fix(db2): bound large LOB column previews with SUBSTR to cap memory

Refs: #10723

crates/dbx-sql-dialect/src/sql_dialect/table_select.rs

@@ -92,6 +92,18 @@ fn large_value_preview_kind(
                 None
             }
         }
+        Some(DatabaseType::Db2) => {
+            if matches!(base.as_str(), "blob" | "binary" | "varbinary" | "longvarbinary") {
+                Some(LargeValuePreviewKind::Binary)
+            } else if matches!(base.as_str(), "clob" | "dbclob")
+                || (matches!(base.as_str(), "char" | "character" | "varchar" | "graphic" | "vargraphic")
+                    && declared_data_type_length(data_type).is_some_and(|length| length > preview_size))
+            {
+                Some(LargeValuePreviewKind::Text)
+            } else {
+                None
+            }
+        }
         _ => None,
     }
 }
@@ -149,6 +161,17 @@ fn build_large_value_preview_columns(options: &TableDataSelectSqlOptions) -> Opt
                 (format!("left({quoted}::text, {prefix_size}) AS {quoted}"), "V")
             }
             Some(DatabaseType::Postgres) => (format!("left({quoted}, {prefix_size}) AS {quoted}"), "T"),
+            // DB2 的 SUBSTR 第三参数越界会报 SQL0138N，而 BLOB/CLOB 长度不定，故用
+            // CASE WHEN LENGTH(..) 把预览长度夹在实际长度内，对短值不报错；SUBSTR 对 BLOB
+            // 返回 BLOB、对 CLOB/字符列返回 VARCHAR，agent 侧 BLOB 再转 0x 前缀的 hex 字符串。
+            Some(DatabaseType::Db2) if kind == LargeValuePreviewKind::Binary => (
+                format!("SUBSTR({quoted}, 1, CASE WHEN LENGTH({quoted}) >= {prefix_size} THEN {prefix_size} ELSE LENGTH({quoted}) END) AS {quoted}"),
+                "B",
+            ),
+            Some(DatabaseType::Db2) => (
+                format!("SUBSTR({quoted}, 1, CASE WHEN LENGTH({quoted}) >= {prefix_size} THEN {prefix_size} ELSE LENGTH({quoted}) END) AS {quoted}"),
+                "T",
+            ),
             _ => return None,
         };
         let marker = if database_type == Some(DatabaseType::Mysql) {

crates/dbx-sql-dialect/src/sql_dialect/tests.rs

@@ -1034,6 +1034,37 @@ fn builds_postgres_table_data_large_value_previews() {
     assert!(sql.contains("'T:8192' AS \"__DBX_LARGE_VALUE_BYTES_K_2\""));
 }
 
+#[test]
+fn builds_db2_table_data_large_value_previews_with_bounded_substr() {
+    let sql = build_table_data_select_sql(TableDataSelectSqlOptions {
+        database_type: Some(DatabaseType::Db2),
+        schema: Some("DB2INST1".to_string()),
+        table_name: "large_rows".to_string(),
+        primary_keys: vec!["id".to_string()],
+        columns: vec!["id".to_string(), "doc".to_string(), "raw_value".to_string()],
+        column_types: vec!["integer".to_string(), "clob".to_string(), "blob".to_string()],
+        large_value_preview_size: Some(4096),
+        limit: Some(100),
+        ..Default::default()
+    });
+
+    // DB2 的 SUBSTR 第三参数越界会报 SQL0138N，故用 CASE WHEN LENGTH(..) 把预览长度
+    // 夹在实际长度内：CLOB → 文本预览、BLOB → 二进制预览，都不对短值报错。
+    assert!(
+        sql.contains("SUBSTR(\"doc\", 1, CASE WHEN LENGTH(\"doc\") >= 4097 THEN 4097 ELSE LENGTH(\"doc\") END) AS \"doc\""),
+        "clob preview sql: {sql}"
+    );
+    assert!(sql.contains("'T:4096' AS "), "clob marker sql: {sql}");
+    assert!(
+        sql.contains("SUBSTR(\"raw_value\", 1, CASE WHEN LENGTH(\"raw_value\") >= 4097 THEN 4097 ELSE LENGTH(\"raw_value\") END) AS \"raw_value\""),
+        "blob preview sql: {sql}"
+    );
+    assert!(sql.contains("'B:4096' AS "), "blob marker sql: {sql}");
+    // 主键列不做预览，且不得生成裸 SUBSTR(col,1,n)（会越界报错）
+    assert!(!sql.contains("SUBSTR(\"id\""), "pk should not be previewed: {sql}");
+    assert!(!sql.contains("SUBSTR(\"doc\", 1, 4097)"), "must not emit unbounded substr: {sql}");
+}
+
 #[test]
 fn preserves_postgres_array_types_in_large_value_previews() {
     let sql = build_table_data_select_sql(TableDataSelectSqlOptions {