"Joe Conway" <[EMAIL PROTECTED]> writes: > Sorry for my ignorance, but I haven't been able to keep up lately -- > what is the difference between pg_detoast_datum_packed and pg_detoast_datum, > and how do I know when to use each? E.g. I notice that the related macro > PG_GETARG_TEXT_PP is used in place of PG_GETARG_TEXT_P in many (but not all) > places in the backend.
We now use only 1 byte for varlena length headers when the datum is up to 126 bytes long. This saves 3-6 bytes since we also don't have to do the four byte alignment that 4-byte headers require. This gets expanded into a regular 4-byte header by pg_detoast_datum() so that all regular data type functions never see the packed varlenas with 1-byte headers. That lets them use the regular VARDATA() and VARSIZE() macros and lets them assume 4-byte alignment. It's always safe to just use the old PG_DETOAST_DATUM() even on a datatype like text. In heavily used functions on data types such as text which don't care about alignment we can avoid having to allocate memory to hold a 4-byte header copy of the packed varlenas. But we still have to detoast externally stored or compressed data. The interface to do so is to use PG_DETOAST_DATUM_PACKED() and then use VARDATA_ANY() and VARSIZE_ANY_EXHDR() instead of VARDATA and VARSIZE. This detoasts large data but keeps small data packed and lets you work with either 1-byte or 4-byte headers without knowing which you have. There's a comment i fmgr.h above pg_detoast_datum which says most of this and more detailed comments in postgres.h. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
