Doc. prewhere example

docs/en/sql-reference/statements/select/prewhere.md

@@ -26,3 +26,44 @@ The `PREWHERE` section is executed before `FINAL`, so the results of `FROM ... F
 ## Limitations
 
 `PREWHERE` is only supported by tables from the [*MergeTree](../../../engines/table-engines/mergetree-family/index.md) family.
+
+## Example
+
+```sql
+CREATE TABLE mydata
+(
+    `A` Int64,
+    `B` Int8,
+    `C` String
+)
+ENGINE = MergeTree
+ORDER BY A AS
+SELECT
+    number,
+    0,
+    if(number between 1000 and 2000, 'x', toString(number))
+FROM numbers(10000000);
+
+SELECT count()
+FROM mydata
+WHERE (B = 0) AND (C = 'x');
+
+1 row in set. Elapsed: 0.074 sec. Processed 10.00 million rows, 168.89 MB (134.98 million rows/s., 2.28 GB/s.)
+
+-- let's enable tracing to see which predicate are moved to PREWHERE
+set send_logs_level='debug';
+
+MergeTreeWhereOptimizer: condition "B = 0" moved to PREWHERE  
+-- Clickhouse moves automatically `B = 0` to PREWHERE, but it has no sense because B is always 0.
+
+-- Let's move other predicate `C = 'x'` 
+
+SELECT count()
+FROM mydata
+PREWHERE C = 'x'
+WHERE B = 0;
+
+1 row in set. Elapsed: 0.069 sec. Processed 10.00 million rows, 158.89 MB (144.90 million rows/s., 2.30 GB/s.)
+
+-- This query with manual `PREWHERE` processes slightly less data: 158.89 MB VS 168.89 MB
+```