Re: Emitting JSON to file using COPY TO

On 2026-03-08 Su 12:16 PM, jian he wrote:
> hi.
>
> V27-0002 is still not bullet-proof.
>
> drop table if exists t1;
> create table t1(a int);
> insert into t1 values (1);
> copy (select * from t1) to stdout json;
> {"a":1}
> WARNING: resource was not closed: TupleDesc 0x7171d0ca3440 (18239,-1)
>
> Also see ExecAssignScanProjectionInfo->ExecConditionalAssignProjectionInfo
> So in v28-0002, I changed to
> + /*
> + * composite_to_json() requires a stable TupleDesc. Since the slot's
> + * descriptor (slot->tts_tupleDescriptor) can change during the execution
> + * of a SELECT query, we use cstate->queryDesc->tupDesc instead. This
> + * precaution is only necessary when the output slot's TupleDesc is of
> + * type RECORDOID.
> + */
> + if (!cstate->rel && slot->tts_tupleDescriptor->tdtypeid == RECORDOID)
> + slot->tts_tupleDescriptor = cstate->queryDesc->tupDesc;

Hmm. But should we be scribbling on slot->tts_tupleDescriptor like that?
How about something like this?:

-        * Full table or query without column list.  Ensure the slot uses
-        * cstate->tupDesc so that the datum is stamped with the right type;
-        * for queries output type is RECORDOID this must be the blessed
-        * descriptor so that composite_to_json can look it up via
-        * lookup_rowtype_tupdesc.
+        * Full table or query without column list.  For queries, the slot's
+        * TupleDesc may carry RECORDOID, which is not registered in the
type
+        * cache and would cause composite_to_json's lookup_rowtype_tupdesc
+        * call to fail.  Build a HeapTuple stamped with the blessed
+        * descriptor so the type can be looked up correctly.
         */
        if (!cstate->rel && slot->tts_tupleDescriptor->tdtypeid ==
RECORDOID)
-           slot->tts_tupleDescriptor = cstate->queryDesc->tupDesc;
+       {
+           HeapTuple   tup;

-       rowdata = ExecFetchSlotHeapTupleDatum(slot);
+           tup = heap_form_tuple(cstate->tupDesc,
+                                 slot->tts_values,
+                                 slot->tts_isnull);
+           rowdata = HeapTupleGetDatum(tup);
+       }
+       else
+       {
+           rowdata = ExecFetchSlotHeapTupleDatum(slot);
+       }

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com