Re: CustomScan in a larger structure (RE: CustomScan support on readfuncs.c) - Mailing list pgsql-hackers

On Wed, Dec 2, 2015 at 4:06 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2015-12-02 08:52:20 +0000, Kouhei Kaigai wrote:
>> diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
>> index 26264cb..c4bb76e 100644
>> --- a/src/backend/nodes/copyfuncs.c
>> +++ b/src/backend/nodes/copyfuncs.c
>> @@ -635,8 +635,12 @@ _copyWorkTableScan(const WorkTableScan *from)
>>  static ForeignScan *
>>  _copyForeignScan(const ForeignScan *from)
>>  {
>> -     ForeignScan *newnode = makeNode(ForeignScan);
>> -
>> +     const ExtensibleNodeMethods *methods
>> +             = GetExtensibleNodeMethods(from->extnodename, true);
>> +     ForeignScan *newnode = (ForeignScan *) newNode(!methods
>> +                                                                                                ?
sizeof(ForeignScan)
>> +                                                                                                :
methods->node_size,
>> +                                                                                                T_ForeignScan);
>
> Changing the FDW API seems to put this squarely into 9.6
> territory. Agreed?

I don't think anybody thought this patch was 9.5 material.  I didn't,
anyway.  The FDW changes for the join-pushdown-EPQ problem are another
issue, but that can be discussed on the other thread.

>>       /*
>>        * copy node superclass fields
>>        */
>> @@ -645,6 +649,7 @@ _copyForeignScan(const ForeignScan *from)
>>       /*
>>        * copy remainder of node
>>        */
>> +     COPY_STRING_FIELD(extnodename);
>>       COPY_SCALAR_FIELD(fs_server);
>>       COPY_NODE_FIELD(fdw_exprs);
>>       COPY_NODE_FIELD(fdw_private);
>> @@ -652,6 +657,8 @@ _copyForeignScan(const ForeignScan *from)
>>       COPY_NODE_FIELD(fdw_recheck_quals);
>>       COPY_BITMAPSET_FIELD(fs_relids);
>>       COPY_SCALAR_FIELD(fsSystemCol);
>> +     if (methods && methods->nodeCopy)
>> +             methods->nodeCopy((Node *) newnode, (const Node *) from);
>
> I wonder if we shouldn't instead "recurse" into a generic routine for
> extensible nodes here.

I don't know what that means.

>> +void
>> +RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *methods)
>> +{
>> +     uint32                  hash;
>> +     int                             index;
>> +     ListCell           *lc;
>> +     MemoryContext   oldcxt;
>> +
>> +     if (!xnodes_methods_slots)
>> +             xnodes_methods_slots = MemoryContextAllocZero(TopMemoryContext,
>> +                                                                               sizeof(List *) * XNODES_NUM_SLOTS);
>> +
>> +     hash = hash_any(methods->extnodename, strlen(methods->extnodename));
>> +     index = hash % XNODES_NUM_SLOTS;
>> +
>> +     /* duplication check */
>> +     foreach (lc, xnodes_methods_slots[index])
>> +     {
>> +             const ExtensibleNodeMethods *curr = lfirst(lc);
>> +
>> +             if (strcmp(methods->extnodename, curr->extnodename) == 0)
>> +                     ereport(ERROR,
>> +                                     (errcode(ERRCODE_DUPLICATE_OBJECT),
>> +                                      errmsg("ExtensibleNodeMethods \"%s\" already exists",
>> +                                                     methods->extnodename)));
>> +     }
>> +     /* ok, register this entry */
>> +     oldcxt = MemoryContextSwitchTo(TopMemoryContext);
>> +     xnodes_methods_slots[index] = lappend(xnodes_methods_slots[index],
>> +                                                                               (void *)methods);
>> +     MemoryContextSwitchTo(oldcxt);
>> +}
>
> Why aren't you using dynahash here, and instead reimplement a hashtable?

I had thought we might assume that the number of extensible nodes was
small enough that we could use an array for this and just use linear
search.  But if we want to keep the door open to having enough
extensible nodes that this would be inefficient, then I agree that
dynahash is better than a hand-rolled hash table.

>>  static ForeignScan *
>>  _readForeignScan(void)
>>  {
>> +     const ExtensibleNodeMethods *methods;
>>       READ_LOCALS(ForeignScan);
>>
>>       ReadCommonScan(&local_node->scan);
>>
>> +     READ_STRING_FIELD(extnodename);
>>       READ_OID_FIELD(fs_server);
>>       READ_NODE_FIELD(fdw_exprs);
>>       READ_NODE_FIELD(fdw_private);
>> @@ -1804,6 +1806,19 @@ _readForeignScan(void)
>>       READ_BITMAPSET_FIELD(fs_relids);
>>       READ_BOOL_FIELD(fsSystemCol);
>>
>> +     methods = GetExtensibleNodeMethods(local_node->extnodename, true);
>> +     if (methods && methods->nodeRead)
>> +     {
>> +             ForeignScan        *local_temp = palloc0(methods->node_size);
>> +
>> +             Assert(methods->node_size >= sizeof(ForeignScan));
>> +
>> +             memcpy(local_temp, local_node, sizeof(ForeignScan));
>> +             methods->nodeRead((Node *) local_temp);
>> +
>> +             pfree(local_node);
>> +             local_node = local_temp;
>> +     }
>>       READ_DONE();
>>  }
>
> This imo pretty clearly shows why the 'extensible node' handling should
> be delegated to separate routines.

This does look ugly.  I'm not sure off-hand how to fix it.

> I wonder if we shouldn't, before committing this, at least write a PoC
> implementation of actual extensible nodes. I.e. new node types that are
> registered at runtime by extensions. ISTM those are relatively closely
> linked, and the above doesn't yet support them nicely afaics.

How is what you have in mind different from the pgstrom patch KaiGai attached?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company