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postgresql/src/include/optimizer/placeholder.h

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Add a concept of "placeholder" variables to the planner. These are variables that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.
2008-10-21 16:42:53 -04:00
/*-------------------------------------------------------------------------
*
* placeholder.h
* prototypes for optimizer/util/placeholder.c.
*
*
Update copyright for 2025 Backpatch-through: 13
2025-01-01 11:21:55 -05:00
* Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
Add a concept of "placeholder" variables to the planner. These are variables that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.
2008-10-21 16:42:53 -04:00
* Portions Copyright (c) 1994, Regents of the University of California
*
Remove cvs keywords from all files.
2010-09-20 16:08:53 -04:00
* src/include/optimizer/placeholder.h
Add a concept of "placeholder" variables to the planner. These are variables that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.
2008-10-21 16:42:53 -04:00
*
*-------------------------------------------------------------------------
*/
#ifndef PLACEHOLDER_H
#define PLACEHOLDER_H
Rename nodes/relation.h to nodes/pathnodes.h. The old name of this file was never a very good indication of what it was for. Now that there's also access/relation.h, we have a potential confusion hazard as well, so let's rename it to something more apropos. Per discussion, "pathnodes.h" is reasonable, since a good fraction of the file is Path node definitions. While at it, tweak a couple of other headers that were gratuitously importing relation.h into modules that don't need it. Discussion: https://postgr.es/m/7719.1548688728@sss.pgh.pa.us
2019-01-29 16:49:25 -05:00
#include "nodes/pathnodes.h"
Add a concept of "placeholder" variables to the planner. These are variables that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.
2008-10-21 16:42:53 -04:00
extern PlaceHolderVar *make_placeholder_expr(PlannerInfo *root, Expr *expr,
Relids phrels);
extern PlaceHolderInfo *find_placeholder_info(PlannerInfo *root,
Use an explicit state flag to control PlaceHolderInfo creation. Up to now, callers of find_placeholder_info() were required to pass a flag indicating if it's OK to make a new PlaceHolderInfo. That'd be fine if the callers had free choice, but they do not. Once we begin deconstruct_jointree() it's no longer OK to make more PHIs; while callers before that always want to create a PHI if it's not there already. So there's no freedom of action, only the opportunity to cause bugs by creating PHIs too late. Let's get rid of that in favor of adding a state flag PlannerInfo.placeholdersFrozen, which we can set at the point where it's no longer OK to make more PHIs. This patch also simplifies a couple of call sites that were using complicated logic to avoid calling find_placeholder_info() as much as possible. Now that that lookup is O(1) thanks to the previous commit, the extra bitmap manipulations are probably a net negative. Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us
2022-08-17 15:52:53 -04:00
PlaceHolderVar *phv);
Revisit handling of UNION ALL subqueries with non-Var output columns. In commit 57664ed25e5dea117158a2e663c29e60b3546e1c I tried to fix a bug reported by Teodor Sigaev by making non-simple-Var output columns distinct (by wrapping their expressions with dummy PlaceHolderVar nodes). This did not work too well. Commit b28ffd0fcc583c1811e5295279e7d4366c3cae6c fixed some ensuing problems with matching to child indexes, but per a recent report from Claus Stadler, constraint exclusion of UNION ALL subqueries was still broken, because constant-simplification didn't handle the injected PlaceHolderVars well either. On reflection, the original patch was quite misguided: there is no reason to expect that EquivalenceClass child members will be distinct. So instead of trying to make them so, we should ensure that we can cope with the situation when they're not. Accordingly, this patch reverts the code changes in the above-mentioned commits (though the regression test cases they added stay). Instead, I've added assorted defenses to make sure that duplicate EC child members don't cause any problems. Teodor's original problem ("MergeAppend child's targetlist doesn't match MergeAppend") is addressed more directly by revising prepare_sort_from_pathkeys to let the parent MergeAppend's sort list guide creation of each child's sort list. In passing, get rid of add_sort_column; as far as I can tell, testing for duplicate sort keys at this stage is dead code. Certainly it doesn't trigger often enough to be worth expending cycles on in ordinary queries. And keeping the test would've greatly complicated the new logic in prepare_sort_from_pathkeys, because comparing pathkey list entries against a previous output array requires that we not skip any entries in the list. Back-patch to 9.1, like the previous patches. The only known issue in this area that wasn't caused by the ill-advised previous patches was the MergeAppend planning failure, which of course is not relevant before 9.1. It's possible that we need some of the new defenses against duplicate child EC entries in older branches, but until there's some clear evidence of that I'm going to refrain from back-patching further.
2012-03-16 13:11:12 -04:00
extern void find_placeholders_in_jointree(PlannerInfo *root);
Fix PlaceHolderVar mechanism's interaction with outer joins. The point of a PlaceHolderVar is to allow a non-strict expression to be evaluated below an outer join, after which its value bubbles up like a Var and can be forced to NULL when the outer join's semantics require that. However, there was a serious design oversight in that, namely that we didn't ensure that there was actually a correct place in the plan tree to evaluate the placeholder :-(. It may be necessary to delay evaluation of an outer join to ensure that a placeholder that should be evaluated below the join can be evaluated there. Per recent bug report from Kirill Simonov. Back-patch to 8.4 where the PlaceHolderVar mechanism was introduced.
2010-09-28 12:08:56 -04:00
extern void fix_placeholder_input_needed_levels(PlannerInfo *root);
Recalculate where-needed data accurately after a join removal. Up to now, remove_rel_from_query() has done a pretty shoddy job of updating our where-needed bitmaps (per-Var attr_needed and per-PlaceHolderVar ph_needed relid sets). It removed direct mentions of the to-be-removed baserel and outer join, which is the minimum amount of effort needed to keep the data structures self-consistent. But it didn't account for the fact that the removed join ON clause probably mentioned Vars of other relations, and those Vars might now not be needed as high up in the join tree as before. It's easy to show cases where this results in failing to remove a lower outer join that could also have been removed. To fix, recalculate the where-needed bitmaps from scratch after each successful join removal. This sounds expensive, but it seems to add only negligible planner runtime. (We cheat a little bit by preserving "relation 0" entries in the bitmaps, allowing us to skip re-scanning the targetlist and HAVING qual.) The submitted test case drew attention because we had successfully optimized away the lower join prior to v16. I suspect that that's somewhat accidental and there are related cases that were never optimized before and now can be. I've not tried to come up with one, though. Perhaps we should back-patch this into v16 and v17 to repair the performance regression. However, since it took a year for anyone to notice the problem, it can't be affecting too many people. Let's let the patch bake awhile in HEAD, and see if we get more complaints. Per bug #18627 from Mikaël Gourlaouen. No back-patch for now. Discussion: https://postgr.es/m/18627-44f950eb6a8416c2@postgresql.org
2024-09-27 16:04:04 -04:00
extern void rebuild_placeholder_attr_needed(PlannerInfo *root);
Rework join-removal logic as per recent discussion. In particular this fixes things so that it works for cases where nested removals are possible. The overhead of the optimization should be significantly less, as well.
2010-03-28 18:59:34 -04:00
extern void add_placeholders_to_base_rels(PlannerInfo *root);
Add an explicit representation of the output targetlist to Paths. Up to now, there's been an assumption that all Paths for a given relation compute the same output column set (targetlist). However, there are good reasons to remove that assumption. For example, an indexscan on an expression index might be able to return the value of an expensive function "for free". While we have the ability to generate such a plan today in simple cases, we don't have a way to model that it's cheaper than a plan that computes the function from scratch, nor a way to create such a plan in join cases (where the function computation would normally happen at the topmost join node). Also, we need this so that we can have Paths representing post-scan/join steps, where the targetlist may well change from one step to the next. Therefore, invent a "struct PathTarget" representing the columns we expect a plan step to emit. It's convenient to include the output tuple width and tlist evaluation cost in this struct, and there will likely be additional fields in future. While Path nodes that actually do have custom outputs will need their own PathTargets, it will still be true that most Paths for a given relation will compute the same tlist. To reduce the overhead added by this patch, keep a "default PathTarget" in RelOptInfo, and allow Paths that compute that column set to just point to their parent RelOptInfo's reltarget. (In the patch as committed, actually every Path is like that, since we do not yet have any cases of custom PathTargets.) I took this opportunity to provide some more-honest costing of PlaceHolderVar evaluation. Up to now, the assumption that "scan/join reltargetlists have cost zero" was applied not only to Vars, where it's reasonable, but also PlaceHolderVars where it isn't. Now, we add the eval cost of a PlaceHolderVar's expression to the first plan level where it can be computed, by including it in the PathTarget cost field and adding that to the cost estimates for Paths. This isn't perfect yet but it's much better than before, and there is a way forward to improve it more. This costing change affects the join order chosen for a couple of the regression tests, changing expected row ordering.
2016-02-18 20:01:49 -05:00
extern void add_placeholders_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel,
Make Vars be outer-join-aware. 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
2023-01-30 13:16:20 -05:00
RelOptInfo *outer_rel, RelOptInfo *inner_rel,
SpecialJoinInfo *sjinfo);
extern bool contain_placeholder_references_to(PlannerInfo *root, Node *clause,
int relid);
Fix some new issues with planning of PlaceHolderVars. In the wake of commit a16ef313f, we need to deal with more cases involving PlaceHolderVars in NestLoopParams than we did before. For one thing, a16ef313f was incorrect to suppose that we could rely on the required-outer relids of the lefthand path to decide placement of nestloop-parameter PHVs. As Richard Guo argued at the time, we must look at the required-outer relids of the join path itself. For another, we have to apply replace_nestloop_params() to such a PHV's expression, in case it contains references to values that will be supplied from NestLoopParams of higher-level nestloops. For another, we need to be more careful about the phnullingrels of the PHV than we were being. identify_current_nestloop_params only bothered to ensure that the phnullingrels didn't contain "too many" relids, but now it has to be exact, because setrefs.c will apply both NRM_SUBSET and NRM_SUPERSET checks in different places. We can compute the correct relids by determining the set of outer joins that should be able to null the PHV and then subtracting whatever's been applied at or below this join. Do the same for plain Vars, too. (This should make it possible to use NRM_EQUAL to process nestloop params in setrefs.c, but I won't risk making such a change in v18 now.) Lastly, if a nestloop parameter PHV was pulled up out of a subquery and it contains a subquery that was originally pushed down from this query level, then that will still be represented as a SubLink, because SS_process_sublinks won't recurse into outer PHVs, so it didn't get transformed during expression preprocessing in the subquery. We can substitute the version of the PHV's expression appearing in its PlaceHolderInfo to ensure that that preprocessing has happened. (Seems like this processing sequence could stand to be redesigned, but again, late in v18 development is not the time for that.) It's not very clear to me why the old have_dangerous_phv join-order restriction prevented us from seeing the last three of these problems. But given the lack of field complaints, it must have done so. Reported-by: Alexander Lakhin <exclusion@gmail.com> Author: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/18953-1c9883a9d4afeb30@postgresql.org
2025-06-29 15:04:32 -04:00
extern Relids get_placeholder_nulling_relids(PlannerInfo *root,
PlaceHolderInfo *phinfo);
Add a concept of "placeholder" variables to the planner. These are variables that represent some expression that we desire to compute below the top level of the plan, and then let that value "bubble up" as though it were a plain Var (ie, a column value). The immediate application is to allow sub-selects to be flattened even when they are below an outer join and have non-nullable output expressions. Formerly we couldn't flatten because such an expression wouldn't properly go to NULL when evaluated above the outer join. Now, we wrap it in a PlaceHolderVar and arrange for the actual evaluation to occur below the outer join. When the resulting Var bubbles up through the join, it will be set to NULL if necessary, yielding the correct results. This fixes a planner limitation that's existed since 7.1. In future we might want to use this mechanism to re-introduce some form of Hellerstein's "expensive functions" optimization, ie place the evaluation of an expensive function at the most suitable point in the plan tree.
2008-10-21 16:42:53 -04:00
#endif /* PLACEHOLDER_H */
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