Обсуждение: Accessing composite type elements
Hi All,
I’m not sure if this is the actual place for this but I guess I can start here. The question I have is, I’ve created a composite type like this:
CREATE TYPE myxml AS { encoding_ int4, xml_ xml };
In my client-side C code I am using PQgetvalue to pull in the data from the row/column. However, since it is a composite type, it is unclear to me how to get the individual members from the data. I have googled but I probably just am not googling the correct term because I cannot find any examples of this being done anywhere. A hex dump of the data gives me this:
0x0000000200000017000000046f0100000000008e000001433c637573746f6d6572696e666f20786d6c6e733d22687474703a2f2f6c7a6c6162732e637573742e636f6d22204369643d22543130303130303031223e3c6e616d653e5461626c6520584d4c…
I can tell that the green portion is my endcoding_ value and the blue section is the xml_ data.
My best guess right now is:
- 0x2 is the number of members
- 0x4 and 0x143 are the lengths of the individual members
- 0x17 and 0x8e are the OID for the member type
Is this the proper layout of these composite types? Can I go ahead and use this without possibly having it broken in the future? Are there any existing supported APIs that I can use instead to get this information?
Regards
G
Garfield Lewis <garfield.lewis@lzlabs.com> writes:
> In my client-side C code I am using PQgetvalue to pull in the data from the row/column. However, since it is a
compositetype, it is unclear to me how to get the individual members from the data.
Binary representations are not too well documented :-(. However,
looking at record_send() helps here.
> My best guess right now is:
> * 0x2 is the number of members
> * 0x4 and 0x143 are the lengths of the individual members
> * 0x17 and 0x8e are the OID for the member type
Right, with the additional proviso that -1 "length" indicates a null
field value.
regards, tom lane
> Binary representations are not too well documented :-(. However,
> looking at record_send() helps here.
will have a look…
> Right, with the additional proviso that -1 "length" indicates a null
> field value.
Thx, Tom… never thought the null field…
--
Regards,
Garfield A. Lewis
> Binary representations are not too well documented :-(. However,
> looking at record_send() helps here.will have a look…
> Right, with the additional proviso that -1 "length" indicates a null
> field value.
Thx, Tom… never thought the null field…
> take a look at libpqtypes. it's client side extension library to libpq that implements the binary protocol.
>
>
> merlin
Thx, Merlin… I’ll have a look see…