What's the layout of postgres array stored in memory? How can I get the real data?
For example, for array[0.1, 0.2, 0.3]::float8[], is the real data (0.1, 0.2, 0.3) stored like a standard c array? Could I use memcpy to copy an existing array? Does the pointer we get use ARR_DATA_PTR refer to the real data?
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for better question/answer see stackoverflow.com/q/9159440/287948Peter Krauss– Peter Krauss2012-05-10 18:21:10 +00:00Commented May 10, 2012 at 18:21
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1 Answer
PostgreSQL uses a mutable C structure - so first n bytes contains a fixed length data and next bytes are used for data. First four bytes holds length, follow number of dimmensions, dataoffset and identification of element's data type - next data should be a bitmap that holds NULLs and after this bitmaps data are serialised.
PostgreSQL array is not compatible with C arrays - resp. in few cases C array is part of PostgreSQL array. ARR_DATA_PTR can or must not to refer to real data. Depends on current state - data should be toasted, detoasted, ...
People usually use a macros and supporting functions when work with pg arrays. There are ways for unpacking to C arrays or iteration over pg array.
