Thread

Re: Proposal: Conflict log history table for Logical Replication

Peter Smith <smithpb2250@gmail.com> — 2025-12-23T06:49:21Z

Hi Dilip.

Here are some review comments after a first pass of patch v15-0001.

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Commit Message

1.
If user choose to log into the table the table will automatically created while
creating the subscription with internal name i.e.
conflict_log_table_$subid$.  The
table will be created in the current search path and table would be
automatically
dropped while dropping the subscription.

English:

/If user choose/
/the table the table/
/and table would/

======
src/backend/commands/subscriptioncmds.c

2.
+#define SUBOPT_CONFLICT_LOG_DESTINATION 0x00040000

For the values, you are using DEST instead of DESTINATION. You can do
the same here to keep the macro name a bit shorter.

~~~

parse_subscription_options:

3.
+ dest = GetLogDestination(val);
+
+ if (dest == CONFLICT_LOG_DEST_INVALID)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("unrecognized conflict_log_destination value: \"%s\"", val),
+ errhint("Valid values are \"log\", \"table\", and \"all\".")));

I don't think CONFLICT_LOG_DEST_INVALID should even exist as an enum
value. Instead, the validation and the ereport(ERROR) should all be
done within GetLogDestination function. So, it should only return
valid values, else give an error.

~~~

CreateSubscription:

4.
+ /* Always set the destination, default will be log. */
+ values[Anum_pg_subscription_sublogdestination - 1] =
+ CStringGetTextDatum(ConflictLogDestLabels[opts.logdest]);
+
+ /*
+ * If the conflict log destination includes 'table', generate an internal
+ * name using the subscription OID and determine the target namespace based
+ * on the current search path. Store the namespace OID and the conflict log
+ * format in the pg_subscription catalog tuple., then  physically create
+ * the table.
+ */

4a.
When referring to these parameter values, you should always
consistently quote them. Currently, there is a mix of lots of formats.
(e.g. log (unquoted), 'table' (single-quoted), "log" (double-quoted)).

Pick one style, and make them all the same. Check for the same everywhere.

~

4b.
Typo "tuple.,"

~~~

5.
+ if (opts.logdest == CONFLICT_LOG_DEST_TABLE ||
+ opts.logdest == CONFLICT_LOG_DEST_ALL)

IIUC, you are effectively treating these parameter values like bits
that can be OR-ed together. And if in the future a "list" is
supported, then that's exactly what you will be doing. So, IMO, they
should be defined that way. See a review comment later in this post.

e.g. this condition would be written more like:
if ((opts.logdest & CONFLICT_LOG_DEST_TABLE) != 0)
or, using the macro
if (IsSet(opts.logdest, CONFLICT_LOG_DEST_TABLE))

~~~

AlterSubscription:

6.
+ if (opts.logdest != old_dest)
+ {
+ bool want_table =
+ (opts.logdest == CONFLICT_LOG_DEST_TABLE ||
+ opts.logdest == CONFLICT_LOG_DEST_ALL);
+ bool has_oldtable =
+ (old_dest == CONFLICT_LOG_DEST_TABLE ||
+ old_dest == CONFLICT_LOG_DEST_ALL);
+


This is more of the same kind of logic that convinces me the code
should be using bitmasks.

SUGGESTION
bool want_table = IsSet(opts.logdest, CONFLICT_LOG_DEST_TABLE);
bool has_oldtable = IsSet(olddest, CONFLICT_LOG_DEST_TABLE);

~~~

create_conflict_log_table:

7.
+/*
+ * Create conflict log table.
+ *
+ * The subscription owner becomes the owner of this table and has all
+ * privileges on it.
+ */
+static Oid
+create_conflict_log_table(Oid subid, char *subname, Oid namespaceId,
+   char *conflictrel)


I felt something like 'relname' is a better name for the char *
conflictrel param. It clearly is the name of the conflict relation
because of the name of the function.

~~~

8.
+ /* Add a comments for the conflict log table. */
+ snprintf(comment, sizeof(comment),
+ "Conflict log table for subscription \"%s\"", subname);
+ CreateComments(relid, RelationRelationId, 0, comment);
+

8a.
typo /Add a comments/Add a comment/

~

8b.
My (previous review) suggestion for adding a table comment/description
made more sense when the CLT was some arbitrary name chosen by the
user. But, now that the CLT is a name like "conflict_log_table_%u",
the idea for a comment seems redundant.

~~~

9.
+/*
+ * Format the standardized internal conflict log table name for a subscription
+ *
+ * Use the OID to prevent collisions during rename operations.
+ */
+void
+GetConflictLogTableName(char *dest, Oid subid)
+{
+ snprintf(dest, NAMEDATALEN, "conflict_log_table_%u", subid);
+}
+

9a.
To emphasise that this is an "internal" table, IMO there should be a
"pg_" prefix for this table name.

~

9b.
Since it is internal anyway, why not make the tablename descriptive to
clarify what that number means?
e.g. "pg_conflict_log_table_for_subid_%u"

BTW, since it is already a TABLE, then why is "table" even part of
this name? Why not just "pg_conflict_log_for_subid_%u"
~~~

10.
+/*
+ * GetLogDestination
+ *
+ * Convert string to enum by comparing against standardized labels.
+ */
+ConflictLogDest
+GetLogDestination(const char *dest)
+{
+ /* Empty string or NULL defaults to LOG. */
+ if (dest == NULL || dest[0] == '\0')
+ return CONFLICT_LOG_DEST_LOG;
+
+ for (int i = CONFLICT_LOG_DEST_LOG; i <= CONFLICT_LOG_DEST_ALL; i++)
+ {
+ if (pg_strcasecmp(dest, ConflictLogDestLabels[i]) == 0)
+ return (ConflictLogDest) i;
+ }
+
+ /* Unrecognized string. */
+ return CONFLICT_LOG_DEST_INVALID;
+}

Mentioned previously: I think there should be no such thing as
CONFLICT_LOG_DEST_INVALID. I also think this function should be
responsible for the ereport(ERROR).


======
src/include/catalog/pg_subscription.h

11.
+ /*
+ * Strategy for logging replication conflicts:
+ * log - server log only,
+ * table - internal table only,
+ * all - both log and table.
+ */
+ text sublogdestination;
+

SUGGEST 'subconflictlogdest'

(see next review comment #12 for why)

~~~

12.
+ Oid conflictrelid; /* conflict log table Oid */
  char    *conninfo; /* Connection string to the publisher */
  char    *slotname; /* Name of the replication slot */
  char    *synccommit; /* Synchronous commit setting for worker */
  List    *publications; /* List of publication names to subscribe to */
  char    *origin; /* Only publish data originating from the
  * specified origin */
+ char    *logdestination; /* Conflict log destination */
 } Subscription;

These don't seem very good member names:

Maybe 'conflictrelid' -> 'conflictlogrelid' (because it's rel of the
log; not the conflict)
Maybe 'logdestination' -> 'conflictlogdest' (because in future there
might be other kinds of subscription logs)

======
src/include/replication/conflict.h

13.
+typedef enum ConflictLogDest
+{
+ CONFLICT_LOG_DEST_INVALID = 0,
+ CONFLICT_LOG_DEST_LOG, /* "log" (default) */
+ CONFLICT_LOG_DEST_TABLE, /* "table" */
+ CONFLICT_LOG_DEST_ALL /* "all" */
+} ConflictLogDest;
+

I didn't like this enum much.

Suggest removing CONFLICT_LOG_DEST_INVALID.
And use bits for the other values.
And you can still have a default enum if you want.

SUGGESTION
typedef enum ConflictLogDest
{
  CONFLICT_LOG_DEST_LOG = 0x001,
  CONFLICT_LOG_DEST_TABLE = 0x010,
  CONFLICT_LOG_DEST_DEFAULT = CONFLICT_LOG_DEST_LOG,
  CONFLICT_LOG_DEST_ALL = CONFLICT_LOG_DEST_LOG | CONFLICT_LOG_DEST_TABLE,
} ConflictLogDest;

BTW, there are only a few values that the array won't exceed length
0x11, so I guess you can still keep your same designated initialiser
for the dest labels.

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Kind Regards,
Peter Smith.
Fujitsu Australia