Re: [PATCH] postgres_fdw connection caching - cause remote sessions linger till the local session exit - Mailing list pgsql-hackers

Hi,

On 2020-11-23 09:48, Bharath Rupireddy wrote:
>> 
>> Here is how I'm making 4 separate patches:
>> 
>> 1. new function and it's documentation.
>> 2. GUC and it's documentation.
>> 3. server level option and it's documentation.
>> 4. test cases for all of the above patches.
>> 
> 
> Hi, I'm attaching the patches here. Note that, though the code changes
> for this feature are small, I divided them up as separate patches to
> make review easy.
> 
> v1-0001-postgres_fdw-function-to-discard-cached-connections.patch
> 

This patch looks pretty straightforward for me, but there are some 
things to be addressed IMO:

+        server = GetForeignServerByName(servername, true);
+
+        if (server != NULL)
+        {

Yes, you return a false if no server was found, but for me it worth 
throwing an error in this case as, for example, dblink does in the 
dblink_disconnect().

+ result = disconnect_cached_connections(FOREIGNSERVEROID,
+     hashvalue,
+     false);

+        if (all || (!all && cacheid == FOREIGNSERVEROID &&
+            entry->server_hashvalue == hashvalue))
+        {
+            if (entry->conn != NULL &&
+                !all && cacheid == FOREIGNSERVEROID &&
+                entry->server_hashvalue == hashvalue)

These conditions look bulky for me. First, you pass FOREIGNSERVEROID to 
disconnect_cached_connections(), but actually it just duplicates 'all' 
flag, since when it is 'FOREIGNSERVEROID', then 'all == false'; when it 
is '-1', then 'all == true'. That is all, there are only two calls of 
disconnect_cached_connections(). That way, it seems that we should keep 
only 'all' flag at least for now, doesn't it?

Second, I think that we should just rewrite this if statement in order 
to simplify it and make more readable, e.g.:

    if ((all || entry->server_hashvalue == hashvalue) &&
        entry->conn != NULL)
    {
        disconnect_pg_server(entry);
        result = true;
    }

+    if (all)
+    {
+        hash_destroy(ConnectionHash);
+        ConnectionHash = NULL;
+        result = true;
+    }

Also, I am still not sure that it is a good idea to destroy the whole 
cache even in 'all' case, but maybe others will have a different 
opinion.

> 
> v1-0002-postgres_fdw-add-keep_connections-GUC-to-not-cache-connections.patch
> 

+            entry->changing_xact_state) ||
+            (entry->used_in_current_xact &&
+            !keep_connections))

I am not sure, but I think, that instead of adding this additional flag 
into ConnCacheEntry structure we can look on entry->xact_depth and use 
local:

bool used_in_current_xact = entry->xact_depth > 0;

for exactly the same purpose. Since we set entry->xact_depth to zero at 
the end of xact, then it was used if it is not zero. It is set to 1 by 
begin_remote_xact() called by GetConnection(), so everything seems to be 
fine.

Otherwise, both patches seem to be working as expected. I am going to 
have a look on the last two patches a bit later.

Regards
-- 
Alexey Kondratov

Postgres Professional https://www.postgrespro.com
Russian Postgres Company