#17502: Filter on field from base class 2 levels up hierarchy causes extra join
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Reporter: dbenamy@… | Owner: lrekucki
Type: Cleanup/optimization | Status: assigned
Component: Uncategorized | Version: 1.3
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by lrekucki):
* status: new => assigned
* stage: Unreviewed => Accepted
Comment:
Going step by step:
1. We start with table "C".
2. {{{Query.setup_inherited_model()}}} adds {{{INNER JOIN}}} for all
parent models. C is joined with B [{{{ ON (C.b_ptr_id = B.a_ptr_id) - B's
parent pointer is it's own primary key). Then C is joined with A and
because of B's PK is the same as B's parent pointer, Django optimizes? by
doing (C.b_ptr_id [= B.a_ptr_id] = A.id).
3. Then the {{{Query.add_filter()}}} lookups the {{{alice}}} field and
setups neccesary joins with {{{Query.setup_joins()}}}. As the field
resides on the base model, the following code is executed:
{{{#!python
for int_model in opts.get_base_chain(model):
if int_model is proxied_model:
opts = int_model._meta
else:
lhs_col = opts.parents[int_model].column
dedupe = lhs_col in opts.duplicate_targets
if dedupe:
exclusions.update(self.dupe_avoidance.get(
(id(opts), lhs_col), ()))
dupe_set.add((opts, lhs_col))
opts = int_model._meta
alias = self.join((alias, opts.db_table, lhs_col,
opts.pk.column), exclusions=exclusions)
joins.append(alias)
exclusions.add(alias)
for (dupe_opts, dupe_col) in dupe_set:
self.update_dupe_avoidance(dupe_opts,
dupe_col,
alias)
}}}
This takes a different approach by joining C with B (alias lookup
succeeds) and then B with A (alias lookup fails, because we previously
used a different connection). This is where the additional join comes
from.
Possible solutions:
a. make the strategy in 2. and 3. consistent. Chaining the joins step by
step instead of skipping should work, but if we select only values from C
and A, we should be able to skip joining B. This would suggest changing
{{{setup_joins to}}} try to reuse the optimized alias first.
b. When doing the {{{setup_inherited_models()}}}, if we did join to B
(i.e. there are selected fields to B), add a fake entry to {{{alias_map}}}
for "B INNER JOIN A", that {{{setup_joins()}}} could reuse later.
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Ticket URL: <https://code.djangoproject.com/ticket/17502#comment:2>
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