Add basic documentation

Author: rschu1ze

docs/en/operations/query-condition-cache.md

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+---
+slug: /operations/query-cache
+sidebar_position: 64
+sidebar_label: Query Condition Cache
+---
+
+# Query Condition Cache
+
+Many practical workloads expose repeated queries against the same data.
+If users are aware of these patterns, they can create primary key indexes, skipping indexes, projections etc., to speed up such queries.
+This however requires careful monitoring of the workload and explicit tuning (i.e., physical schema optimization) from a database administrator.
+
+The query condition cache addresses this by remembering which ranges of the data cannot possibly satisfy filter predicates.
+For example, if the user runs the query `SELECT * FROM tab WHERE col = 'a'`, ClickHouse will scan column `col` for filter predicate `col = 'a'`.
+At the end of the scan, the query condition cache will remember which vertical ranges of table `tab` did not match the predicate.
+When the user runs the same query again, ClickHouse will first check the cache and exclude non-matching data ranges from the subsequent scan.
+
+## Configuration Settings and Usage {#configuration-settings-and-usage}
+
+Setting [use_query_cache](/operations/settings/settings#use_query_condition_cache) controls whether a specific query or all queries of the
+current session should utilize the query condition cache. For example, the first execution of query
+
+```sql
+SELECT col1, col2
+FROM table
+WHERE col1 = 'x'
+SETTINGS use_query_condition_cache = true;
+```
+
+will store ranges of the table which do not satisfy the predicate.
+Subsequent executions of the same query (also with parameter `use_query_condition_cache = true`) will utilize the query condition cache to scan less data.