Re: Disabling vacuum truncate for autovacuum - Mailing list pgsql-hackers

On Thu, Mar 20, 2025 at 02:18:33PM -0700, David G. Johnston wrote:
> So my concern about dump/restore seems to be alleviated but then, why can
> we not just do whatever pg_dump is doing to decide whether the current
> value for vacuum_truncate is its default (and thus would not be dumped) or
> not (and would be dumped)?

pg_dump looks at the pg_class.reloptions array directly.  In the vacuum
code, we look at the pre-parsed rd_options (see RelationParseRelOptions()
in relcache.c), which will have already resolved vacuum_truncate to its
default value if it was not explicitly set.  We could probably look at
pg_class.reloptions directly in the vacuum code if we _really_ wanted to,
but I felt that putting this information into rd_options was much cleaner.

index 645b5c0046..c795df6100 100644
--- a/src/backend/access/common/reloptions.c
+++ b/src/backend/access/common/reloptions.c
@@ -1912,7 +1912,9 @@ default_reloptions(Datum reloptions, bool validate, relopt_kind kind)
                {"vacuum_index_cleanup", RELOPT_TYPE_ENUM,
                offsetof(StdRdOptions, vacuum_index_cleanup)},
                {"vacuum_truncate", RELOPT_TYPE_BOOL,
-               offsetof(StdRdOptions, vacuum_truncate), offsetof(StdRdOptions, vacuum_truncate_set)},
+               offsetof(StdRdOptions, vacuum_truncate),
+               /* vacuum_truncate uses the isset_offset member instead of a sentinel value */
+               offsetof(StdRdOptions, vacuum_truncate_set)},
                {"vacuum_max_eager_freeze_failure_rate", RELOPT_TYPE_REAL,
                offsetof(StdRdOptions, vacuum_max_eager_freeze_failure_rate)}
        };
diff --git a/src/include/access/reloptions.h b/src/include/access/reloptions.h
index 146aed47c2..fddeee0d54 100644
--- a/src/include/access/reloptions.h
+++ b/src/include/access/reloptions.h
@@ -152,7 +152,17 @@ typedef struct
        const char *optname;            /* option's name */
        relopt_type opttype;            /* option's datatype */
        int                     offset;                 /* offset of field in result struct */
-       int                     isset_offset;   /* if > 0, offset of "is set" field */
+       /*
+        * The reloptions system knows whether a reloption has been set by the
+        * DBA or not.  Historically, this information was lost when parsing
+        * the reloptions so we coped by using sentinel values.
+        * This doesn't work for boolean reloptions.  For those, we
+        * need a place for the reloption parser to store its knowledge of
+        * whether the reloption was set.  Set this field to an offset
+        * in the StdRdOptions struct.  The pointed-to location needs to
+        * be a bool.  A value of 0 here is interpreted as "no need to store".
+        */
+       int                     isset_offset;
 } relopt_parse_elt;
 
 /* Local reloption definition */
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index d94fddd7ce..461648aac6 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -344,7 +344,7 @@ typedef struct StdRdOptions
        int                     parallel_workers;       /* max number of parallel workers */
        StdRdOptIndexCleanup vacuum_index_cleanup;      /* controls index vacuuming */
        bool            vacuum_truncate;        /* enables vacuum to truncate a relation */
-       bool            vacuum_truncate_set;    /* whether vacuum_truncate is set */
+       bool            vacuum_truncate_set;    /* reserve space for isset status of vacuum_truncate */
 
        /*
         * Fraction of pages in a relation that vacuum can eagerly scan and fail