http://www.postgresql.org/docs/current/interactive/storage-toast.html
If any of the columns of a table are TOAST-able, the table will have an associated TOAST table, whose OID is stored in the table's pg_class.reltoastrelid entry. Out-of-line TOASTed values are kept in the TOAST table,
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Out-of-line values are divided (after compression if used) into chunks of at most TOAST_MAX_CHUNK_SIZE bytes (by default this value is chosen so that four chunk rows will fit on a page, making it about 2000 bytes). Each chunk is stored as a separate row in the TOAST table for the owning table. Every TOAST table has the columns chunk_id (an OID identifying the particular TOASTed value), chunk_seq (a sequence number for the chunk within its value), and chunk_data (the actual data of the chunk). A unique index on chunk_id and chunk_seq provides fast retrieval of the values. A pointer datum representing an out-of-line TOASTed value therefore needs to store the OID of the TOAST table in which to look and the OID of the specific value (its chunk_id).


A pointer datum representing an out-of-line TOASTed value therefore needs to store the OID of the TOAST table in which to look and the OID of the specific value (its chunk_id).

ну, и там далее видно, что будет две группы записей. два чанка. ссылка на конкретный свой чанк и будет лежать в исходном тупле в конкретной колонке.