fix(postgres): read EXPLAIN JSON output as json column (#279)

* fix(postgres): read EXPLAIN JSON output as json column

EXPLAIN (FORMAT JSON) returns the plan in a column typed as json, not
text, so reading it into a String failed with "error deserializing
column 0" and the Explain Plan feature never worked on Postgres.

Read the column as a serde_json::Value and re-serialize it for the
parser, falling back to a plain String for Postgres-compatible engines
that hand the plan back as text.

Fixes #276

* Update src-tauri/src/drivers/postgres/explain.rs

Co-authored-by: Dominik Spitzli <dominik@spitzli.dev>

---------

Co-authored-by: Dominik Spitzli <dominik@spitzli.dev>

Author: debba

src-tauri/src/drivers/postgres/explain.rs

@@ -29,8 +29,18 @@ pub async fn explain_query(
         return Err("EXPLAIN returned no output".into());
     }
 
-    // PostgreSQL returns a single row with a single text column containing JSON
-    let plan_json_str: String = rows[0].try_get(0).map_err(|e| format_pg_error(&e))?;
+    // `EXPLAIN (FORMAT JSON)` on real PostgreSQL returns a column of type
+    // `json` (not `text`), so reading it as `String` fails with
+    // "error deserializing column 0". Read it as a `serde_json::Value` and
+    // re-serialize. Some Postgres-compatible engines hand back a plain `text`
+    // column instead, so fall back to reading the raw string in that case.
+    let plan_json_str = match rows[0].try_get::<_, serde_json::Value>(0) {
+        Ok(value) => value.to_string(),
+		Err(json_err) => rows[0].try_get::<_, String>(0).map_err(|e| {
+    	log::debug!("EXPLAIN json read failed ({json_err}); text read also failed");
+    	format_pg_error(&e)
+	})?,
+    };
 
     let mut plan = parse_postgres_json(&plan_json_str)?;
     plan.original_query = query.to_string();