Which Greenplum Version your are using?

Did you tried this - Set Optimizer Parameters Explicitly:

**You can try enabling GPORCA explicitly in your session or function using:

SET optimizer = on;

Also consider setting other relevant parameters like:

check these parameters also(it should be ON)

optimizer_force_multistage_agg

optimizer_force_three_stage_scalar_dqa

FYI, As per vendor recommendations to Force GPORCA Usage

Avoid Dynamic SQL When Possible:

**If you can, avoid using EXECUTE with dynamic SQL inside functions. Instead, use static SQL so GPORCA can analyze it at compile time.

Use SQL Functions Instead of PL/pgSQL:

**SQL functions (as opposed to PL/pgSQL) are more likely to be optimized by GPORCA.

Use External Scripts for Complex Queries:

**If the query is very large and complex, consider storing it in a table or file and executing it from a client (e.g., Python, Bash) where GPORCA can fully optimize it.