Обсуждение: what is the meaning of Datum?
Hi all, ok this is a silly question, but I've got a doubt: what is the exact meaning of Datum? I see having a look at the macroes (e.g., PG_RETURN_XXX) that a Datum can be used as a pointer or as a single data, that is it can be a reference or a value. Is this right? So for instance the fact that each stored procedure returns a Datum means that the semantic of the Datum is interpreted depending on how the procedure is defined in the SQL language (i.e., it returns a integer, a tuple, ...). Am I right? Moreover, is there a documentation (aside the source code) that explains and links each internal data structure like HeapTuple, HeapTupleHeader, and so on? Thanks, Luca
Luca Ferrari <fluca1978@infinito.it> writes:
> ok this is a silly question, but I've got a doubt: what is the exact meaning
> of Datum?
It's the backend-internal representation of a single value of any SQL
data type. The code using the Datum has to know which type it is,
since the Datum itself doesn't contain that information. Usually,
C code will work with a value in a "native" representation, and then
convert to or from Datum in order to pass the value through
data-type-independent interfaces.
regards, tom lane
Tom Lane wrote: > Luca Ferrari <fluca1978@infinito.it> writes: > >> ok this is a silly question, but I've got a doubt: what is the exact meaning >> of Datum? >> > > It's the backend-internal representation of a single value of any SQL > data type. The code using the Datum has to know which type it is, > since the Datum itself doesn't contain that information. Usually, > C code will work with a value in a "native" representation, and then > convert to or from Datum in order to pass the value through > data-type-independent interfaces. > > regards, tom lane > > And how this convertions will be made ? Are there internal functions to do this ? Thanks. -Tedy
Excerpts from Luca Ferrari's message of mié jun 23 06:09:28 -0400 2010: > Hi all, > ok this is a silly question, but I've got a doubt: what is the exact meaning > of Datum? I see having a look at the macroes (e.g., PG_RETURN_XXX) that a > Datum can be used as a pointer or as a single data, that is it can be a > reference or a value. Is this right? Yes > So for instance the fact that each stored > procedure returns a Datum means that the semantic of the Datum is interpreted > depending on how the procedure is defined in the SQL language (i.e., it returns > a integer, a tuple, ...). Am I right? Yes > Moreover, is there a documentation (aside the source code) that explains and > links each internal data structure like HeapTuple, HeapTupleHeader, and so on? The source code comments should be plenty. Have you tried reading those? -- Álvaro Herrera <alvherre@commandprompt.com> The PostgreSQL Company - Command Prompt, Inc. PostgreSQL Replication, Consulting, Custom Development, 24x7 support
On Wed, 2010-06-23 at 12:15 -0400, Alvaro Herrera wrote: > > Moreover, is there a documentation (aside the source code) that explains and > > links each internal data structure like HeapTuple, HeapTupleHeader, and so on? > > The source code comments should be plenty. Have you tried reading > those? Here is a nice interface to the code: http://doxygen.postgresql.org/ Sincerely, Joshua D. Drake -- PostgreSQL.org Major Contributor Command Prompt, Inc: http://www.commandprompt.com/ - 509.416.6579 Consulting, Training, Support, Custom Development, Engineering
Teodor Macicas <teodor.macicas@epfl.ch> writes:
> Tom Lane wrote:
>> C code will work with a value in a "native" representation, and then
>> convert to or from Datum in order to pass the value through
>> data-type-independent interfaces.
> And how this convertions will be made ?
> Are there internal functions to do this ?
Usually you should use the appropriate DatumGetXXX or XXXGetDatum
macro, or possibly the PG_GET_XXX or PG_RETURN_XXX wrappers for those.
Whether that's a simple cast or a function call is the macro's business.
regards, tom lane
On Wed, 2010-06-23 at 12:15 -0400, Alvaro Herrera wrote: > > Moreover, is there a documentation (aside the source code) that explains and > > links each internal data structure like HeapTuple, HeapTupleHeader, and so on? > > The source code comments should be plenty. Have you tried reading > those? Here is a nice interface to the code: http://doxygen.postgresql.org/ Sincerely, Joshua D. Drake -- PostgreSQL.org Major Contributor Command Prompt, Inc: http://www.commandprompt.com/ - 509.416.6579 Consulting, Training, Support, Custom Development, Engineering