Re: Use of additional index columns in rows filtering

On 7/15/23 16:20, Tomas Vondra wrote:
>
> ...
> 
> 4) problems with opcintype != opckeytype (name_ops)
> 
> While running the tests, I ran into an issue with name_ops, causing
> failures for \dT and other catalog queries. The root cause is that
> name_ops has opcintype = name, but opckeytype = cstring. The index-only
> clauses are copied from the table, with Vars mutated to reference the
> INDEX_VAR. But the type is not, so when we get to evaluating the
> expressions, CheckVarSlotCompatibility() fails because the Var has name,
> but the iss_IndexSlot (created with index tuple descriptor) has cstring.
> 
> The rebased patch fixes this by explicitly adjusting types of the
> descriptor in ExecInitIndexScan().
> 
> However, maybe this indicates the very idea of evaluating expressions
> using slot with index tuple descriptor is misguided. This made me look
> at regular index-only scan (nodeIndexonlyscan.c), and that uses a slot
> with the "table" structure, and instead of evaluating the expression on
> the index index tuple it expands the index tuple into the table slot.
> Which is what StoreIndexTuple() does.
> 
> So maybe this should do what IOS does - expand the index tuple into
> "table slot" and evaluate the expression on that. That'd also make the
> INDEX_VAR tweak in createplan.c unnecessary - in fact, that seemed a bit
> strange anyway, so ditching fix_indexfilter_mutator would be good.
> 

This kept bothering me, so I looked at it today, and reworked it to use
the IOS approach. It's a bit more complicated because for IOS both slots
have the same overall structure, except for the data types. But for
regular index scans that's not the case - the code has to "expand" the
index tuple into the larger "table slot". This works, and in general I
think the result is much cleaner - in particular, it means we don't need
to switch the Var nodes to reference the INDEX_VAR.

While experimenting with this I realized again that we're not matching
expressions to IOS. So if you have an expression index on (a+b), that
can't be used even if the query only uses this particular expression.
The same limitation applies to index-only filters, of course. It's not
the fault of this patch, but perhaps it'd be an interesting improvement.


regards

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