Re: Proposal: Generic WAL logical messages

Hello,

On 27.02.2016 03:05, Andres Freund wrote:
> Hi,
>
> I'm not really convinced by RegisterStandbyMsgPrefix() et al. There's
> not much documentation about what it actually is supposed to
> acomplish. Afaics you're basically forced to use
> shared_preload_libraries with it right now? Also, iterating through a
> linked list everytime something is logged doesn't seem very satisfying?
>

I have did some tests with a simple plugin. I have used event triggers
to send messages. It works, but I agree with Andres. We have problems if
plugin is not loaded. For example, if you will execute the query:

SELECT 'msg2' FROM pg_logical_send_message(false, 'test', 'msg2');

you will get the error (if plugin which should register a prefix is not
loaded yet):

ERROR: standby message prefix "test" is not registered

Some stylistic note (logical.c):

> +static void message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
> + XLogRecPtr message_lsn,
> + bool transactional, const char *prefix,
> + Size sz, const char *message)
> +{
> + LogicalDecodingContext *ctx = cache->private_data;
> + LogicalErrorCallbackState state;
> + ErrorContextCallback errcallback;

It should be written in the following way:

static void
message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
XLogRecPtr message_lsn,
bool transactional, const char *prefix,
Size sz, const char *message)
{
LogicalDecodingContext *ctx = cache->private_data;
LogicalErrorCallbackState state;
ErrorContextCallback errcallback;

--
Artur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company