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Merge pull request #45089 from ClickHouse/add-cnf-docs

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Add convert_query_to_cnf to docs
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Rich Raposa 2023-01-10 07:11:57 -07:00 committed by GitHub
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docs/en/operations/settings/settings.md
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@ -195,6 +195,75 @@ SELECT * FROM data_01515 WHERE d1 = 0 AND assumeNotNull(d1_null) = 0 SETTINGS fo
Works with tables in the MergeTree family.
## convert_query_to_cnf {#convert_query_to_cnf}
When set to `true`, a `SELECT` query will be converted to conjuctive normal formal (CNF). There are scenarios where rewriting a query in CNF may execute faster (view this [Github issue](https://github.com/ClickHouse/ClickHouse/issues/11749) for an explanation).
For example, notice how the following `SELECT` query is not modified (the default behavior):
```sql
EXPLAIN SYNTAX
SELECT *
FROM
(
SELECT number AS x
FROM numbers(20)
) AS a
WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15))
SETTINGS convert_query_to_cnf = false;
```
The result is:
```response
┌─explain────────────────────────────────────────────────────────┐
│ SELECT x │
│ FROM │
│ ( │
│ SELECT number AS x │
│ FROM numbers(20) │
│ WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15)) │
│ ) AS a │
│ WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15)) │
│ SETTINGS convert_query_to_cnf = 0 │
└────────────────────────────────────────────────────────────────┘
```
Let's set `convert_query_to_cnf` to `true` and see what changes:
```sql
EXPLAIN SYNTAX
SELECT *
FROM
(
SELECT number AS x
FROM numbers(20)
) AS a
WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15))
SETTINGS convert_query_to_cnf = true;
```
Notice the `WHERE` clause is rewritten in CNF, but the result set is the identical - the Boolean logic is unchanged:
```response
┌─explain───────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ SELECT x │
│ FROM │
│ ( │
│ SELECT number AS x │
│ FROM numbers(20) │
│ WHERE ((x <= 15) OR (x <= 5)) AND ((x <= 15) OR (x >= 1)) AND ((x >= 10) OR (x <= 5)) AND ((x >= 10) OR (x >= 1)) │
│ ) AS a │
│ WHERE ((x >= 10) OR (x >= 1)) AND ((x >= 10) OR (x <= 5)) AND ((x <= 15) OR (x >= 1)) AND ((x <= 15) OR (x <= 5)) │
│ SETTINGS convert_query_to_cnf = 1 │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```
Possible values: true, false
Default value: false
## fsync_metadata {#fsync-metadata}
Enables or disables [fsync](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fsync.html) when writing `.sql` files. Enabled by default.