fix(mysql): infer LIMIT placeholders in prepare

Signed-off-by: discord9 <discord9@163.com>

Author: discord9

src/query/src/planner.rs

@@ -494,14 +494,45 @@ impl DfLogicalPlanner {
         Ok(())
     }
 
+    fn infer_limit_placeholder_types(
+        plan: &LogicalPlan,
+        placeholder_types: &mut HashMap<String, Option<DataType>>,
+    ) -> Result<()> {
+        plan.apply(|node| {
+            if let LogicalPlan::Limit(limit) = node {
+                for expr in limit.skip.iter().chain(limit.fetch.iter()) {
+                    expr.apply(|e| {
+                        if let DfExpr::Placeholder(ph) = e {
+                            placeholder_types
+                                .entry(ph.id.clone())
+                                .and_modify(|existing| {
+                                    if existing.is_none() {
+                                        *existing = Some(DataType::Int64);
+                                    }
+                                })
+                                .or_insert(Some(DataType::Int64));
+                        }
+
+                        Ok(TreeNodeRecursion::Continue)
+                    })?;
+                }
+            }
+
+            Ok(TreeNodeRecursion::Continue)
+        })?;
+
+        Ok(())
+    }
+
     /// Gets inferred parameter types from a logical plan.
     /// Returns a map where each parameter ID is mapped to:
     /// - Some(DataType) if the parameter type could be inferred
     /// - None if the parameter type could not be inferred
     ///
     /// This function first uses DataFusion's `get_parameter_types()` to infer types.
     /// If any parameters have `None` values (i.e., DataFusion couldn't infer their types),
-    /// it falls back to using `extract_placeholder_cast_types()` to detect explicit casts.
+    /// it falls back to using `extract_placeholder_cast_types()` to detect explicit casts
+    /// and applies context-specific inference such as LIMIT/OFFSET placeholders.
     ///
     /// This is because datafusion can only infer types for a limited cases.
     ///
@@ -510,19 +541,15 @@ impl DfLogicalPlanner {
     pub fn get_inferred_parameter_types(
         plan: &LogicalPlan,
     ) -> Result<HashMap<String, Option<DataType>>> {
-        let param_types = plan.get_parameter_types().context(PlanSqlSnafu)?;
+        let mut param_types = plan.get_parameter_types().context(PlanSqlSnafu)?;
 
         let has_none = param_types.values().any(|v| v.is_none());
 
-        if !has_none {
-            Ok(param_types)
-        } else {
+        if has_none {
             let cast_types = Self::extract_placeholder_cast_types(plan)?;
 
-            let mut merged = param_types;
-
             for (id, opt_type) in cast_types {
-                merged
+                param_types
                     .entry(id)
                     .and_modify(|existing| {
                         if existing.is_none() {
@@ -532,8 +559,10 @@ impl DfLogicalPlanner {
                     .or_insert(opt_type);
             }
 
-            Ok(merged)
+            Self::infer_limit_placeholder_types(plan, &mut param_types)?;
         }
+
+        Ok(param_types)
     }
 }
 
@@ -793,6 +822,15 @@ mod tests {
         assert_eq!(type_3, &Some(DataType::Int32));
     }
 
+    #[tokio::test]
+    async fn test_get_inferred_parameter_types_limit_offset() {
+        let plan = parse_sql_to_plan("SELECT id FROM test LIMIT $1 OFFSET $2").await;
+        let types = DfLogicalPlanner::get_inferred_parameter_types(&plan).unwrap();
+
+        assert_eq!(types.get("$1"), Some(&Some(DataType::Int64)));
+        assert_eq!(types.get("$2"), Some(&Some(DataType::Int64)));
+    }
+
     #[tokio::test]
     async fn test_plan_pql_applies_extension_rules() {
         for inner_agg in ["count", "sum", "avg", "min", "max", "stddev", "stdvar"] {

src/servers/tests/mysql/mysql_server_test.rs

@@ -516,6 +516,38 @@ async fn test_query_prepared() -> Result<()> {
         _ => unreachable!(),
     }
 
+    // Regression test for #8142: LIMIT ? should work in prepared statements.
+    // The LIMIT placeholder should be inferred as Int64 so the MySQL prepare
+    // response advertises the correct parameter count.
+    {
+        let stmt = connection
+            .prep("SELECT uint32s FROM all_datatypes LIMIT ?")
+            .await
+            .unwrap();
+        let rows: Vec<Row> = connection
+            .exec(stmt, vec![mysql_async::Value::Int(1)])
+            .await
+            .unwrap();
+        assert_eq!(rows.len(), 1, "LIMIT 1 should return 1 row");
+    }
+
+    // Also cover mixed placeholders: the WHERE placeholder is inferred from
+    // the column type and the LIMIT placeholder is inferred from its context.
+    {
+        let stmt = connection
+            .prep("SELECT uint32s FROM all_datatypes WHERE uint32s >= ? LIMIT ?")
+            .await
+            .unwrap();
+        let rows: Vec<Row> = connection
+            .exec(
+                stmt,
+                vec![mysql_async::Value::UInt(0), mysql_async::Value::UInt(1)],
+            )
+            .await
+            .unwrap();
+        assert_eq!(rows.len(), 1, "LIMIT 1 should return 1 row");
+    }
+
     Ok(())
 }