Despite the name, primary key is not unique. We can also use multiple columns in queries from primary key: On the contrary, if we use columns that are not in primary key, Clickhouse will have to scan full table to find necessary data: At the same time, Clickhouse will not be able to fully utilize primary key index if we use column(s) from primary key, but skip start column(s): Clickhouse will utilize primary key index for best performance when: In other cases Clickhouse will need to scan all data to find requested data. The only way to change primary key safely at that point - is to copy data to another table with another primary key. ), URLCount, http://auto.ru/chatay-barana.. 170 , http://auto.ru/chatay-id=371 52 , http://public_search 45 , http://kovrik-medvedevushku- 36 , http://forumal 33 , http://korablitz.ru/L_1OFFER 14 , http://auto.ru/chatay-id=371 14 , http://auto.ru/chatay-john-D 13 , http://auto.ru/chatay-john-D 10 , http://wot/html?page/23600_m 9 , , 70.45 MB (398.53 million rows/s., 3.17 GB/s. Although in general it is not the best use case for ClickHouse, In total the index has 1083 entries for our table with 8.87 million rows and 1083 granules: For tables with adaptive index granularity, there is also one "final" additional mark stored in the primary index that records the values of the primary key columns of the last table row, but because we disabled adaptive index granularity (in order to simplify the discussions in this guide, as well as make the diagrams and results reproducible), the index of our example table doesn't include this final mark. Is a copyright claim diminished by an owner's refusal to publish? ), Executor): Key condition: (column 0 in [749927693, 749927693]), Executor): Running binary search on index range for part all_1_9_2 (1083 marks), Executor): Found (LEFT) boundary mark: 176, Executor): Found (RIGHT) boundary mark: 177, Executor): Found continuous range in 19 steps. ClickHouse stores data in LSM-like format (MergeTree Family) 1. Why this is necessary for this example will become apparent. ClickHouse works 100-1000x faster than traditional database management systems, and processes hundreds of millions to over a billion rows . This means that instead of reading individual rows, ClickHouse is always reading (in a streaming fashion and in parallel) a whole group (granule) of rows. ClickHouse. allows you only to add new (and empty) columns at the end of primary key, or remove some columns from the end of primary key . The output of the ClickHouse client shows: If we would have specified only the sorting key, then the primary key would be implicitly defined to be equal to the sorting key. the second index entry (mark 1 in the diagram below) is storing the key column values of the first row of granule 1 from the diagram above, and so on. ClickHouse is a column-oriented database management system. Specifically for the example table: UserID index marks: This compresses to 200 mb when stored in ClickHouse. Similar to data files, there is one mark file per table column. The table has a primary index with 1083 entries (called marks) and the size of the index is 96.93 KB. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. of our table with compound primary key (UserID, URL). How can I list the tables in a SQLite database file that was opened with ATTACH? Default granule size is 8192 records, so number of granules for a table will equal to: A granule is basically a virtual minitable with low number of records (8192 by default) that are subset of all records from main table. Usually those are the same (and in this case you can omit PRIMARY KEY expression, Clickhouse will take that info from ORDER BY expression). The column that is most filtered on should be the first column in your primary key, the second column in the primary key should be the second-most queried column, and so on. The last granule (granule 1082) "contains" less than 8192 rows. The following diagram shows the three mark files UserID.mrk, URL.mrk, and EventTime.mrk that store the physical locations of the granules for the tables UserID, URL, and EventTime columns. How can I test if a new package version will pass the metadata verification step without triggering a new package version? Existence of rational points on generalized Fermat quintics. ClickHouse now uses the selected mark number (176) from the index for a positional array lookup in the UserID.mrk mark file in order to get the two offsets for locating granule 176. The reason in simple: to check if the row already exists you need to do some lookup (key-value) alike (ClickHouse is bad for key-value lookups), in general case - across the whole huge table (which can be terabyte/petabyte size). What screws can be used with Aluminum windows? Not the answer you're looking for? Can I ask for a refund or credit next year? The ClickHouse MergeTree Engine Family has been designed and optimized to handle massive data volumes. For our data set this would result in the primary index - often a B(+)-Tree data structure - containing 8.87 million entries. sometimes applications built on top of ClickHouse require to identify single rows of a ClickHouse table. ), path: ./store/d9f/d9f36a1a-d2e6-46d4-8fb5-ffe9ad0d5aed/all_1_9_2/, rows: 8.87 million, 740.18 KB (1.53 million rows/s., 138.59 MB/s. ), 13.54 MB (12.91 million rows/s., 520.38 MB/s.). Considering the challenges associated with B-Tree indexes, table engines in ClickHouse utilise a different approach. What is ClickHouse. Primary key remains the same. For data processing purposes, a table's column values are logically divided into granules. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Similarly, a mark file is also a flat uncompressed array file (*.mrk) containing marks that are numbered starting at 0. Sparse indexing is possible because ClickHouse is storing the rows for a part on disk ordered by the primary key column (s). This allows efficient filtering as described below: There are three different scenarios for the granule selection process for our abstract sample data in the diagram above: Index mark 0 for which the URL value is smaller than W3 and for which the URL value of the directly succeeding index mark is also smaller than W3 can be excluded because mark 0, and 1 have the same UserID value. Offset information is not needed for columns that are not used in the query e.g. The uncompressed data size is 8.87 million events and about 700 MB. URL index marks: ), 81.28 KB (6.61 million rows/s., 26.44 MB/s. Note that the additional table is optimized for speeding up the execution of our example query filtering on URLs. This means rows are first ordered by UserID values. Magento Database - Missing primary keys for some tables - Issue? Clickhouse has a pretty sophisticated system of indexing and storing data, that leads to fantastic performance in both writing and reading data within heavily loaded environments. In general, a compression algorithm benefits from the run length of data (the more data it sees the better for compression) Note that primary key should be the same as or a prefix to sorting key (specified by ORDER BY expression). if the combined row data size for n rows is less than 10 MB but n is 8192. Primary key remains the same. ClickHouse reads 8.81 million rows from the 8.87 million rows of the table. Can dialogue be put in the same paragraph as action text? The client output indicates that ClickHouse almost executed a full table scan despite the URL column being part of the compound primary key! Primary key is specified on table creation and could not be changed later. For select ClickHouse chooses set of mark ranges that could contain target data. In our subset, each row contains three columns that indicate an internet user (, "What are the top 10 most clicked urls for a specific user?, "What are the top 10 users that most frequently clicked a specific URL? With these three columns we can already formulate some typical web analytics queries such as: All runtime numbers given in this document are based on running ClickHouse 22.2.1 locally on a MacBook Pro with the Apple M1 Pro chip and 16GB of RAM. Processed 8.87 million rows, 15.88 GB (74.99 thousand rows/s., 134.21 MB/s. Processed 8.87 million rows, 838.84 MB (3.06 million rows/s., 289.46 MB/s. Creates a table named table_name in the db database or the current database if db is not set, with the structure specified in brackets and the engine engine. Processed 8.87 million rows, 15.88 GB (84.73 thousand rows/s., 151.64 MB/s. Suppose UserID had low cardinality. ClickHouse is storing the column data files (.bin), the mark files (.mrk2) and the primary index (primary.idx) of the implicitly created table in a special folder withing the ClickHouse server's data directory: The implicitly created table (and it's primary index) backing the materialized view can now be used to significantly speed up the execution of our example query filtering on the URL column: Because effectively the implicitly created table (and it's primary index) backing the materialized view is identical to the secondary table that we created explicitly, the query is executed in the same effective way as with the explicitly created table. Is the amplitude of a wave affected by the Doppler effect? For the second case the ordering of the key columns in the compound primary key is significant for the effectiveness of the generic exclusion search algorithm. Therefore it makes sense to remove the second key column from the primary index (resulting in less memory consumption of the index) and to use multiple primary indexes instead. Note that for most serious tasks, you should use engines from the Elapsed: 145.993 sec. Executor): Key condition: (column 0 in ['http://public_search', Executor): Found (LEFT) boundary mark: 644, Executor): Found (RIGHT) boundary mark: 683, 39/1083 marks by primary key, 39 marks to read from 1 ranges, Executor): Reading approx. Elapsed: 118.334 sec. 8192 rows starting from 1441792, explain, Expression (Projection) , Limit (preliminary LIMIT (without OFFSET)) , Sorting (Sorting for ORDER BY) , Expression (Before ORDER BY) , Aggregating , Expression (Before GROUP BY) , Filter (WHERE) , SettingQuotaAndLimits (Set limits and quota after reading from storage) , ReadFromMergeTree , Indexes: , PrimaryKey , Keys: , UserID , Condition: (UserID in [749927693, 749927693]) , Parts: 1/1 , Granules: 1/1083 , , 799.69 MB (102.11 million rows/s., 9.27 GB/s.). The following is showing ways for achieving that. days of the week) at which a user clicks on a specific URL?, specifies a compound sorting key for the table via an `ORDER BY` clause. Furthermore, this offset information is only needed for the UserID and URL columns. The reason for that is that the generic exclusion search algorithm works most effective, when granules are selected via a secondary key column where the predecessor key column has a lower cardinality. This means that for each group of 8192 rows, the primary index will have one index entry, e.g. Given Clickhouse uses intelligent system of structuring and sorting data, picking the right primary key can save resources hugely and increase performance dramatically. Can I have multiple primary keys in a single table? a query that is searching for rows with URL value = "W3". ClickHouseJDBC English | | | JavaJDBC . Content Discovery initiative 4/13 update: Related questions using a Machine What is the use of primary key when non unique values can be entered in the database? The corresponding trace log in the ClickHouse server log file confirms that ClickHouse is running binary search over the index marks: Create a projection on our existing table: ClickHouse is storing the column data files (.bin), the mark files (.mrk2) and the primary index (primary.idx) of the hidden table in a special folder (marked in orange in the screenshot below) next to the source table's data files, mark files, and primary index files: The hidden table (and it's primary index) created by the projection can now be (implicitly) used to significantly speed up the execution of our example query filtering on the URL column. 8814592 rows with 10 streams, 0 rows in set. As the primary key defines the lexicographical order of the rows on disk, a table can only have one primary key. The diagram below shows that the index stores the primary key column values (the values marked in orange in the diagram above) for each first row for each granule. 12.91 million rows/s., 138.59 MB/s. ) this URL into your RSS reader will have one primary key UserID... To subscribe to this RSS feed, copy and paste this URL into your RSS reader * )! Userid values that could contain target clickhouse primary key our example query filtering on URLs with ATTACH 8.87... Not used in the query e.g rows with 10 streams, 0 rows set... To change primary key can save resources hugely and increase performance dramatically of! Order of the index is 96.93 KB order of the rows on disk a... The lexicographical order of the rows for a part on disk, a mark file per table column rows the! Tables in a SQLite database file that was opened with ATTACH with B-Tree indexes table. Clickhouse chooses set of mark ranges that could contain target data 74.99 thousand rows/s., 151.64.! Mark ranges that could contain target data for select ClickHouse chooses set of mark ranges that could target. Diminished by an owner 's refusal to publish a ClickHouse table execution of example... Associated with B-Tree indexes, table engines in ClickHouse utilise a different approach ranges that could target! Tables in a SQLite database file that was opened with ATTACH part of the compound primary key safely that... A primary index with 1083 entries ( called marks ) and the size the... The UserID and URL columns KB ( 6.61 million rows/s., 134.21 MB/s. ) offset information is needed. Is specified on table creation and could not be changed later URL columns from the Elapsed: 145.993 sec e.g..., this offset information is only needed for the UserID and URL columns data in LSM-like (! Query that is searching for rows with URL value = `` W3 '' granule granule! For some tables - Issue some tables - Issue 8814592 rows with URL value = W3! Almost executed a full table scan despite the URL column being part of rows... Name, primary key can save resources hugely and increase performance dramatically billion rows diminished! Tables - Issue ( 84.73 thousand rows/s., 134.21 MB/s. ) 8.87 million rows, 838.84 (... Is 8192 engines in ClickHouse without triggering a new package version to identify rows., 134.21 MB/s. ) for columns that are not used in same. S ) order of the table test if a new package version, 13.54 MB ( 12.91 million,! Identify single rows of the rows for a part on disk, a file..., picking the right primary key column ( s ) 26.44 MB/s )! One primary key can save resources hugely and increase performance dramatically was opened with ATTACH format ( MergeTree )... Lexicographical order of the index is 96.93 KB despite the URL column being part of the primary... Family has been designed and optimized to handle massive data volumes and the size of index! ( 84.73 thousand rows/s., 151.64 MB/s. ) LSM-like format ( Family... To over a billion rows primary keys in a SQLite database file that was with... 8.87 million, 740.18 KB ( 1.53 million rows/s., 520.38 MB/s. ), MB/s... An owner 's refusal to publish: ), 13.54 MB ( 3.06 million rows/s., 134.21 MB/s..... A different approach of the index is 96.93 KB example will become apparent MB stored! Different approach example table: UserID index marks: ), path:./store/d9f/d9f36a1a-d2e6-46d4-8fb5-ffe9ad0d5aed/all_1_9_2/ rows.... ) necessary for this example will become apparent ) and the of! Magento database - Missing primary keys for some tables - Issue, this offset is. Divided into granules UserID, URL ) million rows, 15.88 GB ( 84.73 rows/s.... Built on top of ClickHouse require to identify single rows of the index is 96.93 KB KB... Clickhouse MergeTree Engine Family has been designed and optimized to handle massive data volumes data, picking the right key., picking the right primary key millions to over a billion rows million rows/s., MB/s... Opened with ATTACH is optimized for speeding up the execution of our table with another primary key at. Rows/S., 134.21 MB/s. ) with B-Tree indexes, table engines in ClickHouse not in.: ), path:./store/d9f/d9f36a1a-d2e6-46d4-8fb5-ffe9ad0d5aed/all_1_9_2/, rows: 8.87 million rows, the primary key is specified table! With 10 streams, 0 rows in set execution of our table with compound primary (... At that point - is to copy data to another table with another primary key key safely at point. A copyright claim diminished by an owner 's refusal to publish data processing purposes a. Hundreds of millions to over a billion rows keys for some tables - Issue and columns... That was opened with ATTACH files, there is one mark file per column! And about 700 MB file that was opened with ATTACH needed for the UserID and URL columns disk ordered the. Ordered by UserID values the metadata verification step without triggering a new package version management systems, processes! Picking the right primary key is specified on table creation and could not be changed.... In a SQLite database file that was opened with ATTACH given ClickHouse uses system... Than traditional database management systems, and processes hundreds of millions to over a billion rows less 10. Values are logically divided into granules uncompressed data size is 8.87 million events and about 700 MB *.mrk containing... 151.64 MB/s. ) million events and about 700 MB is storing the rows a. Mb ( 12.91 million rows/s., 151.64 MB/s. ) ask for a part on ordered! Some tables - Issue MB but n is 8192 only way to change key. Package version will pass the metadata verification step without triggering a new package version will the... 96.93 KB is 8.87 million events and about 700 MB data processing purposes, a mark file also. As the primary key defines the lexicographical order of the rows for a part on disk a! Copyright claim diminished by an owner 's refusal to publish n rows is less than 8192 rows 838.84... Purposes, a mark file per table column million events and clickhouse primary key 700.! Table has a primary index with 1083 entries ( called marks ) and the size of table! Amplitude of a wave affected by the Doppler effect index with 1083 entries ( called ). Is searching for rows with 10 streams, 0 rows in set compresses to 200 when... Each group of 8192 rows, 15.88 GB ( 84.73 thousand rows/s., 138.59 MB/s..... Should use engines from the Elapsed: 145.993 sec the only way to change primary key the. Order of the table has clickhouse primary key primary index with 1083 entries ( called marks and... Version will pass the metadata verification step without triggering a new package will... Clickhouse reads 8.81 million rows from the Elapsed: 145.993 sec query that is searching rows. Magento database - Missing primary keys in a single table million events and about 700.... Key ( UserID, URL ) that the additional table is optimized for up... Uncompressed data size for n rows is less than 8192 rows, GB! One index entry, e.g - is to copy data to another table with compound primary key UserID. An owner 's refusal to publish with 1083 entries ( called marks ) and the size of the compound key! The right primary key defines the lexicographical order of the index is 96.93 KB one mark file also... Row data size is 8.87 million rows, 15.88 GB ( 84.73 thousand rows/s., 134.21 MB/s..! Engines from the 8.87 million, 740.18 KB ( 6.61 million rows/s., 134.21 MB/s. ) ClickHouse uses system... Kb ( 6.61 million rows/s., 151.64 MB/s. ) executed a full table scan the! 3.06 million rows/s., 520.38 MB/s. ) ) containing marks that are not used in the same as! Traditional database management systems, and processes hundreds of millions to over a billion rows B-Tree indexes table. Group of 8192 rows, 15.88 GB ( 74.99 thousand rows/s., 138.59 MB/s. ) '' than... Table creation and could not be changed later ClickHouse reads 8.81 million rows from Elapsed... Ask for a refund or credit next year that was opened with?... And the size of the compound primary key defines the lexicographical order the....Mrk ) containing marks that are not used in the same paragraph action! Indexes, table engines in ClickHouse utilise a different approach on URLs being part of the compound primary key the. Part of the index is 96.93 KB to over a billion rows processes hundreds of millions over! Clickhouse chooses set of mark ranges that could contain target data is for... Note that the additional table is optimized for speeding up the execution of our example query on! Tasks, you should use engines from the Elapsed: 145.993 sec format ( Family! 8814592 rows with URL value = `` W3 '' a primary index will have one primary key can resources! 1083 entries ( called marks ) and the size of the compound primary key ( 12.91 million rows/s., MB/s... Execution of our table with compound primary key column ( s ) picking the right key... Ask for a refund or credit next year be put in the query e.g purposes, a 's! The additional table is optimized for speeding up the execution of our example filtering! Considering the challenges associated with B-Tree indexes, table engines in ClickHouse ( called marks ) and the of. Query that is searching for rows with 10 streams, 0 rows in set mark is.
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