fix(postgres): cast xml preview columns to text

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

@@ -76,13 +76,17 @@ fn large_value_preview_kind(
                 None
             } else if base == "bytea" {
                 Some(LargeValuePreviewKind::Binary)
-            } else if matches!(base.as_str(), "char" | "character" | "varchar" | "text" | "citext" | "name" | "xml")
+            } else if matches!(base.as_str(), "char" | "character" | "varchar" | "text" | "citext" | "name")
                 || normalized.starts_with("character varying")
             {
                 Some(LargeValuePreviewKind::Text)
             } else if base == "vector" {
                 Some(LargeValuePreviewKind::Vector)
-            } else if matches!(base.as_str(), "json" | "jsonb" | "tsvector") {
+            } else if matches!(base.as_str(), "json" | "jsonb" | "tsvector" | "xml") {
+                // `xml` must be cast to text: there is no `left(xml, int)`
+                // overload and no implicit xml -> text cast, so the preview
+                // would otherwise fail with
+                // `function left(xml, integer) does not exist`.
                 Some(LargeValuePreviewKind::TextCast)
             } else {
                 None

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

@@ -1033,6 +1033,28 @@ fn builds_pgvector_table_data_preview_with_compatible_text_cast() {
     assert!(sql.contains("'V:8192' AS \"__DBX_LARGE_VALUE_BYTES_V_1\""));
 }
 
+#[test]
+fn postgres_xml_preview_casts_to_text() {
+    // `left(xml, int)` does not exist and xml has no implicit cast to text,
+    // so an uncast preview breaks table data reads with
+    // `function left(xml, integer) does not exist`.
+    let sql = build_table_data_select_sql(TableDataSelectSqlOptions {
+        database_type: Some(DatabaseType::Postgres),
+        schema: Some("public".to_string()),
+        table_name: "documents".to_string(),
+        primary_keys: vec!["id".to_string()],
+        columns: vec!["id".to_string(), "xml_value".to_string()],
+        column_types: vec!["integer".to_string(), "xml".to_string()],
+        large_value_preview_size: Some(8192),
+        limit: Some(100),
+        ..Default::default()
+    });
+
+    assert!(sql.contains("left(\"xml_value\"::text, 8193) AS \"xml_value\""), "sql was: {sql}");
+    assert!(sql.contains("'T:8192' AS \"__DBX_LARGE_VALUE_BYTES_T_1\""), "sql was: {sql}");
+    assert!(!sql.contains("left(\"xml_value\","), "sql was: {sql}");
+}
+
 #[test]
 fn table_data_preview_requires_stable_keys_and_parallel_types() {
     let base = TableDataSelectSqlOptions {