Fetching CLOB storing JSON fails on NotSupportedError

[mauropagano]

1. What versions are you using?

3.4.2
DB is 19c but fails on 26ai too as per Anthony

2. Is it an error or a hang or a crash?

Error

3. What error(s) or behavior you are seeing?

oracledb.exceptions.NotSupportedError: DPY-3030: conversion from Oracle Database type DB_TYPE_CLOB to Apache Arrow format is not supported

4. Does your application call init_oracle_client()?

No, but it fails regardless

5. Include a runnable Python script that shows the problem.

import oracledb

conn = oracledb.connect("...")

with conn.cursor() as c:
    c.execute("create table mauro_clob_isjson (body clob, check (body is json))")
    c.execute("insert into mauro_clob_isjson values ('{\"a\":1}')")
    conn.commit()

odf = conn.fetch_df_all("select body from mauro_clob_isjson")

[anthony-tuininga]

I discovered the source of the issue. This patch addresses it:

diff --git a/src/oracledb/impl/base/cursor.pyx b/src/oracledb/impl/base/cursor.pyx
index 2d1da0a7f..7daf3cdac 100644
--- a/src/oracledb/impl/base/cursor.pyx
+++ b/src/oracledb/impl/base/cursor.pyx
@@ -210,14 +210,16 @@ cdef class BaseCursorImpl:
         # applicable
         db_type_num = metadata.dbtype.num
         if db_type_num == DB_TYPE_NUM_NUMBER:
-            if self.fetch_decimals:
+            if self.fetch_decimals and not self.fetching_arrow:
                 var_impl.metadata._py_type_num = PY_TYPE_NUM_DECIMAL
-        elif metadata.is_oson and db_type_num != DB_TYPE_NUM_JSON:
+        elif metadata.is_oson and db_type_num != DB_TYPE_NUM_JSON \
+                and not self.fetching_arrow:
             conn_impl = self._get_conn_impl()
             var_impl.metadata.dbtype = DB_TYPE_LONG_RAW
             var_impl._fetch_metadata.dbtype = DB_TYPE_LONG_RAW
             var_impl.outconverter = conn_impl.decode_oson
-        elif metadata.is_json and db_type_num != DB_TYPE_NUM_JSON:
+        elif metadata.is_json and db_type_num != DB_TYPE_NUM_JSON \
+                and not self.fetching_arrow:
             var_impl.outconverter = self._build_json_converter_fn()
         elif not self.fetch_lobs or self.fetching_arrow:
             if db_type_num == DB_TYPE_NUM_BLOB:

In other words, when fetching Arrow data, the conversion from JSON string to Python object shouldn't be taking place, but some of that code was still functioning and the error message was less than understandable! We are in the process of wrapping up 4.0 but I will push this change after the release and there will be a 4.0.1 later in May.