Re: Emitting JSON to file using COPY TO

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;

+ cstate->json_projvalues = (Datum *) palloc(natts * sizeof(Datum));
+ cstate->json_projnulls = (bool *) palloc(natts * sizeof(bool));
I changed it to
+ cstate->json_projvalues = palloc_array(Datum, natts);
+ cstate->json_projnulls = palloc_array(bool, natts);

+ rowdata = HeapTupleHeaderGetDatum(tup->t_data);
I changed it to
+ rowdata = HeapTupleGetDatum(tup);

Patch v28-0004 adds the json_projvalues and json_projnulls pointers to struct
CopyToStateData. I wondered if adding these would slow the COPY TO with TEXT and
CSV format, so I ran a quick test using a 36-column table.

Surprisingly, v28 actually make COPY TO with TEXT and CSV performs a little bit
faster. But I didn't find out why.
You may also try the attached test script: copyto_json_perfomance_test.nocfbot.

--
jian
https://www.enterprisedb.com/