Thread

Reduce "Var IS [NOT] NULL" quals during constant folding

BharatDB <bharatdbpg@gmail.com> — 2025-08-28T13:01:55Z

*Subject:* Contribution Interest – Bug Fix on Reducing "Var IS [NOT] NULL"
Quals During Constant Folding

Dear Team,

I hope this message finds you well.

With reference to the conversation ongioing in message ID:
(CAMbWs487wxHq0fUu+Cew7Y+n+wpY_B--PT-61syXrE40uZqErw
<https://www.postgresql.org/message-id/CAMbWs487wxHq0fUu%2BCew7Y%2Bn%2BwpY_B--PT-61syXrE40uZqErw%40mail.gmail.com>
 ,
I am writing to express my interest in contributing to the ongoing work on
fixing the bug related to reducing “Var IS [NOT] NULL” quals during
constant folding. As part of my initial efforts, I have been exploring the
planner code paths and introducing an approach to gather early catalog
information for base relations.

Specifically, I have implemented the following:

   -

   Preprocessing Relation RTEs
   -

   Added a new static function preprocess_relation_rtes() in
   src/backend/optimizer/plan/planner.c to collect attribute-level metadata
   (e.g., NOT NULL, generated columns, inheritance flags) at an early stage.
   -

   Hash Table for Attribute Info
   -

   Defined a new planner-local hash table (relattrinfo_htab) for storing
   per-attribute information.
   -

   Created a supporting header (src/include/optimizer/relattrinfo.h) and
   implementation file (src/backend/optimizer/util/relattrinfo.c).
   -

   Planner Integration
   -

   Initialized the attribute info hash table in subquery_planner().
   -

   Ensured preprocessing is invoked before pull_up_sublinks().
   -

   Optimization Step
   -

   Extended the constant folding path to check stored attribute metadata
   during IS [NOT] NULL qual simplification, allowing safe reduction to
   constant true/false where applicable.


1. Declare - *preprocess_relation_rtes()*

File: in subquery_planner() inside *src/backend/optimizer/plan/planner.c*.

   -

   Insert your new step *before* pull_up_sublinks().

*Snippet:*

   /* * Do preprocessing of RTEs before we pull up sublinks */

   preprocess_relation_rtes(root);



2.Create a new static function *preprocess_relation_rtes()* in planner.c:

File: *src/backend/optimizer/plan/planner.c*

*Snippet:*

/* * preprocess_relation_rtes *   Gather early catalog info for each
base relation *   (NOT NULL attrs, generated cols, inheritance flags).
*/static void preprocess_relation_rtes(PlannerInfo *root){    ListCell
*lc;    Relation rel;
    foreach(lc, root->parse->rtable)    {        RangeTblEntry *rte =
(RangeTblEntry *) lfirst(lc);
        if (rte->rtekind == RTE_RELATION)        {            rel =
table_open(rte->relid, AccessShareLock);
            /* Collect attnotnull info */            TupleDesc tupdesc
= RelationGetDescr(rel);            for (int attno = 0; attno <
tupdesc->natts; attno++)            {                Form_pg_attribute
attr = TupleDescAttr(tupdesc, attno);                if
(!attr->attisdropped)                {                    if
(attr->attnotnull)                        store_notnull_info(root,
rte->relid, attno + 1);
                    if (attr->attgenerated)
store_generated_info(root, rte->relid, attno + 1);                }
        }
            /* Inheritance flag */            if
(rel->rd_rel->relhassubclass)                store_inh_info(root,
rte->relid);
            table_close(rel, AccessShareLock);        }    }

}


3. Filename* – **src/include/nodes/pathnodes.h*

*Snippet:*

typedef struct PlannerInfo

{

 HTAB *relattrinfo_htab; /* plannerlocal cache of attr metadata */

} PlannerInfo;


4. Create a new header.

Location: *src/include/optimizer/relattrinfo.h*

*Snippet: *

#ifndef RELATTRINFO_H

#define RELATTRINFO_H

#include "postgres.h"

#include "utils/hsearch.h"


/* key for hash table */

typedef struct RelAttrInfoKey

{

Oid relid; /* relation OID */

AttrNumber attno; /* attribute number (1-based) */

} RelAttrInfoKey;


/* value stored in hash table */

typedef struct RelAttrInfoEntry

{

RelAttrInfoKey key;

bool notnull;

bool generated;

} RelAttrInfoEntry;

extern HTAB *create_relattrinfo_htab(void);

#endif /* RELATTRINFO_H */


5. Created a new file.

Location: *src/backend/optimizer/util/relattrinfo.c*

*Snippet:*

#include "postgres.h"

#include "optimizer/relattrinfo.h"

HTAB *create_relattrinfo_htab(void)

{

HASHCTL ctl;

memset(&ctl, 0, sizeof(ctl));

ctl.keysize = sizeof(RelAttrInfoKey);

ctl.entrysize = sizeof(RelAttrInfoEntry);

return hash_create("Planner relattr info cache",

128, /* initial size */

&ctl,

HASH_ELEM | HASH_BLOBS);

}


6. File: *src/backend/optimizer/plan/planner.c* in *subquery_planner()*
(this runs before most rewrites).

*Snippet:*

*/** Create hash table for early attr info */

root->relattrinfo_htab = create_relattrinfo_htab();


/* Collect early info before pull_up_sublinks */

preprocess_relation_rtes(root);



7. *C**reate**d** a hash table*

File: *src/backend/optimizer/plan/planner.c*

*Example:*

typedef struct RelAttrInfoKey{    Oid relid;    AttrNumber attno;}
RelAttrInfoKey;
typedef struct RelAttrInfoEntry{    RelAttrInfoKey key;    bool
notnull;    bool generated;} RelAttrInfoEntry;

Initialize it in *subquery_planner()*:

root->relattrinfo_htab = create_relattrinfo_htab();


8. Constant folding

if (IsA(arg, Var)){    Var *var = (Var *) arg;    if
(lookup_notnull_info(root, var->varno, var->varattno))    {        if
(ntest->nulltesttype == IS_NOT_NULL)            return (Expr *)
makeBoolConst(true, false);        else            return (Expr *)
makeBoolConst(false, false);    }}

With this groundwork, my plan is to continue refining the approach,
validating correctness against inheritance cases, and ensuring
compliance with planner invariants. I would also like to engage with
reviewers to align this with the broader design direction.

Please let me know how best I can proceed with submitting my patch for
review and contributing further to this fix.

Thank you for your time and consideration.


Best Regards,

Soumya