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# Integrating Rust libraries
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Rust library integration will be described based on BLAKE3 hash-function integration.
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---
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---
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# Kerberos
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Existing and properly configured ClickHouse users can be authenticated via Kerberos authentication protocol.
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---
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---
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# LDAP
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LDAP server can be used to authenticate ClickHouse users. There are two different approaches for doing this:
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---
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---
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# SSL X.509 certificate authentication
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[SSL 'strict' option](../server-configuration-parameters/settings.md#server_configuration_parameters-openssl) enables mandatory certificate validation for the incoming connections. In this case, only connections with trusted certificates can be established. Connections with untrusted certificates will be rejected. Thus, certificate validation allows to uniquely authenticate an incoming connection. `Common Name` field of the certificate is used to identify connected user. This allows to associate multiple certificates with the same user. Additionally, reissuing and revoking of the certificates does not affect the ClickHouse configuration.
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---
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---
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# Memory overcommit
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Memory overcommit is an experimental technique intended to allow to set more flexible memory limits for queries.
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---
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---
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# MergeTree tables settings
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The values of `merge_tree` settings (for all MergeTree tables) can be viewed in the table `system.merge_tree_settings`, they can be overridden in `config.xml` in the `merge_tree` section, or set in the `SETTINGS` section of each table.
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---
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---
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# asynchronous_metric_log
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Contains the historical values for `system.asynchronous_metrics`, which are saved once per minute. Enabled by default.
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# asynchronous_metrics
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Contains metrics that are calculated periodically in the background. For example, the amount of RAM in use.
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---
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# clusters
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Contains information about clusters available in the config file and the servers in them.
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---
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# columns
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Contains information about columns in all the tables.
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# contributors
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Contains information about contributors. The order is random at query execution time.
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# crash_log
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Contains information about stack traces for fatal errors. The table does not exist in the database by default, it is created only when fatal errors occur.
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---
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# current_roles
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Contains active roles of a current user. `SET ROLE` changes the contents of this table.
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# data_skipping_indices
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Contains information about existing data skipping indices in all the tables.
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---
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---
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# data_type_families
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Contains information about supported [data types](../../sql-reference/data-types/index.md).
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# databases
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Contains information about the databases that are available to the current user.
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# detached_parts
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Contains information about detached parts of [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables. The `reason` column specifies why the part was detached.
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# dictionaries
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Contains information about [external dictionaries](../../sql-reference/dictionaries/external-dictionaries/external-dicts.md).
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# disks
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Contains information about disks defined in the [server configuration](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-multiple-volumes_configure).
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---
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# distributed_ddl_queue
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Contains information about [distributed ddl queries (ON CLUSTER clause)](../../sql-reference/distributed-ddl.md) that were executed on a cluster.
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# distribution_queue
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Contains information about local files that are in the queue to be sent to the shards. These local files contain new parts that are created by inserting new data into the Distributed table in asynchronous mode.
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# enabled_roles
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Contains all active roles at the moment, including current role of the current user and granted roles for current role.
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# errors
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Contains error codes with the number of times they have been triggered.
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# events
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Contains information about the number of events that have occurred in the system. For example, in the table, you can find how many `SELECT` queries were processed since the ClickHouse server started.
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# functions
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Contains information about normal and aggregate functions.
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# grants
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Privileges granted to ClickHouse user accounts.
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# graphite_retentions
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Contains information about parameters [graphite_rollup](../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-graphite) which are used in tables with [\*GraphiteMergeTree](../../engines/table-engines/mergetree-family/graphitemergetree.md) engines.
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# INFORMATION_SCHEMA
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`INFORMATION_SCHEMA` (`information_schema`) is a system database that contains views. Using these views, you can get information about the metadata of database objects. These views read data from the columns of the [system.columns](../../operations/system-tables/columns.md), [system.databases](../../operations/system-tables/databases.md) and [system.tables](../../operations/system-tables/tables.md) system tables.
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# licenses
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Сontains licenses of third-party libraries that are located in the [contrib](https://github.com/ClickHouse/ClickHouse/tree/master/contrib) directory of ClickHouse sources.
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# merge_tree_settings
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Contains information about settings for `MergeTree` tables.
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# merges
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Contains information about merges and part mutations currently in process for tables in the MergeTree family.
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# metric_log
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Contains history of metrics values from tables `system.metrics` and `system.events`, periodically flushed to disk.
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# metrics
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Contains metrics which can be calculated instantly, or have a current value. For example, the number of simultaneously processed queries or the current replica delay. This table is always up to date.
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# mutations
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The table contains information about [mutations](../../sql-reference/statements/alter/index.md#mutations) of [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables and their progress. Each mutation command is represented by a single row.
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# numbers
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This table contains a single UInt64 column named `number` that contains almost all the natural numbers starting from zero.
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# numbers_mt
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The same as [system.numbers](../../operations/system-tables/numbers.md) but reads are parallelized. The numbers can be returned in any order.
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# one
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This table contains a single row with a single `dummy` UInt8 column containing the value 0.
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# opentelemetry_span_log
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Contains information about [trace spans](https://opentracing.io/docs/overview/spans/) for executed queries.
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# part_log
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The `system.part_log` table is created only if the [part_log](../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-part-log) server setting is specified.
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# parts {#system_tables-parts}
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Contains information about parts of [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables.
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# parts_columns
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Contains information about parts and columns of [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables.
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# processes
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This system table is used for implementing the `SHOW PROCESSLIST` query.
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# query_log
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Contains information about executed queries, for example, start time, duration of processing, error messages.
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# query_thread_log
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Contains information about threads that execute queries, for example, thread name, thread start time, duration of query processing.
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# query_views_log
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Contains information about the dependent views executed when running a query, for example, the view type or the execution time.
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# quota_limits
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Contains information about maximums for all intervals of all quotas. Any number of rows or zero can correspond to one quota.
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# quota_usage
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Quota usage by the current user: how much is used and how much is left.
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# quotas
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Contains information about [quotas](../../operations/system-tables/quotas.md).
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# quotas_usage
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Quota usage by all users.
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# replicas
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Contains information and status for replicated tables residing on the local server.
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# replicated_fetches
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Contains information about currently running background fetches.
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# replication_queue
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Contains information about tasks from replication queues stored in ClickHouse Keeper, or ZooKeeper, for tables in the `ReplicatedMergeTree` family.
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# role_grants
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Contains the role grants for users and roles. To add entries to this table, use `GRANT role TO user`.
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# roles
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Contains information about configured [roles](../../operations/access-rights.md#role-management).
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# row_policies
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Contains filters for one particular table, as well as a list of roles and/or users which should use this row policy.
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# session_log
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Contains information about all successful and failed login and logout events.
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# settings
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Contains information about session settings for current user.
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# settings_profile_elements
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Describes the content of the settings profile:
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# settings_profiles
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Contains properties of configured setting profiles.
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# stack_trace
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Contains stack traces of all server threads. Allows developers to introspect the server state.
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# storage_policies
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Contains information about storage policies and volumes defined in the [server configuration](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-multiple-volumes_configure).
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# table_engines
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Contains description of table engines supported by server and their feature support information.
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# tables
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Contains metadata of each table that the server knows about.
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# text_log
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Contains logging entries. The logging level which goes to this table can be limited to the `text_log.level` server setting.
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# time_zones
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Contains a list of time zones that are supported by the ClickHouse server. This list of timezones might vary depending on the version of ClickHouse.
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# trace_log
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Contains stack traces collected by the sampling query profiler.
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# users
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Contains a list of [user accounts](../../operations/access-rights.md#user-account-management) configured at the server.
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# zookeeper
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The table does not exist unless ClickHouse Keeper or ZooKeeper is configured. The `system.zookeeper` table exposes data from the Keeper cluster defined in the config.
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# zookeeper_log
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This table contains information about the parameters of the request to the ZooKeeper server and the response from it.
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# clickhouse-compressor
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# clickhouse-format
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Allows formatting input queries.
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# clickhouse-obfuscator
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A simple tool for table data obfuscation.
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---
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# clickhouse-odbc-bridge
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Simple HTTP-server which works like a proxy for ODBC driver. The main motivation
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---
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---
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# SimpleAggregateFunction
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`SimpleAggregateFunction(name, types_of_arguments…)` data type stores current value of the aggregate function, and does not store its full state as [`AggregateFunction`](../../sql-reference/data-types/aggregatefunction.md) does. This optimization can be applied to functions for which the following property holds: the result of applying a function `f` to a row set `S1 UNION ALL S2` can be obtained by applying `f` to parts of the row set separately, and then again applying `f` to the results: `f(S1 UNION ALL S2) = f(f(S1) UNION ALL f(S2))`. This property guarantees that partial aggregation results are enough to compute the combined one, so we do not have to store and process any extra data.
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# Distance functions
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## L1Norm
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# EXISTS
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The `EXISTS` operator checks how many records are in the result of a subquery. If it is empty, then the operator returns `0`. Otherwise, it returns `1`.
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# IN Operators
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The `IN`, `NOT IN`, `GLOBAL IN`, and `GLOBAL NOT IN` operators are covered separately, since their functionality is quite rich.
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