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FROM Clause
The FROM clause specifies the source to read data from:
JOIN and ARRAY JOIN clauses may also be used to extend the functionality of the FROM clause.
Subquery is another SELECT query that may be specified in parenthesis inside FROM clause.
FROM clause can contain multiple data sources, separated by commas, which is equivalent of performing CROSS JOIN on them.
FINAL Modifier
When FINAL is specified, ClickHouse fully merges the data before returning the result and thus performs all data transformations that happen during merges for the given table engine.
It is applicable when selecting data from tables that use the MergeTree-engine family. Also supported for:
- Replicated versions of
MergeTreeengines. - View, Buffer, Distributed, and MaterializedView engines that operate over other engines, provided they were created over
MergeTree-engine tables.
Now SELECT queries with FINAL are executed in parallel and slightly faster. But there are drawbacks (see below). The max_final_threads setting limits the number of threads used.
Drawbacks
Queries that use FINAL are executed slightly slower than similar queries that do not, because:
- Data is merged during query execution.
- Queries with
FINALread primary key columns in addition to the columns specified in the query.
In most cases, avoid using FINAL. The common approach is to use different queries that assume the background processes of the MergeTree engine have’t happened yet and deal with it by applying aggregation (for example, to discard duplicates).
Implementation Details
If the FROM clause is omitted, data will be read from the system.one table.
The system.one table contains exactly one row (this table fulfills the same purpose as the DUAL table found in other DBMSs).
To execute a query, all the columns listed in the query are extracted from the appropriate table. Any columns not needed for the external query are thrown out of the subqueries.
If a query does not list any columns (for example, SELECT count() FROM t), some column is extracted from the table anyway (the smallest one is preferred), in order to calculate the number of rows.