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docs/integrations/data-ingestion/dbms/postgresql/postgres-vs-clickhouse.md

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While topologies with multiple shards and replicas are possible without a distributed table, these advanced deployments typically have their own routing infrastructure. We therefore assume deployments with more than one shard are using a Distributed table (distributed tables can be used with single shard deployments but are usually unnecessary).
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In this case, users should ensure consistent node routing is performed based on a property e.g. `session_id` or `user_id`. The settings [`prefer_localhost_replica=0`](/operations/settings/settings#prefer-localhost-replica), [`load_balancing=in_order`](/operations/settings/settings#load_balancing) should be [set in the query](/operations/settings/query-level). This will ensure any local replicas of shards are preferred, with replicas preferred as listed in the configuration otherwise - provided they have the same number of errors - failover will occur with random selection if errors are higher. [`load_balancing=nearest_hostname`](/operations/settings/settings#load_balancing) can also be used as an alternative for this deterministic shard selection.
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In this case, users should ensure consistent node routing is performed based on a property e.g. `session_id` or `user_id`. The settings [`prefer_localhost_replica=0`](/operations/settings/settings#prefer_localhost_replica), [`load_balancing=in_order`](/operations/settings/settings#load_balancing) should be [set in the query](/operations/settings/query-level). This will ensure any local replicas of shards are preferred, with replicas preferred as listed in the configuration otherwise - provided they have the same number of errors - failover will occur with random selection if errors are higher. [`load_balancing=nearest_hostname`](/operations/settings/settings#load_balancing) can also be used as an alternative for this deterministic shard selection.
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> When creating a Distributed table, users will specify a cluster. This cluster definition, specified in config.xml, will list the shards (and their replicas) - thus allowing users to control the order in which they are used from each node. Using this, users can ensure selection is deterministic.
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docs/integrations/data-ingestion/s3/index.md

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SETTINGS index_granularity = 8192
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```
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Note the use of [partitioning](/engines/table-engines/mergetree-family/custom-partitioning-key.md/#custom-partitioning-key) on the `pickup_date` field. Usually a partition key is for data management, but later on we will use this key to parallelize writes to S3.
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Note the use of [partitioning](/engines/table-engines/mergetree-family/custom-partitioning-key) on the `pickup_date` field. Usually a partition key is for data management, but later on we will use this key to parallelize writes to S3.
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Each entry in our taxi dataset contains a taxi trip. This anonymized data consists of 20M records compressed in the S3 bucket https://datasets-documentation.s3.eu-west-3.amazonaws.com/ under the folder **nyc-taxi**. The data is in the TSV format with approximately 1M rows per file.
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The following notes cover the implementation of S3 interactions with ClickHouse. Whilst generally only informative, it may help the readers when [Optimizing for Performance](#s3-optimizing-performance):
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* By default, the maximum number of query processing threads used by any stage of the query processing pipeline is equal to the number of cores. Some stages are more parallelizable than others, so this value provides an upper bound. Multiple query stages may execute at once since data is streamed from the disk. The exact number of threads used for a query may thus exceed this. Modify through the setting [max_threads](/operations/settings/settings.md/#settings-max_threads).
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* By default, the maximum number of query processing threads used by any stage of the query processing pipeline is equal to the number of cores. Some stages are more parallelizable than others, so this value provides an upper bound. Multiple query stages may execute at once since data is streamed from the disk. The exact number of threads used for a query may thus exceed this. Modify through the setting [max_threads](/operations/settings/settings#max_threads).
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* Reads on S3 are asynchronous by default. This behavior is determined by setting `remote_filesystem_read_method`, set to the value `threadpool` by default. When serving a request, ClickHouse reads granules in stripes. Each of these stripes potentially contain many columns. A thread will read the columns for their granules one by one. Rather than doing this synchronously, a prefetch is made for all columns before waiting for the data. This offers significant performance improvements over synchronous waits on each column. Users will not need to change this setting in most cases - see [Optimizing for Performance](#s3-optimizing-performance).
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* Writes are performed in parallel, with a maximum of 100 concurrent file writing threads. `max_insert_delayed_streams_for_parallel_write`, which has a default value of 1000, controls the number of S3 blobs written in parallel. Since a buffer is required for each file being written (~1MB), this effectively limits the memory consumption of an INSERT. It may be appropriate to lower this value in low server memory scenarios.
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docs/integrations/language-clients/python/index.md

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in a similar form.) The size of a block returned from a query is governed by two user settings that can be set at several levels
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(user profile, user, session, or query). They are:
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- [max_block_size](/operations/settings/settings.md/#setting-max_block_size) -- Limit on the size of the block in rows. Default 65536.
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- [preferred_block_size_bytes](/operations/settings/settings.md/#preferred-block-size-bytes) -- Soft limit on the size of the block in bytes. Default 1,000,0000.
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- [max_block_size](/operations/settings/settings#max_block_size) -- Limit on the size of the block in rows. Default 65536.
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- [preferred_block_size_bytes](/operations/settings/settings#preferred_block_size_bytes) -- Soft limit on the size of the block in bytes. Default 1,000,0000.
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Regardless of the `preferred_block_size_setting`, each block will never be more than `max_block_size` rows. Depending on the
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type of query, the actual blocks returned can be of any size. For example, queries to a distributed table covering many shards

images/knowledgebase/async_vs_optimize_read_in_order.md

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# Synchronous data reading
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Normally the [max_threads](https://clickhouse.com/docs/operations/settings/settings/#settings-max_threads) setting [controls](https://clickhouse.com/company/events/query-performance-introspection) the number of parallel reading threads and parallel query processing threads:
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Normally the [max_threads](/operations/settings/settings#max_threads) setting [controls](https://clickhouse.com/company/events/query-performance-introspection) the number of parallel reading threads and parallel query processing threads:
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![Untitled scene](https://user-images.githubusercontent.com/97666923/212138072-5410b684-d00d-4218-93c5-6f49523928a5.png)
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