What caused that outage

I don't want to speak for pg, but I highly doubt this. He chose the architecture he did for good reason. Looking things up in memory is much faster than looking them up in the database. The only issue is that the site is too big to fit entirely into memory now, so he needs to write something that cleanly manages the memory space. (BerkeleyDB calls this a pager; data is stored in pages that can be in memory or disk.)

With that in place, the site should perform fine.

Here's a page with some more details: http://news.ycombinator.com/item?id=452005

For example, if you have an string type, which keeps track of it's length, then you might need 8 bytes. 4 for the pointer to the string of chars, 4 for the integer to keep count.

Here's a better example from tinyscheme:

    struct cell {
      unsigned int _flag;
      union {
        struct {
          char   *_svalue;
          int   _length;
        } _string;
        num _number;
        port *_port;
        foreign_func _ff;
        struct {
          struct cell *_car;
          struct cell *_cdr;
        } _cons;
      } _object;
    };

    typedef struct num {
       char is_fixnum;
       union {
          long ivalue;
          double rvalue;
       } value;
    } num;