thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-09-21 01:00:48 +00:00
Code Issues Packages Projects Releases Wiki Activity

Update docs/en/engines/table-engines/mergetree-family/mergetree.md

Browse Source 
Co-authored-by: Alexey Boykov <33257111+mathalex@users.noreply.github.com>
This commit is contained in:
Vladimir Goncharov 2021-07-26 17:56:40 +03:00 committed by GitHub
parent a1acc56718
commit 933d0b7e3b
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
docs/en/engines/table-engines/mergetree-family/mergetree.md
View File

@ -399,7 +399,7 @@ A projection query is what defines a projection. It has the following grammar:
It implicitly selects data from the parent table.
#### Storage {#projection-storage}
Projections are stored inside the part directory. It's similar to an index but contains an subdirectory which stores an anonymous MergeTree table's part. The table is induced by the definition query of the projection. If there is a GROUP BY clause, the underlying storage engine becomes AggregatedMergeTree, and all aggregate functions are converted to AggregateFunction. If there is an ORDER BY clause, the MergeTree table will use it as its primary key expression. During the merge process, the projection part will be merged via its storage's merge routine. The checksum of the parent table's part will combine the projection's part. Other maintenance jobs are similar to skip indices.
Projections are stored inside the part directory. It's similar to an index but contains a subdirectory that stores an anonymous MergeTree table's part. The table is induced by the definition query of the projection. If there is a GROUP BY clause, the underlying storage engine becomes AggregatedMergeTree, and all aggregate functions are converted to AggregateFunction. If there is an ORDER BY clause, the MergeTree table will use it as its primary key expression. During the merge process, the projection part will be merged via its storage's merge routine. The checksum of the parent table's part will combine the projection's part. Other maintenance jobs are similar to skip indices.
#### Query Analysis {#projection-query-analysis}
1. Check if the projection can be used to answer the given query, that is, it generates the same answer as querying the base table.