docs: Add barrier function documentation and use it in KNN join example (#72)

Author: zhangfengcdt

docs/reference/sql-joins.md

@@ -52,3 +52,37 @@ FROM cities AS cities_l
 INNER JOIN cities AS cities_r
 ON ST_KNN(cities_l.geometry, cities_r.geometry, 5, false)
 ```
+
+## Optimization Barrier
+
+Use the `barrier` function to prevent filter pushdown and control predicate evaluation order in complex spatial joins. This function creates an optimization barrier by evaluating boolean expressions at runtime.
+
+The `barrier` function takes a boolean expression as a string, followed by pairs of variable names and their values that will be substituted into the expression:
+
+```sql
+barrier(expression, var_name1, var_value1, var_name2, var_value2, ...)
+```
+
+The placement of filters relative to KNN joins changes the semantic meaning of the query:
+
+- **Filter before KNN**: First filters the data, then finds K nearest neighbors from the filtered subset. This answers "What are the K nearest high-rated restaurants?"
+- **Filter after KNN**: First finds K nearest neighbors from all data, then filters those results. This answers "Of the K nearest restaurants, which ones are high-rated?"
+
+### Example
+
+Find the 3 nearest high-rated restaurants to luxury hotels, ensuring the KNN join completes before filtering.
+
+```sql
+SELECT
+    h.name AS hotel,
+    r.name AS restaurant,
+    r.rating
+FROM hotels AS h
+INNER JOIN restaurants AS r
+ON ST_KNN(h.geometry, r.geometry, 3, false)
+WHERE barrier('rating > 4.0 AND stars >= 4',
+              'rating', r.rating,
+              'stars', h.stars)
+```
+
+With the barrier function, this query first finds the 3 nearest restaurants to each hotel (regardless of rating), then filters to keep only those pairs where the restaurant has rating > 4.0 and the hotel has stars >= 4. Without the barrier, an optimizer might push the filters down, changing the query to first filter for high-rated restaurants and luxury hotels, then find the 3 nearest among those filtered sets.