Re: extensible external toast tuple support & snappy prototype

I took a look at them a little.  This proposal is a super set of patch #1127.
https://commitfest.postgresql.org/action/patch_view?id=1127

- <endian.h> is not found in my mac.  Commented it out, it builds clean.

- I don't see what the added is_inline flag means in toast_compress_datum().  Obviously not used, but I wonder what was the intention.

- By this,

     * compression method. We could just use the two bytes to store 3 other
     * compression methods but maybe we better don't paint ourselves in a
     * corner again.

you mean two bits, not two bytes?

And patch adds snappy-c in src/common.  I definitely like the idea to have pluggability for different compression algorithm for datum, but I am not sure if this location is a good place to add it.  Maybe we want a modern algorithm other than pglz for different components across the system in core, and it's better to let users choose which to add more.  The mapping between the index number and compression algorithm should be consistent for the entire life of database, so it should be defined at initdb time.  From core maintainability perspective and binary size of postgres, I don't think we want to put dozenes of different algorithms into core in the future.  And maybe someone will want to try BSD-incompatible algorithm privately.