Re: Emitting JSON to file using COPY TO - Mailing list pgsql-hackers

On Mon, Mar 9, 2026 at 3:44 AM Andrew Dunstan <andrew@dunslane.net> wrote:

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);
+       }

This is better. I've tried to get rid of json_projvalues and json_projnulls.
Just using heap_form_tuple, but it won't work.

I incorporated the v28-0004 COPY column list into v9-0002.
With this patch set, we added four fields to the struct CopyToStateData.

+    StringInfo    json_buf;        /* reusable buffer for JSON output,
+                                 * initialized in BeginCopyTo */
+    TupleDesc    tupDesc;        /* Descriptor for JSON output; for a column
+                                 * list this is a projected descriptor */
+    Datum       *json_projvalues;    /* pre-allocated projection values, or
+                                     * NULL */
+    bool       *json_projnulls; /* pre-allocated projection nulls, or NULL */

Using the script in
https://www.postgresql.org/message-id/CACJufxFFZqxC3p4WjpTEi4riaJm%3DpADX%2Bpy0yQ0%3DRWTn5cqK3Q%40mail.gmail.com
I tested it again on macOS and Linux, and there are no regressions for
COPY TO with the TEXT and CSV formats.

OK, I think we're really close now. Here is a tiny fixup patch that fixes an error message and a comment, and adds a missing test case.

cheers

andrew

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