Unprocessed SubLink from whole-row join alias expansion - #416
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If a subquery has been flattened into its parent, the joinaliasvars entries of a join above it can be arbitrary expressions rather than plain Vars, so expanding a reference to such a join alias may insert a SubLink into a lower-level subquery. flatten_join_alias_vars_mutator detects that and sets the subquery's hasSubLinks flag, but only in the single-column code path; the whole-row path just asserted in a comment that its recursive call would handle this, which is true only when the alias entry is itself a Var referencing another join. Hence a whole-row reference to such a join appearing in a sub-select left that sub-select's hasSubLinks false, so preprocess_expression skipped SS_process_sublinks for it, and the unprocessed SubLink reached code that is not prepared for one: this produced "cannot handle unplanned sub-select" from cost_qual_eval, an assertion failure in preprocess_aggrefs, or "unrecognized node type" at execution, depending on where in the sub-select the SubLink ended up. To fix, make the same check in the whole-row expansion path.
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Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com> via pgsql-hackers · original email Hi,
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pgsql-hackersCAMbWs49PgeNFhQTZ2gSzwatF0_LXyegMPGt++jSfxUN7NZuQ5Q@mail.gmail.comPatch files:
Further fuzzing with Claude on the join alias found this bug. The
following queries fail in various ways on master and all supported
branches:
create table t (a int, b int);
select 1 from ((select (select 1) as x) s cross join t) j
where (select 1 where j is null) is null;
ERROR: cannot handle unplanned sub-select
select 1 from ((select (select 1) as x) s cross join t) j
where (1, 1) in (select (j is null)::int, count(*) from t);
TRAP: failed Assert("!IsA(node, SubLink)"), File: "prepagg.c"
select 1 from ((select (select 1) as x) s cross join t) j
where exists (select 1 from t tablesample system ((j is null)::int * 100));
ERROR: unrecognized node type: 22
Once subquery s is flattened, the joinaliasvars entry for j.x is no
longer a Var but the SubLink (select 1), so expanding a reference to j
inside a sub-select inserts a SubLink into that sub-select.
But flatten_join_alias_vars_mutator fails to notice this and thus does
not set the sub-select's hasSubLinks flag. So preprocess_expression
skips SS_process_sublinks, and the SubLink survives into code that
can't cope with one.
The fix is to make the same checkExprHasSubLink() test in the
whole-row path. See attached.