Re: pg_amcheck contrib application

> On Mar 16, 2021, at 10:48 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Mar 16, 2021 at 12:51 PM Mark Dilger
>> <mark(dot)dilger(at)enterprisedb(dot)com> wrote:
>>> It shows them all has having attalign = 'd', but for some array types the alignment will be 'i', not 'd'. So it's lying a bit about the contents. But I'm now confused why this caused problems on the two hosts where integer and double have the same alignment? It seems like that would be the one place where the bug would not happen, not the one place where it does.
>
>> Wait, so the value in the attalign column isn't the alignment we're
>> actually using? I can understand how we might generate tuples like
>> that, if we pass the actual type to construct_array(), but shouldn't
>> we then get garbage when we deform the tuple?
>
> No. What should be happening there is that some arrays in the column
> get larger alignment than they actually need, but that shouldn't cause
> a problem (and has not done so, AFAIK, in the decades that it's been
> like this). As you say, deforming the tuple is going to rely on the
> table's tupdesc for alignment; it can't know what is in the data.
>
> I'm not entirely sure what's going on, but I think coming at this
> with the mindset that "amcheck has detected some corruption" is
> just going to lead you astray. Almost certainly, it's "amcheck
> is incorrectly claiming corruption". That might be from mis-decoding
> a TOAST-referencing datum. (Too bad the message doesn't report the
> TOAST OID it probed for, so we can see if that's sane or not.)

I've added that and now get the toast pointer's va_valueid in the message:

mark(dot)dilger(at)laptop280-ma-us amcheck % pg_amcheck -t "pg_catalog.pg_statistic" postgres
heap table "postgres"."pg_catalog"."pg_statistic", block 4, offset 2, attribute 29:
toasted value id 13227 for attribute 29 missing from toast table
heap table "postgres"."pg_catalog"."pg_statistic", block 4, offset 5, attribute 27:
toasted value id 13228 for attribute 27 missing from toast table

diff --git a/contrib/amcheck/verify_heapam.c b/contrib/amcheck/verify_heapam.c
index 5ccae46375..a0be71bb7f 100644
--- a/contrib/amcheck/verify_heapam.c
+++ b/contrib/amcheck/verify_heapam.c
@@ -1111,8 +1111,8 @@ check_tuple_attribute(HeapCheckContext *ctx)
}
if (!found_toasttup)
report_corruption(ctx,
- psprintf("toasted value for attribute %u missing from toast table",
- ctx->attnum));
+ psprintf("toasted value id %u for attribute %u missing from toast table",
+ toast_pointer.va_valueid, ctx->attnum));
else if (ctx->chunkno != (ctx->endchunk + 1))
report_corruption(ctx,
psprintf("final toast chunk number %u differs from expected value %u",

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company