_drop` limit to apply when trying to delete a `MATERIALIZED VIEW` looking at an explicitly specified table.
* Fixed incompatibility with old clients (old clients were sometimes sent data with the `DateTime('timezone')` type, which they do not understand).
* Fixed a bug when reading `Nested` column elements of structures that were added using `ALTER` but that are empty for the old partitions, when the conditions for these columns moved to `PREWHERE`.
* Fixed a bug when filtering tables by virtual `_table` columns in queries to `Merge` tables.
* Fixed a bug when using `ALIAS` columns in `Distributed` tables.
* Fixed a bug that made dynamic compilation impossible for queries with aggregate functions from the `quantile` family.
* Fixed a race condition in the query execution pipeline that occurred in very rare cases when using `Merge` tables with a large number of tables, and when using `GLOBAL` subqueries.
* Fixed a crash when passing arrays of different sizes to an `arrayReduce` function when using aggregate functions from multiple arguments.
* Prohibited the use of queries with `UNION ALL` in a `MATERIALIZED VIEW`.
## Backward incompatible changes:
* Removed the `distributed_ddl_allow_replicated_alter` option. This behavior is enabled by default.
* Added macros support for defining cluster names in distributed DDL queries and constructors of Distributed tables: `CREATE TABLE distr ON CLUSTER '{cluster}' (...) ENGINE = Distributed('{cluster}', 'db', 'table')`.
* Fixed a regression in 1.1.54337: if the default user has readonly access, then the server refuses to start up with the message `Cannot create database in readonly mode`.
* Fixed a regression in 1.1.54337: on systems with `systemd`, logs are always written to syslog regardless of the configuration; the watchdog script still uses `init.d`.
* Fixed a regression in 1.1.54337: wrong default configuration in the Docker image.
* Fixed nondeterministic behaviour of GraphiteMergeTree (you can notice it in log messages `Data after merge is not byte-identical to data on another replicas`).
* Fixed a bug that may lead to inconsistent merges after OPTIMIZE query to Replicated tables (you may notice it in log messages `Part ... intersects previous part`).
* Added support for table functions in `DESCRIBE` and `INSERT` queries. Added support for subqueries in `DESCRIBE`. Examples: `DESC TABLE remote('host', default.hits)`; `DESC TABLE (SELECT 1)`; `INSERT INTO TABLE FUNCTION remote('host', default.hits)`. Support for `INSERT INTO TABLE` syntax in addition to `INSERT INTO`.
* Improved support for timezones. The `DateTime` data type can be annotated with the timezone that is used for parsing and formatting in text formats. Example: `DateTime('Europe/Moscow')`. When timezones are specified in functions for DateTime arguments, the return type will track the timezone, and the value will be displayed as expected.
* Added the functions `toTimeZone`, `timeDiff`, `toQuarter`, `toRelativeQuarterNum`. The `toRelativeHour`/`Minute`/`Second` functions can take a value of type `Date` as an argument. The name of the `now` function has been made case-insensitive.
* Added the `toStartOfFifteenMinutes` function (Kirill Shvakov).
* Added the `clickhouse format` tool for formatting queries.
* Added the `format_schema_path` configuration parameter (Marek Vavruša). It is used for specifying a schema in `Cap'n'Proto` format. Schema files can be located only in the specified directory.
* Added a column with documentation for the `system.settings` table (Kirill Shvakov).
* Added the `system.parts_columns` table with information about column sizes in each data part of `MergeTree` tables.
* Added the `system.models` table with information about loaded `CatBoost` machine learning models.
* Added the `mysql` and `odbc` table functions along with the corresponding `MySQL` and `ODBC` table engines for working with foreign databases. This feature is in the beta stage.
* Added the possibility to pass an argument of type `AggregateFunction` for the `groupArray` aggregate function (so you can create an array of states of some aggregate function).
* Removed restrictions on various combinations of aggregate function combinators. For example, you can use `avgForEachIf` as well as `avgIfForEach` aggregate functions, which have different behaviors.
* The `-ForEach` aggregate function combinator is extended for the case of aggregate functions of multiple arguments.
* Added support for aggregate functions of `Nullable` arguments even for cases when the function returns a non-`Nullable` result (added with the contribution of Silviu Caragea). Examples: `groupArray`, `groupUniqArray`, `topK`.
* Added the `max_client_network_bandwidth` command line parameter for `clickhouse-client` (Kirill Shvakov).
* Users with the `readonly = 2` setting are allowed to work with TEMPORARY tables (CREATE, DROP, INSERT...) (Kirill Shvakov).
* Added support for using multiple consumers with the `Kafka` engine. Extended configuration options for `Kafka` (Marek Vavruša).
* Added the `intExp2` and `intExp10` functions.
* Added the `sumKahan` aggregate function (computationally stable summation of floating point numbers).
* Added to*Number*OrNull functions, where *Number* is a numeric type.
* Added support for the `WITH` clause for an `INSERT SELECT` query (by zhang2014).
* Added the settings `http_connection_timeout`, `http_send_timeout`, and `http_receive_timeout`. In particular, these settings are used for downloading data parts for replication. Changing these settings allows for faster failover if the network is overloaded.
* Added support for the `ALTER` query for tables of type `Null` (Anastasiya Tsarkova). Tables of type `Null` are often used with materialized views.
* The `reinterpretAsString` function is extended for all data types that are stored contiguously in memory.
* Added the `--silent` option for the `clickhouse-local` tool. It suppresses printing query execution info in stderr.
* Added support for reading values of type `Date` from text in a format where the month and/or day of the month is specified using a single digit instead of two digits (Amos Bird).
* Lowered memory usage for `JOIN` in the case when the left and right parts have columns with identical names that are not contained in `USING`.
* Improved performance of `varSamp`, `varPop`, `stddevSamp`, `stddevPop`, `covarSamp`, `covarPop`, and `corr` aggregate functions by reducing computational stability. The old functions are available under the names: `varSampStable`, `varPopStable`, `stddevSampStable`, `stddevPopStable`, `covarSampStable`, `covarPopStable`, `corrStable`.
* Fixed data deduplication after running a `DROP PARTITION` query. In the previous version, dropping a partition and INSERTing the same data again was not working because INSERTed blocks were considered duplicates.
* Fixed a bug that could lead to incorrect interpretation of the `WHERE` clause for `CREATE MATERIALIZED VIEW` queries with `POPULATE`.
* Fixed a bug in using the `root_path` parameter in the `zookeeper_servers` configuration.
* Fixed unexpected results of passing the `Date` argument to `toStartOfDay`.
* Added missing support for the `UUID` data type for `DISTINCT`, `JOIN`, and `uniq` aggregate functions and external dictionaries (Evgeniy Ivanov). Support for `UUID` is still incomplete.
* Support for `Nullable` types is completely reworked. Fixed bugs that may lead to a server crash. Fixed almost all other bugs related to NULL support: incorrect type conversions in INSERT SELECT, insufficient support for Nullable in HAVING and PREWHERE, `join_use_nulls` mode, Nullable types as arguments of OR operator, etc.
* Fixed various bugs related to internal semantics of data types. Examples: unnecessary summing of `Enum` type fields in `SummingMergeTree`; alignment of `Enum` types in Pretty formats, etc.
* Stricter checks for allowed combinations of composite columns. Fixed several bugs that could lead to a server crash.
* Fixed the overflow when specifying a very large parameter for the `FixedString` data type.
* Fixed a bug in the `topK` aggregate function in a generic case.
* Added the missing check for equality of array sizes in arguments of n-ary variants of aggregate functions with an `-Array` combinator.
* Fixed the `--pager` option for `clickhouse-client` (by ks1322).
* Fixed the precision of the `exp10` function.
* Fixed the behavior of the `visitParamExtract` function for better compliance with documentation.
* Fixed the crash when incorrect data types are specified.
* Fixed the behavior of `DISTINCT` in the case when all columns are constants.
* Fixed query formatting in the case of using the `tupleElement` function with a complex constant expression as the tuple element index.
* Fixed the `Dictionary` table engine for dictionaries of type `range_hashed`.
* Fixed a bug that leads to excessive rows in the result of `FULL` and `RIGHT JOIN` (Amos Bird).
* Fixed a server crash when creating and removing temporary files in `config.d` directories during config reload.
* Fixed the `SYSTEM DROP DNS CACHE` query: the cache was flushed but addresses of cluster nodes were not updated.
* Fixed the behavior of `MATERIALIZED VIEW` after executing `DETACH TABLE` for the table under the view (Marek Vavruša).
* A single build is used for different OS versions. Packages and binaries have been made compatible with a wide range of Linux systems.
* Added the `clickhouse-test` package. It can be used to run functional tests.
* The source tarball can now be published to the repository. It can be used to reproduce the build without using GitHub.
* Added limited integration with Travis CI. Due to limits on build time in Travis, only the debug build is tested and a limited subset of tests are run.
* Added support for `Cap'n'Proto` in the default build.
* Changed the format of documentation sources from `Restructured Text` to `Markdown`.
* Added support for `systemd` (Vladimir Smirnov). It is disabled by default due to incompatibility with some OS images and can be enabled manually.
* For dynamic code generation, `clang` and `lld` are embedded into the `clickhouse` binary. They can also be invoked as `clickhouse clang` and `clickhouse lld`.
* The format for marks in `Log` type tables that contain `Nullable` columns was changed in a backward incompatible way. If you have these tables, you should convert them to the `TinyLog` type before starting up the new server version. To do this, replace `ENGINE = Log` with `ENGINE = TinyLog` in the corresponding `.sql` file in the `metadata` directory. If your table doesn't have `Nullable` columns or if the type of your table is not `Log`, then you don't need to do anything.
* Removed the `experimental_allow_extended_storage_definition_syntax` setting. Now this feature is enabled by default.
* Removed the `FROM ARRAY JOIN arr` syntax when ARRAY JOIN is specified directly after FROM with no table (Amos Bird).
* Removed the `BlockTabSeparated` format that was used solely for demonstration purposes.
* Changed the serialization format of intermediate states of the aggregate functions `varSamp`, `varPop`, `stddevSamp`, `stddevPop`, `covarSamp`, `covarPop`, and `corr`. If you have stored states of these aggregate functions in tables (using the AggregateFunction data type or materialized views with corresponding states), please write to clickhouse-feedback@yandex-team.com.
* In previous server versions there was an undocumented feature: if an aggregate function depends on parameters, you can still specify it without parameters in the AggregateFunction data type. Example: `AggregateFunction(quantiles, UInt64)` instead of `AggregateFunction(quantiles(0.5, 0.9), UInt64)`. This feature was lost. Although it was undocumented, we plan to support it again in future releases.
* When doing a rolling update on a cluster, at the point when some of the replicas are running the old version of ClickHouse and some are running the new version, replication is temporarily stopped and the message `unknown parameter 'shard'` appears in the log. Replication will continue after all replicas of the cluster are updated.
* If you have different ClickHouse versions on the cluster, you can get incorrect results for distributed queries with the aggregate functions `varSamp`, `varPop`, `stddevSamp`, `stddevPop`, `covarSamp`, `covarPop`, and `corr`. You should update all cluster nodes.
* Fixed bug with possible race condition in replication that could lead to data loss. This issue affects versions 1.1.54310 and 1.1.54318. If you use one of these versions with Replicated tables, the update is strongly recommended. This issue shows in logs in Warning messages like `Part ... from own log doesn't exist.` The issue is relevant even if you don't see these messages in logs.
* The processing logic for Nested columns with names ending in -Map in a SummingMergeTree table was extracted to the sumMap aggregate function. You can now specify such columns explicitly.
* Fixed hangups when synchronously inserting into a Distributed table.
* Fixed nonatomic adding and removing of parts in Replicated tables.
* Data inserted into a materialized view is not subjected to unnecessary deduplication.
* Executing a query to a Distributed table for which the local replica is lagging and remote replicas are unavailable does not result in an error anymore.
* Users don't need access permissions to the `default` database to create temporary tables anymore.
* Fixed crashing when specifying the Array type without arguments.
* Fixed hangups when the disk volume containing server logs is full.
* Fixed an overflow in the `toRelativeWeekNum` function for the first week of the Unix epoch.
## Build improvements:
* Several third-party libraries (notably Poco) were updated and converted to git submodules.
* Added the `pointInPolygon` function for working with coordinates on a coordinate plane.
* Added the `sumMap` aggregate function for calculating the sum of arrays, similar to `SummingMergeTree`.
* Added the `trunc` function. Improved performance of the rounding functions (`round`, `floor`, `ceil`, `roundToExp2`) and corrected the logic of how they work. Changed the logic of the `roundToExp2` function for fractions and negative numbers.
* The ClickHouse executable file is now less dependent on the libc version. The same ClickHouse executable file can run on a wide variety of Linux systems. Note: There is still a dependency when using compiled queries (with the setting `compile = 1`, which is not used by default).
*`SYSTEM` queries for server administration: `SYSTEM RELOAD DICTIONARY`, `SYSTEM RELOAD DICTIONARIES`, `SYSTEM DROP DNS CACHE`, `SYSTEM SHUTDOWN`, `SYSTEM KILL`.
* Added functions for working with arrays: `concat`, `arraySlice`, `arrayPushBack`, `arrayPushFront`, `arrayPopBack`, `arrayPopFront`.
* Added the `root` and `identity` parameters for the ZooKeeper configuration. This allows you to isolate individual users on the same ZooKeeper cluster.
* Added the aggregate functions `groupBitAnd`, `groupBitOr`, and `groupBitXor` (for compatibility, they can also be accessed with the names `BIT_AND`, `BIT_OR`, and `BIT_XOR`).
* Improved the process for deleting old nodes in ZooKeeper. Previously, old nodes sometimes didn't get deleted if there were very frequent inserts, which caused the server to be slow to shut down, among other things.
* Fixed randomization when choosing hosts for the connection to ZooKeeper.
* Fixed the exclusion of lagging replicas in distributed queries if the replica is localhost.
* Fixed an error where a data part in a `ReplicatedMergeTree` table could be broken after running `ALTER MODIFY` on an element in a `Nested` structure.
* You can use `libc++` instead of `libstdc++` for builds on Linux.
* Added instructions for using static code analysis tools: `Coverity`, `clang-tidy`, and `cppcheck`.
## Please note when upgrading:
* There is now a higher default value for the MergeTree setting `max_bytes_to_merge_at_max_space_in_pool` (the maximum total size of data parts to merge, in bytes): it has increased from 100 GiB to 150 GiB. This might result in large merges running after the server upgrade, which could cause an increased load on the disk subsystem. If the free space available on the server is less than twice the total amount of the merges that are running, this will cause all other merges to stop running, including merges of small data parts. As a result, INSERT requests will fail with the message "Merges are processing significantly slower than inserts." Use the `SELECT * FROM system.merges` request to monitor the situation. You can also check the `DiskSpaceReservedForMerge` metric in the `system.metrics` table, or in Graphite. You don't need to do anything to fix this, since the issue will resolve itself once the large merges finish. If you find this unacceptable, you can restore the previous value for the `max_bytes_to_merge_at_max_space_in_pool` setting (to do this, go to the `<merge_tree>` section in config.xml, set `<max_bytes_to_merge_at_max_space_in_pool>107374182400</max_bytes_to_merge_at_max_space_in_pool>` and restart the server).
* You can use an optional WITH clause in a SELECT query. Example query: `WITH 1+1 AS a SELECT a, a*a`
* INSERT can be performed synchronously in a Distributed table: OK is returned only after all the data is saved on all the shards. This is activated by the setting insert_distributed_sync=1.
* Added the UUID data type for working with 16-byte identifiers.
* Added aliases of CHAR, FLOAT and other types for compatibility with the Tableau.
* Added the functions toYYYYMM, toYYYYMMDD, and toYYYYMMDDhhmmss for converting time into numbers.
* You can use IP addresses (together with the hostname) to identify servers for clustered DDL queries.
* Added support for non-constant arguments and negative offsets in the function `substring(str, pos, len).`
* Added the max_size parameter for the `groupArray(max_size)(column)` aggregate function, and optimized its performance.
* Improved error messages for queries with invalid syntax.
* Significantly reduced memory consumption and improved performance when merging large sections of MergeTree data.
* Significantly increased the performance of data merges for the ReplacingMergeTree engine.
* Improved performance for asynchronous inserts from a Distributed table by batching multiple source inserts. To enable this functionality, use the setting distributed_directory_monitor_batch_inserts=1.
## Backward incompatible changes:
* Changed the binary format of aggregate states of `groupArray(array_column)` functions for arrays.
## Complete list of changes:
* Added the `output_format_json_quote_denormals` setting, which enables outputting nan and inf values in JSON format.
* Optimized thread allocation when reading from a Distributed table.
* Settings can be modified in readonly mode if the value doesn't change.
* Added the ability to read fractional granules of the MergeTree engine in order to meet restrictions on the block size specified in the preferred_block_size_bytes setting. The purpose is to reduce the consumption of RAM and increase cache locality when processing queries from tables with large columns.
* Efficient use of indexes that contain expressions like `toStartOfHour(x)` for conditions like `toStartOfHour(x) op сonstexpr.`
* Added new settings for MergeTree engines (the merge_tree section in config.xml):
- replicated_deduplication_window_seconds sets the size of deduplication window in seconds for Replicated tables.
- cleanup_delay_period sets how often to start cleanup to remove outdated data.
- replicated_can_become_leader can prevent a replica from becoming the leader (and assigning merges).
* Accelerated cleanup to remove outdated data from ZooKeeper.
* Multiple improvements and fixes for clustered DDL queries. Of particular interest is the new setting distributed_ddl_task_timeout, which limits the time to wait for a response from the servers in the cluster.
* Improved display of stack traces in the server logs.
* Added the "none" value for the compression method.
* You can use multiple dictionaries_config sections in config.xml.
* It is possible to connect to MySQL through a socket in the file system.
* The `system.parts` table has a new column with information about the size of marks, in bytes.
## Bug fixes:
* Distributed tables using a Merge table now work correctly for a SELECT query with a condition on the _table field.
* Fixed a rare race condition in ReplicatedMergeTree when checking data parts.
* Fixed possible freezing on "leader election" when starting a server.
* The max_replica_delay_for_distributed_queries setting was ignored when using a local replica of the data source. This has been fixed.
* Fixed incorrect behavior of `ALTER TABLE CLEAR COLUMN IN PARTITION` when attempting to clean a non-existing column.
* Fixed an exception in the multiIf function when using empty arrays or strings.
* Fixed excessive memory allocations when deserializing Native format.
* Fixed incorrect auto-update of Trie dictionaries.
* Fixed an exception when running queries with a GROUP BY clause from a Merge table when using SAMPLE.
* Fixed a crash of GROUP BY when using distributed_aggregation_memory_efficient=1.
* Now you can specify the database.table in the right side of IN and JOIN.
* Too many threads were used for parallel aggregation. This has been fixed.
* Fixed how the "if" function works with FixedString arguments.
* SELECT worked incorrectly from a Distributed table for shards with a weight of 0. This has been fixed.
* Crashes no longer occur when running `CREATE VIEW IF EXISTS.`
* Fixed incorrect behavior when input_format_skip_unknown_fields=1 is set and there are negative numbers.
* Fixed an infinite loop in the `dictGetHierarchy()` function if there is some invalid data in the dictionary.
* Fixed `Syntax error: unexpected (...)` errors when running distributed queries with subqueries in an IN or JOIN clause and Merge tables.
* Fixed the incorrect interpretation of a SELECT query from Dictionary tables.
* Fixed the "Cannot mremap" error when using arrays in IN and JOIN clauses with more than 2 billion elements.
* Fixed the failover for dictionaries with MySQL as the source.
## Improved workflow for developing and assembling ClickHouse:
* Builds can be assembled in Arcadia.
* You can use gcc 7 to compile ClickHouse.
* Parallel builds using ccache+distcc are faster now.
