>one idea could be to actually prepare a query using SPI for "select * from
>table order by <cols>" and then peek inside
> to see which plan was generated.
I like that!!!
Here's a first attempt, it looks like it's working...
(I still have to skip non-btree indexes and expression indexes, plus
add a ASC/DESC to the select)
static bool use_index_scan(Relation OldHeap, Oid indexOid)
{
HeapTuple indexTuple;
Form_pg_index indexForm;
struct _SPI_plan *spiPlan;
char st[(NAMEDATALEN+1)*INDEX_MAX_KEYS+NAMEDATALEN+100];
int i;
TupleDesc oldTupDesc;
bool retval = true;
PlannedStmt *plan;
CachedPlanSource *cps;
oldTupDesc = RelationGetDescr(OldHeap);
sprintf(st, "select * from %s order by ", OldHeap->rd_rel->relname.data);
indexTuple = SearchSysCache(INDEXRELID, ObjectIdGetDatum(indexOid),0, 0, 0);
if (!HeapTupleIsValid(indexTuple))
elog(ERROR, "cache lookup failed for index %u", indexOid);
indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
for (i = 0; i < indexForm->indnatts; i++)
{
strcat(st, oldTupDesc->attrs[indexForm->indkey.values[i]-1]->attname.data);
if (i+1 < indexForm->indnatts)
{
strcat(st, ",");
}
}
ReleaseSysCache(indexTuple);
if (SPI_connect() != SPI_OK_CONNECT)
return false;
spiPlan = SPI_prepare(st, 0, NULL);
if (spiPlan == NULL)
{
SPI_finish();
return false;
}
cps = (CachedPlanSource*)(list_head(spiPlan->plancache_list)->data.ptr_value);
plan = (PlannedStmt*)(list_head(cps->plan->stmt_list)->data.ptr_value);
if (IsA(plan->planTree, Sort))
{
retval = false;
}
SPI_freeplan(spiPlan);
SPI_finish();
return retval;
}
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