Re: Extended Statistics set/restore/clear functions. - Mailing list pgsql-hackers

After 16 rounds of revisions, v16 looks solid and robust. Only got a few small comments:

> On Nov 21, 2025, at 07:08, Corey Huinker <corey.huinker@gmail.com> wrote:
>
>
<v16-0001-Add-working-input-function-for-pg_ndistinct.patch><v16-0002-Add-working-input-function-for-pg_dependencies.patch><v16-0003-Expose-attribute-statistics-functions-for-use-in.patch><v16-0004-Add-extended-statistics-support-functions.patch><v16-0005-Include-Extended-Statistics-in-pg_dump.patch>

1 - 0001
```
+    /*
+     * We need at least two attribute numbers for a ndistinct item, anything
+     * less is malformed.
+     */
+    natts = list_length(parse->attnum_list);
+    if ((natts < 2) || (natts > STATS_MAX_DIMENSIONS))
+    {
+        errsave(parse->escontext,
+                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+                errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
+                errdetail("The \"%s\" key must contain an array of at least %d "
+                          "and no more than %d attributes.",
+                          PG_NDISTINCT_KEY_ATTRIBUTES, 2, STATS_MAX_DIMENSIONS));
+        return JSON_SEM_ACTION_FAILED;
+    }
```

Do we also need to reset item state vars similar to the success case?

2 - 0001
```
*
+ * Arrays can never be empty.
+ */
+static JsonParseErrorType
+ndistinct_array_end(void *state)
+{
```

This function comment is too simple, and doesn’t match to the function name. Reading the function body, the logic is
clear,so I would suggest either just remove the comment or enhance it. 

3  - 0001
```
static JsonParseErrorType
+ndistinct_array_element_start(void *state, bool isnull)
+{
+    NDistinctParseState *parse = state;
```

Nit: NDistinctParseState * can be const.

4 - 0001
```
+static bool
+valid_subsequent_attnum(const AttrNumber prev, const AttrNumber cur)
```

AttrNumber is int16, thus “const” here is unnecessary.

5 - 0001
```
+            return JSON_SUCCESS;
+            break;
```

“Break” after “return” is unnecessary. I tried to delete the “break” and didn’t see a compile warning, it should be
safeto delete the “break”. 

6 - 0001
```
+/*
+ * Compare the attribute arrays of two MVNDistinctItem values,
+ * looking for duplicate sets.

+ */
+static bool
+has_duplicate_attributes(const MVNDistinctItem *a,
+                         const MVNDistinctItem *b)
```

The function comment says “looking for duplicate sets”, seems to imply the function would return the set, but the
functionreturns a bool, which is a little confusing. 

7 - 0002
```
+    for (int i = 0; i < natts; i++)
+    {
+        dep->attributes[i] = (AttrNumber) list_nth_int(parse->attnum_list, i);
+        if (dep->attributes[i] == parse->dependency)
+        {
+            errsave(parse->escontext,
+                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+                    errmsg("malformed pg_dependencies: \"%s\"", parse->str),
+                    errdetail("Item \"%s\" value %d found in the \"%s\" list.",
+                              PG_DEPENDENCIES_KEY_DEPENDENCY, parse->dependency,
+                                 PG_DEPENDENCIES_KEY_ATTRIBUTES));
+            return JSON_SEM_ACTION_FAILED;
+        }
```

Similar to comment 1, do we need to reset state vars upon failure?

8 - 0002
```
+                errdetail("The \"%s\" key must contain an array of at least %d "
+                          " and no than %d elements.",
```

I believe “more” is missed between “no than”.

9 - 0002
```
+static JsonParseErrorType
+dependencies_array_start(void *state)
+{
+    DependenciesParseState *parse = state;
```

Similar to commit 3, DependenciesParseState * can be const.

10 - 0002
```
static bool
valid_subsequent_attnum(const AttrNumber prev, const AttrNumber cur)
```

This function is a dup to the one in 0001, can we put it to a common place?

11 - 0003
```
+extern void statatt_get_type(Oid reloid, AttrNumber attnum,
+                             Oid *atttypid, int32 *atttypmod,
+                             char *atttyptype, Oid *atttypcoll,
+                             Oid *eq_opr, Oid *lt_opr);
+extern void statatt_init_empty_tuple(Oid reloid, int16 attnum, bool inherited,
+                                     Datum *values, bool *nulls, bool *replaces);
+
+extern void statatt_set_slot(Datum *values, bool *nulls, bool *replaces,
+                             int16 stakind, Oid staop, Oid stacoll,
+                             Datum stanumbers, bool stanumbers_isnull,
+                             Datum stavalues, bool stavalues_isnull);
+
+extern Datum text_to_stavalues(const char *staname, FmgrInfo *array_in, Datum d,
+                               Oid typid, int32 typmod, bool *ok);
+extern bool statatt_get_elem_type(Oid atttypid, char atttyptype,
+                                  Oid *elemtypid, Oid *elem_eq_opr);
```

Several functions are made external visible, they are all renamed with adding a prefix “statatt_”, why
text_to_stavaluesis an exception? 

11 - 0004
```
+bool
+pg_dependencies_validate_deps(MVDependencies *dependencies, int2vector *stxkeys, int numexprs, int elevel)
+{
+    int        attnum_expr_lowbound = 0 - numexprs;
+
+    for (int i = 0; i < dependencies->ndeps; i++)
+    {
+        MVDependency *dep = dependencies->deps[i];
```

This MVDependency * can be const.

12 - 0004
```
static void
upsert_pg_statistic_ext_data(Datum *values, bool *nulls, bool *replaces)
{
```

This function pass values, nulls and replaces to heap_modify_tuple() and heap_form_tuple(), the both functions take all
constpointers as parameters. So, here values, nulls and replaces can all be const. 

13 - 0004
```
+        if (!exprs_nulls[offset + AVG_WIDTH_ELEM])
+        {
+            ok = text_to_int4(exprs_elems[offset + AVG_WIDTH_ELEM],
+                              &values[Anum_pg_statistic_stawidth - 1]);
+
+            if (!ok)
+            {
+                char       *s = TextDatumGetCString(exprs_elems[offset + NULL_FRAC_ELEM]);
+
+                ereport(WARNING,
+                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+                         errmsg("Expression %s element \"%s\" does not match expected input type.",
+                                extexprarginfo[AVG_WIDTH_ELEM].argname, s)));
+                pfree(s);
+                return (Datum) 0;
+            }
+        }
```

Here: char    *s = TextDatumGetCString(exprs_elems[offset + NULL_FRAC_ELEM]);
Should NULL_FRAC_ELEM be AVG_WIDTH_ELEM?

14 - 0004
```
+        if (!exprs_nulls[offset + N_DISTINCT_ELEM])
+        {
+            ok = text_to_float4(exprs_elems[offset + N_DISTINCT_ELEM],
+                                &values[Anum_pg_statistic_stadistinct - 1]);
+
+            if (!ok)
+            {
+                char       *s = TextDatumGetCString(exprs_elems[offset + NULL_FRAC_ELEM]);
+
+                ereport(WARNING,
+                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+                         errmsg("Expression %s element \"%s\" does not match expected input type.",
+                                extexprarginfo[N_DISTINCT_ELEM].argname, s)));
+                pfree(s);
+                return (Datum) 0;
+            }
+        }
```

Similar to comment 13, should NULL_FRAC_ELEM be N_DISTINCT_ELEM?

15 - 0004
```
+extern Datum import_mcvlist(HeapTuple tup, int elevel, int numattrs,
+                            Oid *atttypids, int32 *atttypmods, Oid *atttypcolls,
+                            int nitems, Datum *mcv_elems, bool *mcv_nulls,
+                            bool *mcv_elem_nulls, float8 *freqs, float8 *base_freqs);
+
+extern Datum import_mcvlist(HeapTuple tup, int elevel, int numattrs,
+                            Oid *atttypids, int32 *atttypmods, Oid *atttypcolls,
+                            int nitems, Datum *mcv_elems, bool *mcv_nulls,
+                            bool *mcv_elem_nulls, float8 *freqs, float8 *base_freqs);
```

import_mcvlist is declared twice, looks like a copy-paste mistake.

16 - 0005
```
+    PREPQUERY_DUMPEXTSTATSSTATS,
```

“statsstats” looks weird, I’d suggest PREPQUERY_DUMPEXTSTATSDATA.

Best regards,
—
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/