Accessing composite type elements

From: Garfield Lewis <garfield(dot)lewis(at)lzlabs(dot)com>
To: "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org>
Subject: Accessing composite type elements
Date: 2022-06-02 15:20:57
Message-ID: VI1PR0502MB398150C23E5450CCC30BE666E3DE9@VI1PR0502MB3981.eurprd05.prod.outlook.com
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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

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