Re: row filtering for logical replication

Hi,

Are there any recent performance evaluations of the overhead of row filters? I
think it'd be good to get some numbers comparing:

1) $workload with master
2) $workload with patch, but no row filters
3) $workload with patch, row filter matching everything
4) $workload with patch, row filter matching few rows

For workload I think it'd be worth testing:
a) bulk COPY/INSERT into one table
b) Many transactions doing small modifications to one table
c) Many transactions targetting many different tables
d) Interspersed DDL + small changes to a table


> +/*
> + * Initialize for row filter expression execution.
> + */
> +static ExprState *
> +pgoutput_row_filter_init_expr(Node *rfnode)
> +{
> +    ExprState  *exprstate;
> +    Expr       *expr;
> +
> +    /*
> +     * This is the same code as ExecPrepareExpr() but that is not used because
> +     * we want to cache the expression. There should probably be another
> +     * function in the executor to handle the execution outside a normal Plan
> +     * tree context.
> +     */
> +    expr = expression_planner((Expr *) rfnode);
> +    exprstate = ExecInitExpr(expr, NULL);
> +
> +    return exprstate;
> +}

In what memory context does this run? Are we taking care to deal with leaks?
I'm pretty sure the planner relies on cleanup via memory contexts.


> +    memset(entry->exprstate, 0, sizeof(entry->exprstate));
> +
> +    schemaId = get_rel_namespace(entry->publish_as_relid);
> +    schemaPubids = GetSchemaPublications(schemaId);

Isn't this stuff that we've already queried before? If we re-fetch a lot of
information it's not clear to me that it's actually a good idea to defer
building the row filter.


> +    am_partition = get_rel_relispartition(entry->publish_as_relid);

All this stuff likely can cause some memory "leakage" if you run it in a
long-lived memory context.


> +    /*
> +     * Find if there are any row filters for this relation. If there are,
> +     * then prepare the necessary ExprState and cache it in
> +     * entry->exprstate. To build an expression state, we need to ensure
> +     * the following:
> +     *
> +     * All publication-table mappings must be checked.
> +     *
> +     * If the relation is a partition and pubviaroot is true, use the row
> +     * filter of the topmost partitioned table instead of the row filter of
> +     * its own partition.
> +     *
> +     * Multiple publications might have multiple row filters for this
> +     * relation. Since row filter usage depends on the DML operation, there
> +     * are multiple lists (one for each operation) to which row filters
> +     * will be appended.
> +     *
> +     * FOR ALL TABLES implies "don't use row filter expression" so it takes
> +     * precedence.
> +     *
> +     * ALL TABLES IN SCHEMA implies "don't use row filter expression" if
> +     * the schema is the same as the table schema.
> +     */
> +    foreach(lc, data->publications)
> +    {
> +        Publication *pub = lfirst(lc);
> +        HeapTuple    rftuple = NULL;
> +        Datum        rfdatum = 0;
> +        bool        pub_no_filter = false;
> +
> +        if (pub->alltables)
> +        {
> +            /*
> +             * If the publication is FOR ALL TABLES then it is treated the
> +             * same as if this table has no row filters (even if for other
> +             * publications it does).
> +             */
> +            pub_no_filter = true;
> +        }
> +        else if (list_member_oid(schemaPubids, pub->oid))
> +        {
> +            /*
> +             * If the publication is FOR ALL TABLES IN SCHEMA and it overlaps
> +             * with the current relation in the same schema then this is also
> +             * treated same as if this table has no row filters (even if for
> +             * other publications it does).
> +             */
> +            pub_no_filter = true;

Isn't this basically O(schemas * publications)?




> +    if (has_filter)
> +    {
> +        /* Create or reset the memory context for row filters */
> +        if (entry->cache_expr_cxt == NULL)
> +            entry->cache_expr_cxt = AllocSetContextCreate(CacheMemoryContext,
> +                                                          "Row filter expressions",
> +                                                          ALLOCSET_DEFAULT_SIZES);
> +        else
> +            MemoryContextReset(entry->cache_expr_cxt);

I see this started before this patch, but I don't think it's a great idea that
pgoutput does a bunch of stuff in CacheMemoryContext. That makes it
unnecessarily hard to debug leaks.

Seems like all this should live somwhere below ctx->context, allocated in
pgoutput_startup()?

Consider what happens in a long-lived replication connection, where
occasionally there's a transient error causing streaming to stop. At that
point you'll just loose all knowledge of entry->cache_expr_cxt, no?


> +
> +/* Inialitize the slot for storing new and old tuple */
> +static void
> +init_tuple_slot(Relation relation, RelationSyncEntry *entry)
> +{
> +    MemoryContext    oldctx;
> +    TupleDesc        oldtupdesc;
> +    TupleDesc        newtupdesc;
> +
> +    oldctx = MemoryContextSwitchTo(CacheMemoryContext);
> +
> +    /*
> +     * Create tuple table slots. Create a copy of the TupleDesc as it needs to
> +     * live as long as the cache remains.
> +     */
> +    oldtupdesc = CreateTupleDescCopy(RelationGetDescr(relation));
> +    newtupdesc = CreateTupleDescCopy(RelationGetDescr(relation));
> +
> +    entry->old_slot = MakeSingleTupleTableSlot(oldtupdesc, &TTSOpsHeapTuple);
> +    entry->new_slot = MakeSingleTupleTableSlot(newtupdesc, &TTSOpsHeapTuple);
> +
> +    MemoryContextSwitchTo(oldctx);
> +}

This *definitely* shouldn't be allocated in CacheMemoryContext. It's one thing
to have a named context below CacheMemoryContext, that's still somewhat
identifiable. But allocating directly in CacheMemoryContext is almost always a
bad idea.

What is supposed to clean any of this up in case of error?


I guess I'll start a separate thread about memory handling in pgoutput :/


> +    /*
> +     * We need this map to avoid relying on ReorderBufferChangeType enums
> +     * having specific values.
> +     */
> +    static int map_changetype_pubaction[] = {
> +        [REORDER_BUFFER_CHANGE_INSERT] = PUBACTION_INSERT,
> +        [REORDER_BUFFER_CHANGE_UPDATE] = PUBACTION_UPDATE,
> +        [REORDER_BUFFER_CHANGE_DELETE] = PUBACTION_DELETE
> +    };

Why is this "static"? Function-local statics only really make sense for
variables that are changed and should survive between calls to a function.


> +    Assert(*action == REORDER_BUFFER_CHANGE_INSERT ||
> +           *action == REORDER_BUFFER_CHANGE_UPDATE ||
> +           *action == REORDER_BUFFER_CHANGE_DELETE);
> +
> +    Assert(new_slot || old_slot);
> +
> +    /* Get the corresponding row filter */
> +    filter_exprstate = entry->exprstate[map_changetype_pubaction[*action]];
> +
> +    /* Bail out if there is no row filter */
> +    if (!filter_exprstate)
> +        return true;
> +
> +    elog(DEBUG3, "table \"%s.%s\" has row filter",
> +         get_namespace_name(RelationGetNamespace(relation)),
> +         RelationGetRelationName(relation));
> +
> +    estate = create_estate_for_relation(relation);
> +    ecxt = GetPerTupleExprContext(estate);

So we do this for each filtered row? That's a *lot* of
overhead. CreateExecutorState() creates its own memory context, allocates an
EState, then GetPerTupleExprContext() allocates an ExprContext, which then
creates another memory context.

I don't really see any need to allocate this over-and-over?

>          case REORDER_BUFFER_CHANGE_INSERT:
>              {
> -                HeapTuple    tuple = &change->data.tp.newtuple->tuple;
> +                /*
> +                 * Schema should be sent before the logic that replaces the
> +                 * relation because it also sends the ancestor's relation.
> +                 */
> +                maybe_send_schema(ctx, change, relation, relentry);
> +
> +                new_slot = relentry->new_slot;
> +
> +                ExecClearTuple(new_slot);
> +                ExecStoreHeapTuple(&change->data.tp.newtuple->tuple,
> +                                   new_slot, false);

Why? This isn't free, and you're doing it unconditionally. I'd bet this alone
is noticeable slowdown over the current state.


Greetings,

Andres Freund