Re: batch fdw insert bug (Postgres 14)

On 5/7/21 2:46 PM, houzj(dot)fnst(at)fujitsu(dot)com wrote:
>
>> I am testing new features in Postgres 14, and I found bug
>> EXPLAIN ANALYZE VERBOSE  for insert to FDW table with batch_size 1000 returns
>> -------------------------------------------------------------------------------------------------------------------------------
>>  Insert on public.vzdalena_tabulka2  (cost=0.00..175.00 rows=0 width=0) (actual time=30.269..30.270 rows=0 loops=1)
>>    Remote SQL: \x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F
>>    Batch Size: 1000
>>    ->  Function Scan on pg_catalog.generate_series g  (cost=0.00..175.00 rows=10000 width=36) (actual time=0.453..1.988 rows=10
>>          Output: g.i, ('AHOJ'::text || (g.i)::text)
>>          Function Call: generate_series(1, 10000)
>>  Planning Time: 0.075 ms
>>  Execution Time: 31.032 ms
>> (8 rows)
>> reproducer
>
> I can reproduce the issue and did some basic analysis on it.
>
> The "Remote SQL" is built from the following code:
>
> ----------------
> char *sql = strVal(list_nth(fdw_private,
> FdwModifyPrivateUpdateSql));
>
> ExplainPropertyText("Remote SQL", sql, es);
> ---------------
>
> It use the query string stored in list fdw_private.
> However, the "fmstate->query" will also point to the string in fdw_private,
> by postgresBeginForeignModify --> create_foreign_modify --> "fmstate->query = query;"
>
> And in execute_foreign_modify(), " fmstate->query " will be freed when rebuild the query
> string to do the batch insert like the following:
>
> ----------------
> if (operation == CMD_INSERT && fmstate->num_slots != *numSlots)
> {
> ...
> /* Build INSERT string with numSlots records in its VALUES clause. */
> initStringInfo(&sql);
> rebuildInsertSql(&sql, fmstate->orig_query, fmstate->values_end,
> fmstate->p_nums, *numSlots - 1)
> ** pfree(fmstate->query);
> fmstate->query = sql.data;
> ----------------
>
> So, it output the freed pointer as "Remote SQL".
>
> For the fix.
> The query string could be rebuilt depending on the numSlots,
> which query string should be output ?
> should we just output the original query string like the attached patch ?
> Or should we output the last one?
>

Yeah. The problem is we build fdw_private list once (which references
the SQL string), and during execution we may pfree() it. But then
EXPLAIN ANALYZE gets the same fdw_private list and tries to use the SQL
string which we pfreed() already.

I think the simplest fix is simply to pstrdup() the query when building
fmstate in create_foreign_modify. We've already been doing that for
orig_query anyway. That seems cleaner than printing the last query we
build (which would be confusing I think).

I've pushed a fix doing that. We only need that for INSERT queries, and
we might even restrict that to cases with batching if needed.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company