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* update presentations * CLICKHOUSE-2936: redirect from clickhouse.yandex.ru and clickhouse.yandex.com * update submodule * lost files * CLICKHOUSE-2981: prefer sphinx docs over original reference * CLICKHOUSE-2981: docs styles more similar to main website + add flags to switch language links * update presentations * Less confusing directory structure (docs -> doc/reference/) * Minify sphinx docs too * Website release script: fail fast + pass docker hash on deploy * Do not underline links in docs * shorter * cleanup docker images * tune nginx config * CLICKHOUSE-3043: get rid of habrastorage links * Lost translation * CLICKHOUSE-2936: temporary client-side redirect * behaves weird in test * put redirect back * CLICKHOUSE-3047: copy docs txts to public too * move to proper file * remove old pages to avoid confusion * Remove reference redirect warning for now * Refresh README.md * Yellow buttons in docs * Use svg flags instead of unicode ones in docs * fix test website instance * Put flags to separate files * wrong flag * Copy Yandex.Metrica introduction from main page to docs * Yet another home page structure change, couple new blocks (CLICKHOUSE-3045) * Update Contacts section * CLICKHOUSE-2849: more detailed legal information * CLICKHOUSE-2978 preparation - split by files * More changes in Contacts block * Tune texts on index page * update presentations * One more benchmark * Add usage sections to index page, adapted from slides * Get the roadmap started, based on slides from last ClickHouse Meetup * CLICKHOUSE-2977: some rendering tuning * Get rid of excessive section in the end of getting started * Make headers linkable * CLICKHOUSE-2981: links to editing reference - https://github.com/yandex/ClickHouse/issues/849 * CLICKHOUSE-2981: fix mobile styles in docs * Ban crawling of duplicating docs * Open some external links in new tab * Ban old docs too * Lots of trivial fixes in english docs * Lots of trivial fixes in russian docs * Remove getting started copies in markdown * Add Yandex.Webmaster * Fix some sphinx warnings * More warnings fixed in english docs * More sphinx warnings fixed * Add code-block:: text * More code-block:: text * These headers look not that well * Better switch between documentation languages * merge use_case.rst into ya_metrika_task.rst * Edit the agg_functions.rst texts * Add lost empty lines * Lost blank lines * Add new logo sizes * update presentations * Next step in migrating to new documentation * Fix all warnings in en reference * Fix all warnings in ru reference * Re-arrange existing reference * Move operation tips to main reference * Fix typos noticed by milovidov@ * Get rid of zookeeper.md * Looks like duplicate of tutorial.html * Fix some mess with html tags in tutorial * No idea why nobody noticed this before, but it was completely not clear whet to get the data * Match code block styling between main and tutorial pages (in favor of the latter) * Get rid of some copypaste in tutorial * Normalize header styles * Move example_datasets to sphinx * Move presentations submodule to website * Move and update README.md * No point in duplicating articles from habrahabr here * Move development-related docs as is for now * doc/reference/ -> docs/ (to match the URL on website) * Adapt links to match the previous commit * Adapt development docs to rst (still lacks translation and strikethrough support) * clean on release * blacklist presentations in gulp * strikethrough support in sphinx * just copy development folder for now * fix weird introduction in style article * Style guide translation (WIP) * Finish style guide translation to English * gulp clean separately * Update year in LICENSE * Initial CONTRIBUTING.md * Fix remaining links to old docs in tutorial * Some tutorial fixes * Typo * Another typo * Update list of authors from yandex-team accoding to git log
38 lines
2.5 KiB
ReStructuredText
38 lines
2.5 KiB
ReStructuredText
Merge
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-----
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The Merge engine (not to be confused with MergeTree) does not store data itself, but allows reading from any number of other tables simultaneously.
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Reading is automatically parallelized. Writing to a table is not supported. When reading, the indexes of tables that are actually being read are used, if they exist.
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The Merge engine accepts parameters: the database name and a regular expression for tables. Example:
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.. code-block:: text
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Merge(hits, '^WatchLog')
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- Data will be read from the tables in the 'hits' database with names that match the regex ``'^WatchLog'``.
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Instead of the database name, you can use a constant expression that returns a string. For example, ``currentDatabase()``.
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Regular expressions are re2 (similar to PCRE), case-sensitive. See the notes about escaping symbols in regular expressions in the "match" section.
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When selecting tables to read, the Merge table itself will not be selected, even if it matches the regex. This is to avoid loops.
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It is possible to create two Merge tables that will endlessly try to read each others' data. But don't do this.
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The typical way to use the Merge engine is for working with a large number of TinyLog tables as if with a single table.
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Virtual columns
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~~~~~~~~~~~~~~~
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Virtual columns are columns that are provided by the table engine, regardless of the table definition. In other words, these columns are not specified in CREATE TABLE, but they are accessible for SELECT.
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Virtual columns differ from normal columns in the following ways:
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- They are not specified in table definitions.
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- Data can't be added to them with ``INSERT``.
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- When using ``INSERT`` without specifying the list of columns, virtual columns are ignored.
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- They are not selected when using the asterisk (``SELECT *``).
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- Virtual columns are not shown in ``SHOW CREATE TABLE`` and ``DESC TABLE`` queries.
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A Merge table contains the virtual column **_table** of the String type. (If the table already has a '_table' column, the virtual column is named '_table1', and if it already has '_table1', it is named '_table2', and so on.) It contains the name of the table that data was read from.
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If the WHERE or PREWHERE clause contains conditions for the '_table' column that do not depend on other table columns (as one of the conjunction elements, or as an entire expression), these conditions are used as an index. The conditions are performed on a data set of table names to read data from, and the read operation will be performed from only those tables that the condition was triggered on.
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