Re: Emitting JSON to file using COPY TO

On 2026-03-06 Fr 4:38 AM, jian he wrote:
> COPY (SELECT 1 UNION ALL SELECT 2) TO stdout WITH (format json);
>
> still fails with v25-0002, json_tupledesc_ready is not helpful.
> I think I figured it out. We need to use BlessTupleDesc in BeginCopyTo.
> Then let slot->tts_tupleDescriptor point to cstate->queryDesc->tupDesc
> in CopyToJsonOneRow
>
> * CSV, text and json formats share the same TextLike routines except for the
> * one-row callback.
> This comment is not useful, I want to delete it.
>
> CopyToTextLikeStart
> + /* JSON-specific initialization */
> + if (cstate->opts.format == COPY_FORMAT_JSON)
> + {
> + MemoryContext oldcxt;
> +
> + /* Allocate reusable JSON output buffer in long-lived context */
> + oldcxt = MemoryContextSwitchTo(cstate->copycontext);
> + initStringInfo(&cstate->json_buf);
> + MemoryContextSwitchTo(oldcxt);
> + }
> We ca just add
>
> cstate->json_buf = makeStringInfo();
>
> in BeginCopyTo.
>
> v25-0004-COPY-TO-JSON-build-JSON-per-column-support-colum.patch
> added several fields to the CopyToStateData.
> Actually, there is a simpler way (construct a new Tupdesc and let
> composite_to_json do the job), please see my v26-0004.
>

Yeah, I got some crashes using this, but reworked it a bit, and all now
seems good. I was apparently mistaken about the supposed speedup, but
the performance is steady.

cheers

andrew

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