The initial building of RestrictInfos and SpecialJoinInfos tends to
create structures that share relid sets (such as syn_lefthand and
outer_relids). There's nothing wrong with that in itself, but when
we modify those relid sets during join removal, we have to be sure
not to corrupt the values that other structures are pointing at.
remove_rel_from_query() was careless about this. It accidentally
worked anyway because (1) we'd never be reducing the sets to empty,
so they wouldn't get pfree'd; and (2) the in-place modification is the
same one that we did or will apply to the other struct's relid set,
so that there wasn't visible corruption at the end of the process.
While there's no live bug in a standard build, of course this is way
too fragile to accept going forward. (Maybe we should back-patch
this change too for safety, but I've refrained for now.)
This bug was exposed by the recent invention of REALLOCATE_BITMAPSETS.
Commit e0477837c had installed a fix, but that went away with the
reversion of SJE, so now we need to fix it again.
David Rowley and Tom Lane
Discussion: https://postgr.es/m/CACJufxFVQmr4=JWHAOSLuKA5Zy9H26nY6tVrRFBOekHoALyCkQ@mail.gmail.com
as determined by include-what-you-use (IWYU)
While IWYU also suggests to *add* a bunch of #include's (which is its
main purpose), this patch does not do that. In some cases, a more
specific #include replaces another less specific one.
Some manual adjustments of the automatic result:
- IWYU currently doesn't know about includes that provide global
variable declarations (like -Wmissing-variable-declarations), so
those includes are being kept manually.
- All includes for port(ability) headers are being kept for now, to
play it safe.
- No changes of catalog/pg_foo.h to catalog/pg_foo_d.h, to keep the
patch from exploding in size.
Note that this patch touches just *.c files, so nothing declared in
header files changes in hidden ways.
As a small example, in src/backend/access/transam/rmgr.c, some IWYU
pragma annotations are added to handle a special case there.
Discussion: https://www.postgresql.org/message-id/flat/af837490-6b2f-46df-ba05-37ea6a6653fc%40eisentraut.org
This commit introduces a new field 'sublevels_up' in ReplaceVarnoContext,
and enhances replace_varno_walker() to:
1) recurse into subselects with sublevels_up increased, and
2) perform the replacement only when varlevelsup is equal to sublevels_up.
This commit also fixes some outdated comments. And besides adding relevant
test cases, it makes some unification over existing SJE test cases.
Discussion: https://postgr.es/m/CAMbWs4-%3DPO6Mm9gNnySbx0VHyXjgnnYYwbN9dth%3DTLQweZ-M%2Bg%40mail.gmail.com
Author: Richard Guo
Reviewed-by: Andrei Lepikhov, Alexander Korotkov
When removing a useless join, we'd remove PHVs that are not used at join
partner rels or above the join. A PHV that references the join's relid
in ph_eval_at is logically "above" the join and thus should not be
removed. We have the following check for that:
!bms_is_member(ojrelid, phinfo->ph_eval_at)
However, in the case of SJE removing a useless inner join, 'ojrelid' is
set to -1, which would trigger the "negative bitmapset member not
allowed" error in bms_is_member().
Fix it by skipping examining ojrelid for inner joins in this check.
Reported-by: Zuming Jiang
Bug: #18260
Discussion: https://postgr.es/m/18260-1b6a0c4ae311b837%40postgresql.org
Author: Richard Guo
Reviewed-by: Andrei Lepikhov
The target relation for INSERT/UPDATE/DELETE/MERGE has a different behavior
than other relations in EvalPlanQual() and RETURNING clause. This is why we
forbid target relation to be either source or target relation in SJE.
It's not clear if we could ever support this.
Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/b9e8f460-f9a6-0e9b-e8ba-60d59f0bc22c%40gmail.com
When SJE uses RelOptInfo.unique_for_rels cache, it passes filtered quals to
innerrel_is_unique_ext(). That might lead to an invalid match to cache entries
made by previous non self-join checking calls. Add UniqueRelInfo.self_join
flag to prevent such cases. Also, fix that SJE should require a strict match
of outerrelids to make sure UniqueRelInfo.extra_clauses are valid.
Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/4788f781-31bd-9796-d7d6-588a751c8787%40gmail.com
When the SJE code handles the transfer of qual clauses from the removed
relation to the remaining one, it replaces the Vars of the removed
relation with the Vars of the remaining relation for each clause, and
then reintegrates these clauses into the appropriate restriction or join
clause lists, while attempting to avoid duplicates.
However, the code compares RestrictInfo->clause to determine if two
clauses are duplicates. This is just flat wrong. Two RestrictInfos
with the same clause can have different required_relids,
incompatible_relids, is_pushed_down, and so on. This can cause qual
clauses to be mistakenly omitted, leading to wrong results.
This patch fixes it by comparing the entire RestrictInfos not just their
clauses ignoring 'rinfo_serial' field (otherwise almost all RestrictInfos will
be unique). Making 'rinfo_serial' equal_ignore would break other code. This
is why this commit implements our own comparison function for checking the
equality of RestrictInfos.
Reported-by: Zuming Jiang
Bug: #18261
Discussion: https://postgr.es/m/18261-2a75d748c928609b%40postgresql.org
Author: Richard Guo
There are a lot of situations when we share the same pointer to a Bitmapset
structure across different places. In order to evade undesirable side effects
replace_relid() function should always return a copy.
Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_wJthNtYBL%2BSsebpgF-5L2r5zFFk6xYbS0A78GKOTFHw%40mail.gmail.com
Reviewed-by: Richard Guo, Andres Freund, Ashutosh Bapat, Andrei Lepikhov
Self-join removal appears to be safe to apply with placeholder variables
as long as we handle PlaceHolderVar in replace_varno_walker() and replace
relid in phinfo->ph_lateral.
Discussion: https://postgr.es/m/18187-831da249cbd2ff8e%40postgresql.org
Author: Richard Guo
Reviewed-by: Andrei Lepikhov
This commit also retires sje_walker. This increases the generalty of replacing
varno in the parse tree and simplifies the code.
Discussion: https://postgr.es/m/18187-831da249cbd2ff8e%40postgresql.org
Author: Richard Guo
Reviewed-by: Andrei Lepikhov
It seems that a PHV evaluated/needed at or below the self join should not have
a problem if we remove the self join. But this requires further investigation.
For now, we just do not remove self joins if the rel to be removed is laterally
referenced by PHVs.
Discussion: https://postgr.es/m/CAMbWs4-ns73VF9gi37q61G3dS6Xuos+HtryMaBh37WQn=BsaJw@mail.gmail.com
Author: Richard Guo
d3d55ce571 changed RelOptInfo.unique_for_rels from the list of Relid sets to
the list of UniqueRelInfo's. But it didn't make UniqueRelInfo a node.
This commit makes UniqueRelInfo a node. Also this commit revises some
comments related to RelOptInfo.unique_for_rels.
Reported-by: Tom Lane
Discussion: https://postgr.es/m/flat/1189851.1698340331%40sss.pgh.pa.us
The Self Join Elimination (SJE) feature removes an inner join of a plain table
to itself in the query tree if is proved that the join can be replaced with
a scan without impacting the query result. Self join and inner relation are
replaced with the outer in query, equivalence classes, and planner info
structures. Also, inner restrictlist moves to the outer one with removing
duplicated clauses. Thus, this optimization reduces the length of the range
table list (this especially makes sense for partitioned relations), reduces
the number of restriction clauses === selectivity estimations, and potentially
can improve total planner prediction for the query.
The SJE proof is based on innerrel_is_unique machinery.
We can remove a self-join when for each outer row:
1. At most one inner row matches the join clause.
2. Each matched inner row must be (physically) the same row as the outer one.
In this patch we use the next approach to identify a self-join:
1. Collect all merge-joinable join quals which look like a.x = b.x
2. Add to the list above the baseretrictinfo of the inner table.
3. Check innerrel_is_unique() for the qual list. If it returns false, skip
this pair of joining tables.
4. Check uniqueness, proved by the baserestrictinfo clauses. To prove
the possibility of self-join elimination inner and outer clauses must have
an exact match.
The relation replacement procedure is not trivial and it is partly combined
with the one, used to remove useless left joins. Tests, covering this feature,
were added to join.sql. Some regression tests changed due to self-join removal
logic.
Discussion: https://postgr.es/m/flat/64486b0b-0404-e39e-322d-0801154901f3%40postgrespro.ru
Author: Andrey Lepikhov, Alexander Kuzmenkov
Reviewed-by: Tom Lane, Robert Haas, Andres Freund, Simon Riggs, Jonathan S. Katz
Reviewed-by: David Rowley, Thomas Munro, Konstantin Knizhnik, Heikki Linnakangas
Reviewed-by: Hywel Carver, Laurenz Albe, Ronan Dunklau, vignesh C, Zhihong Yu
Reviewed-by: Greg Stark, Jaime Casanova, Michał Kłeczek, Alena Rybakina
Reviewed-by: Alexander Korotkov
Here we adjust relation_has_unique_index_for() so that it no longer makes
use of partial unique indexes as uniqueness proofs. It is incorrect to
use these as the predicates used by check_index_predicates() to set
predOK makes use of not only baserestrictinfo quals as proofs, but also
qual from join conditions. For relation_has_unique_index_for()'s case, we
need to know the relation is unique for a given set of columns before any
joins are evaluated, so if predOK was only set to true due to some join
qual, then it's unsafe to use such indexes in
relation_has_unique_index_for(). The final plan may not even make use
of that index, which could result in reading tuples that are not as
unique as the planner previously expected them to be.
Bug: #17975
Reported-by: Tor Erik Linnerud
Backpatch-through: 11, all supported versions
Discussion: https://postgr.es/m/17975-98a90c156f25c952%40postgresql.org
Since commit b448f1c8d, we've been able to remove left joins
(that are otherwise removable) even when they are underneath
other left joins, a case that was previously prevented by a
delay_upper_joins check. This is a clear improvement, but
it has a surprising side-effect: it's now possible that there
are EquivalenceClasses whose relid sets mention the removed
baserel and/or outer join. If we fail to clean those up,
we may drop essential join quals due to not having any join
level that appears to satisfy their relid sets.
(It's not quite 100% clear that this was impossible before.
But the lack of complaints since we added join removal a dozen
years ago strongly suggests that it was impossible.)
Richard Guo and Tom Lane, per bug #17976 from Zuming Jiang
Discussion: https://postgr.es/m/17976-4b638b525e9a983b@postgresql.org
A placeholder that references the outer join's relid in ph_eval_at
is logically "above" the join, and therefore we can't remove its
PlaceHolderInfo: it might still be used somewhere in the query.
This was not an issue pre-v16 because we failed to remove the join
at all in such cases. The new outer-join-aware-Var infrastructure
permits deducing that it's okay to remove the join, but then we
have to clean up correctly afterwards.
Report and fix by Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_tuVn9EwwMcggGiZJWWstdXX_ci8FeEU17vs+4nLgw3w@mail.gmail.com
When we're deleting a no-op LEFT JOIN from the query, we must remove
the join's joinclauses from surviving relations' joininfo lists.
The invention of "cloned" clauses in 2489d76c4 broke the logic for
that; it'd fail to remove clones that include OJ relids outside the
doomed join's min relid sets, which could happen if that join was
previously discovered to commute with some other join.
This accidentally failed to cause problems in the majority of cases,
because we'd never decide that such a cloned clause was evaluatable at
any surviving join. However, Richard Guo discovered a case where that
did happen, leading to "no relation entry for relid" errors later.
Also, adding assertions that a non-removed clause contains no Vars from
the doomed join exposes that there are quite a few existing regression
test cases where the problem happens but is accidentally not exposed.
The fix for this is just to include the target join's commute_above_r
and commute_below_l sets in the relid set we test against when
deciding whether a join clause is "pushed down" and thus not
removable.
While at it, do a little refactoring: the join's relid set can be
computed inside remove_rel_from_query rather than in the caller.
Patch by me; thanks to Richard Guo for review.
Discussion: https://postgr.es/m/CAMbWs4_PHrRqTKDNnTRsxxQy6BtYCVKsgXm1_gdN2yQ=kmcO5g@mail.gmail.com
In commit 9df8f903e I (tgl) switched join_is_removable() from
using the min relid sets of the join under consideration to
using its full syntactic relid sets. This was a mistake,
as it allowed join removal in cases where a reference to the
join output would survive in some syntactically-lower join
condition. Revert to the former coding.
Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-EU9uBGSP7G-iTwLBhRQ=rnZKvFDhD+n+xhajokyPCKg@mail.gmail.com
After applying outer-join identity 3 in the forward direction,
it was possible for the planner to mistakenly apply a qual clause
from above the two outer joins at the now-lower join level.
This can give the wrong answer, since a value that would get nulled
by the now-upper join might not yet be null.
To fix, when we perform such a transformation, consider that the
now-lower join hasn't really completed the outer join it's nominally
responsible for and thus its relid set should not include that OJ's
relid (nor should its output Vars have that nullingrel bit set).
Instead we add those bits when the now-upper join is performed.
The existing rules for qual placement then suffice to prevent
higher qual clauses from dropping below the now-upper join.
There are a few complications from needing to consider transitive
closures in case multiple pushdowns have happened, but all in all
it's not a very complex patch.
This is all new logic (from 2489d76c4) so no need to back-patch.
The added test cases all have the same results as in v15.
Tom Lane and Richard Guo
Discussion: https://postgr.es/m/0b819232-4b50-f245-1c7d-c8c61bf41827@postgrespro.ru
I've had a bee in my bonnet for some time about getting rid of
RestrictInfo.is_pushed_down, because it's squishily defined and
requires not-inexpensive extra tests to use (cf RINFO_IS_PUSHED_DOWN).
In commit 2489d76c4, I tried to make remove_rel_from_query() not
depend on that macro; but the replacement test is buggy,
as exposed by a report from Rushabh Lathia and Robert Haas.
That change was pretty incidental to the main goal of 2489d76c4,
so let's just revert it for now. (Getting rid of is_pushed_down
is still far away, anyway.)
Discussion: https://postgr.es/m/CA+TgmoYco=hmg+iX1CW9Y1_CzNoSL81J03wUG-d2_3=rue+L2A@mail.gmail.com
It's possible, in admittedly-rather-contrived cases, for an eclass
to generate a derived "join" qual that constrains the post-outer-join
value(s) of some RHS variable(s) without mentioning the LHS at all.
While the mechanisms were set up to work for this, we fell foul of
the "get_common_eclass_indexes" filter installed by commit 3373c7155:
it could decide that such an eclass wasn't relevant to the join, so
that the required qual clause wouldn't get emitted there or anywhere
else.
To fix, apply get_common_eclass_indexes only at inner joins, where
its rule is still valid. At an outer join, fall back to examining all
eclasses that mention either input (or the OJ relid, though it should
be impossible for an eclass to mention that without mentioning either
input). Perhaps we can improve on that later, but the cost/benefit of
adding more complexity to skip some irrelevant eclasses is dubious.
To allow cheaply distinguishing outer from inner joins, pass the
ojrelid to generate_join_implied_equalities as a separate argument.
This also allows cleaning up some sloppiness that had crept into
the definition of its join_relids argument, and it allows accurate
calculation of nominal_join_relids for a child outer join. (The
latter oversight seems not to have been a live bug, but it certainly
could have caused problems in future.)
Also fix what might be a live bug in check_index_predicates: it was
being sloppy about what it passed to generate_join_implied_equalities.
Per report from Richard Guo.
Discussion: https://postgr.es/m/CAMbWs4-DsTBfOvXuw64GdFss2=M5cwtEhY=0DCS7t2gT7P6hSA@mail.gmail.com
This was not something that required consideration before MERGE
was invented; but MERGE builds a join tree that left-joins to the
result relation, meaning that remove_useless_joins will consider
removing it. That should generally be stopped by the query's use
of output variables from the result relation. However, if the
result relation is inherited (e.g. a partitioned table) then
we don't add any row identity variables to the query until
expand_inherited_rtentry, which happens after join removal.
This was exposed as of commit 3c569049b, which made it possible
to deduce that a partitioned table could contain at most one row
matching a join key, enabling removal of the not-yet-expanded
result relation. Ooops.
To fix, let's just teach join_is_removable that the query result
rel is never removable. It's a cheap enough test in any case,
and it'll save some cycles that we'd otherwise expend in proving
that it's not removable, even in the cases we got right.
Back-patch to v15 where MERGE was added. Although I think the
case cannot be reached in v15, this seems like cheap insurance.
Per investigation of a report from Alexander Lakhin.
Discussion: https://postgr.es/m/36bee393-b351-16ac-93b2-d46d83637e45@gmail.com
In commit b78f6264e I opined that it was "too risky" to delete a
relation's RelOptInfo from the planner's data structures when we have
realized that we don't need to join to it; so instead we just marked
it as a dead relation. In hindsight that judgment seems flawed: any
subsequent access to such a dead relation is arguably a bug in
itself, so leaving the RelOptInfo present just helps to mask bugs.
Let's delete it instead, allowing removal of the whole notion of a
"dead relation". So far as the regression tests can find, this
requires no other code changes, except for one Assert in equivclass.c
that was very dubiously not complaining about access to a dead rel.
Discussion: https://postgr.es/m/229905.1676062220@sss.pgh.pa.us
analyzejoins.c took care to clean out removed relids from the
clause_relids and required_relids of RestrictInfos associated with
the doomed rel ... but it paid no attention to the fact that if such a
RestrictInfo contains an OR clause, there will be sub-RestrictInfos
containing similar fields.
I'm more than a bit surprised that this oversight hasn't caused
visible problems before. In any case, it's certainly broken now,
so add logic to clean out the sub-RestrictInfos recursively.
We might need to back-patch this someday.
Per bug #17786 from Robins Tharakan.
Discussion: https://postgr.es/m/17786-f1ea7fbdab97daec@postgresql.org
The portion of join_is_removable() that checks PlaceHolderVars
can be made a little more accurate and intelligible than it was.
The key point is that we can allow join removal even if a PHV
mentions the target rel in ph_eval_at, if that mention was only
added as a consequence of forcing the PHV up to a join level
that's at/above the outer join we're trying to get rid of.
We can check that by testing for the OJ's relid appearing in
ph_eval_at, indicating that it's supposed to be evaluated after
the outer join, plus the existing test that the contained
expression doesn't actually mention the target rel.
While here, add an explicit check that there'll be something left
in ph_eval_at after we remove the target rel and OJ relid. There
is an Assert later on about that, and I'm not too sure that the
case could happen for a PHV satisfying the other constraints,
but let's just check. (There was previously a bms_is_subset test
that meant to cover this risk, but it's broken now because it
doesn't account for the fact that we'll also remove the OJ relid.)
The real reason for revisiting this code though is that the
Assert I left behind in 8538519db turns out to be easily
reachable, because if a PHV of this sort appears in an upper-level
qual clause then that clause's clause_relids will include the
PHV's ph_eval_at relids. This is a mirage though: we have or soon
will remove these relids from the PHV's ph_eval_at, and therefore
they no longer belong in qual clauses' clause_relids either.
Remove that Assert in join_is_removable, and replace the similar
one in remove_rel_from_query with code to remove the deleted relids
from clause_relids.
Per bug #17773 from Robins Tharakan.
Discussion: https://postgr.es/m/17773-a592e6cedbc7bac5@postgresql.org
If we have a RestrictInfo that mentions both the removal-candidate
relation and the outer join's relid, then that is a pushed-down
condition not a join condition, so it should be grounds for deciding
that we can't remove the outer join. In commit 2489d76c4, I'd blindly
included the OJ's relid into "joinrelids" as per the new standard
convention, but the checks of attr_needed and ph_needed should only
allow the join's input rels to be mentioned.
Having done that, the check for references in pushed-down quals
a few lines further down should be redundant. I left it in place
as an Assert, though.
While researching this I happened across a couple of comments that
worried about the effects of update_placeholder_eval_levels.
That's gone as of b448f1c8d, so we can remove some worry.
Per bug #17769 from Robins Tharakan. The submitted test case
triggers this more or less accidentally because we flatten out
a LATERAL sub-select after we've done join strength reduction;
if we did that in the other order, this problem would be masked
because the outer join would get simplified to an inner join.
To ensure that the committed test case will continue to test
what it means to even if we make that happen someday, use a
test clause involving COALESCE(), which will prevent us from
using it to do join strength reduction.
Patch by me, but thanks to Richard Guo for initial investigation.
Discussion: https://postgr.es/m/17769-e4f7a5c9d84a80a7@postgresql.org
Remove RestrictInfo.nullable_relids, along with a good deal of
infrastructure that calculated it. One use-case for it was in
join_clause_is_movable_to, but we can now replace that usage with
a check to see if the clause's relids include any outer join
that can null the target relation. The other use-case was in
join_clause_is_movable_into, but that test can just be dropped
entirely now that the clause's relids include outer joins.
Furthermore, join_clause_is_movable_into should now be
accurate enough that it will accept anything returned by
generate_join_implied_equalities, so we can restore the Assert
that was diked out in commit 95f4e59c3.
Remove the outerjoin_delayed mechanism. We needed this before to
prevent quals from getting evaluated below outer joins that should
null some of their vars. Now that we consider varnullingrels while
placing quals, that's taken care of automatically, so throw the
whole thing away.
Teach remove_useless_result_rtes to also remove useless FromExprs.
Having done that, the delay_upper_joins flag serves no purpose any
more and we can remove it, largely reverting 11086f2f2.
Use constant TRUE for "dummy" clauses when throwing back outer joins.
This improves on a hack I introduced in commit 6a6522529. If we
have a left-join clause l.x = r.y, and a WHERE clause l.x = constant,
we generate r.y = constant and then don't really have a need for the
join clause. But we must throw the join clause back anyway after
marking it redundant, so that the join search heuristics won't think
this is a clauseless join and avoid it. That was a kluge introduced
under time pressure, and after looking at it I thought of a better
way: let's just introduce constant-TRUE "join clauses" instead,
and get rid of them at the end. This improves the generated plans for
such cases by not having to test a redundant join clause. We can also
get rid of the ugly hack used to mark such clauses as redundant for
selectivity estimation.
Patch by me; thanks to Richard Guo for review.
Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us
Traditionally we used the same Var struct to represent the value
of a table column everywhere in parse and plan trees. This choice
predates our support for SQL outer joins, and it's really a pretty
bad idea with outer joins, because the Var's value can depend on
where it is in the tree: it might go to NULL above an outer join.
So expression nodes that are equal() per equalfuncs.c might not
represent the same value, which is a huge correctness hazard for
the planner.
To improve this, decorate Var nodes with a bitmapset showing
which outer joins (identified by RTE indexes) may have nulled
them at the point in the parse tree where the Var appears.
This allows us to trust that equal() Vars represent the same value.
A certain amount of klugery is still needed to cope with cases
where we re-order two outer joins, but it's possible to make it
work without sacrificing that core principle. PlaceHolderVars
receive similar decoration for the same reason.
In the planner, we include these outer join bitmapsets into the relids
that an expression is considered to depend on, and in consequence also
add outer-join relids to the relids of join RelOptInfos. This allows
us to correctly perceive whether an expression can be calculated above
or below a particular outer join.
This change affects FDWs that want to plan foreign joins. They *must*
follow suit when labeling foreign joins in order to match with the
core planner, but for many purposes (if postgres_fdw is any guide)
they'd prefer to consider only base relations within the join.
To support both requirements, redefine ForeignScan.fs_relids as
base+OJ relids, and add a new field fs_base_relids that's set up by
the core planner.
Large though it is, this commit just does the minimum necessary to
install the new mechanisms and get check-world passing again.
Follow-up patches will perform some cleanup. (The README additions
and comments mention some stuff that will appear in the follow-up.)
Patch by me; thanks to Richard Guo for review.
Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us
Up to now we've just searched the placeholder_list when we want to
find the PlaceHolderInfo with a given ID. While there's no evidence
of that being a problem in the field, an upcoming patch will add
find_placeholder_info() calls in build_joinrel_tlist(), which seems
likely to make it more of an issue: a joinrel emitting lots of
PlaceHolderVars would incur O(N^2) cost, and we might be building
a lot of joinrels in complex queries. Hence, add an array that
can be indexed directly by phid to make the lookups constant-time.
Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us
Previously, pull_varnos() took the relids of a PlaceHolderVar as being
equal to the relids in its contents, but that fails to account for the
possibility that we have to postpone evaluation of the PHV due to outer
joins. This could result in a malformed plan. The known cases end up
triggering the "failed to assign all NestLoopParams to plan nodes"
sanity check in createplan.c, but other symptoms may be possible.
The right value to use is the join level we actually intend to evaluate
the PHV at. We can get that from the ph_eval_at field of the associated
PlaceHolderInfo. However, there are some places that call pull_varnos()
before the PlaceHolderInfos have been created; in that case, fall back
to the conservative assumption that the PHV will be evaluated at its
syntactic level. (In principle this might result in missing some legal
optimization, but I'm not aware of any cases where it's an issue in
practice.) Things are also a bit ticklish for calls occurring during
deconstruct_jointree(), but AFAICS the ph_eval_at fields should have
reached their final values by the time we need them.
The main problem in making this work is that pull_varnos() has no
way to get at the PlaceHolderInfos. We can fix that easily, if a
bit tediously, in HEAD by passing it the planner "root" pointer.
In the back branches that'd cause an unacceptable API/ABI break for
extensions, so leave the existing entry points alone and add new ones
with the additional parameter. (If an old entry point is called and
encounters a PHV, it'll fall back to using the syntactic level,
again possibly missing some valid optimization.)
Back-patch to v12. The computation is surely also wrong before that,
but it appears that we cannot reach a bad plan thanks to join order
restrictions imposed on the subquery that the PlaceHolderVar came from.
The error only became reachable when commit 4be058fe9 allowed trivial
subqueries to be collapsed out completely, eliminating their join order
restrictions.
Per report from Stephan Springl.
Discussion: https://postgr.es/m/171041.1610849523@sss.pgh.pa.us
Originally, Postgres Lists were a more or less exact reimplementation of
Lisp lists, which consist of chains of separately-allocated cons cells,
each having a value and a next-cell link. We'd hacked that once before
(commit d0b4399d8) to add a separate List header, but the data was still
in cons cells. That makes some operations -- notably list_nth() -- O(N),
and it's bulky because of the next-cell pointers and per-cell palloc
overhead, and it's very cache-unfriendly if the cons cells end up
scattered around rather than being adjacent.
In this rewrite, we still have List headers, but the data is in a
resizable array of values, with no next-cell links. Now we need at
most two palloc's per List, and often only one, since we can allocate
some values in the same palloc call as the List header. (Of course,
extending an existing List may require repalloc's to enlarge the array.
But this involves just O(log N) allocations not O(N).)
Of course this is not without downsides. The key difficulty is that
addition or deletion of a list entry may now cause other entries to
move, which it did not before.
For example, that breaks foreach() and sister macros, which historically
used a pointer to the current cons-cell as loop state. We can repair
those macros transparently by making their actual loop state be an
integer list index; the exposed "ListCell *" pointer is no longer state
carried across loop iterations, but is just a derived value. (In
practice, modern compilers can optimize things back to having just one
loop state value, at least for simple cases with inline loop bodies.)
In principle, this is a semantics change for cases where the loop body
inserts or deletes list entries ahead of the current loop index; but
I found no such cases in the Postgres code.
The change is not at all transparent for code that doesn't use foreach()
but chases lists "by hand" using lnext(). The largest share of such
code in the backend is in loops that were maintaining "prev" and "next"
variables in addition to the current-cell pointer, in order to delete
list cells efficiently using list_delete_cell(). However, we no longer
need a previous-cell pointer to delete a list cell efficiently. Keeping
a next-cell pointer doesn't work, as explained above, but we can improve
matters by changing such code to use a regular foreach() loop and then
using the new macro foreach_delete_current() to delete the current cell.
(This macro knows how to update the associated foreach loop's state so
that no cells will be missed in the traversal.)
There remains a nontrivial risk of code assuming that a ListCell *
pointer will remain good over an operation that could now move the list
contents. To help catch such errors, list.c can be compiled with a new
define symbol DEBUG_LIST_MEMORY_USAGE that forcibly moves list contents
whenever that could possibly happen. This makes list operations
significantly more expensive so it's not normally turned on (though it
is on by default if USE_VALGRIND is on).
There are two notable API differences from the previous code:
* lnext() now requires the List's header pointer in addition to the
current cell's address.
* list_delete_cell() no longer requires a previous-cell argument.
These changes are somewhat unfortunate, but on the other hand code using
either function needs inspection to see if it is assuming anything
it shouldn't, so it's not all bad.
Programmers should be aware of these significant performance changes:
* list_nth() and related functions are now O(1); so there's no
major access-speed difference between a list and an array.
* Inserting or deleting a list element now takes time proportional to
the distance to the end of the list, due to moving the array elements.
(However, it typically *doesn't* require palloc or pfree, so except in
long lists it's probably still faster than before.) Notably, lcons()
used to be about the same cost as lappend(), but that's no longer true
if the list is long. Code that uses lcons() and list_delete_first()
to maintain a stack might usefully be rewritten to push and pop at the
end of the list rather than the beginning.
* There are now list_insert_nth...() and list_delete_nth...() functions
that add or remove a list cell identified by index. These have the
data-movement penalty explained above, but there's no search penalty.
* list_concat() and variants now copy the second list's data into
storage belonging to the first list, so there is no longer any
sharing of cells between the input lists. The second argument is
now declared "const List *" to reflect that it isn't changed.
This patch just does the minimum needed to get the new implementation
in place and fix bugs exposed by the regression tests. As suggested
by the foregoing, there's a fair amount of followup work remaining to
do.
Also, the ENABLE_LIST_COMPAT macros are finally removed in this
commit. Code using those should have been gone a dozen years ago.
Patch by me; thanks to David Rowley, Jesper Pedersen, and others
for review.
Discussion: https://postgr.es/m/11587.1550975080@sss.pgh.pa.us
Create a new header optimizer/optimizer.h, which exposes just the
planner functions that can be used "at arm's length", without need
to access Paths or the other planner-internal data structures defined
in nodes/relation.h. This is intended to provide the whole planner
API seen by most of the rest of the system; although FDWs still need
to use additional stuff, and more thought is also needed about just
what selfuncs.c should rely on.
The main point of doing this now is to limit the amount of new
#include baggage that will be needed by "planner support functions",
which I expect to introduce later, and which will be in relevant
datatype modules rather than anywhere near the planner.
This commit just moves relevant declarations into optimizer.h from
other header files (a couple of which go away because everything
got moved), and adjusts #include lists to match. There's further
cleanup that could be done if we want to decide that some stuff
being exposed by optimizer.h doesn't belong in the planner at all,
but I'll leave that for another day.
Discussion: https://postgr.es/m/11460.1548706639@sss.pgh.pa.us
Tom Lane