PostgreSQL C Extension Function returning a Vector of Pairs or a Text Array

Question

I want to return an vector of pairs using a C extension. This is a simple code I have:

extern "C" { 
Datum pair(PG_FUNCTION_ARGS){ 

        // get the input
        text *t1 = PG_GETARG_TEXT_PP(0); 
        text *t2 = PG_GETARG_TEXT_PP(1); 
        std::string localT1 = text_to_cstring(t1); 
        std::string localT2 = text_to_cstring(t2); 

        // Return vector of pairs 
        std::vector<std::pair&lt;std::string, std::string>> ret; 
        ret.emplace_back(encodedLocalT1, encodedLocalT2); 
        PG_RETURN_ARRAYTYPE_P(ret); 

}; 
PG_FUNCTION_INFO_V1(pair); 
}

But it doesn't work like it and I don't know if it's even possible. So I tried to return a text[] but it doesn't work either:

extern "C" { 
Datum pair(PG_FUNCTION_ARGS){ 

        // Get the input 
        text *t1 = PG_GETARG_TEXT_PP(0); 
        text *t2 = PG_GETARG_TEXT_PP(1); 
        std::string localT1 = text_to_cstring(t1); 
        std::string localT2 = text_to_cstring(t2); 

        // Return array with 2 strings 
        ArrayType *array;
        Datum elements[2];
        int16 typlen;
        bool typbyval;
        char typalign;

        elements[0] = cstring_to_text(localT1.c_str());
        elements[1] = cstring_to_text(localT2.c_str());

        get_typlenbyvalalign(TEXTOID, &typlen, &typbyval, &typalign);
        array = construct_array(elements, 2, TEXTOID, typlen, typbyval, typalign);

        PG_RETURN_ARRAYTYPE_P(array);

}; 
PG_FUNCTION_INFO_V1(pair); 
}

I use those includes in the extension:

extern "C" { // C Headers must be inside exter "C" { } block.
#include <postgres.h>
#include <fmgr.h>
#include <utils/builtins.h>
#include <catalog/pg_type.h>
#include <utils/rel.h>
#include <utils/array.h>
#include <stdlib.h>
#include <stdint.h>
#include <utils/lsyscache.h>

PG_MODULE_MAGIC;
}

// CPP Header must be outside extern "C" { } block.
#include <string>
#include <vector>
#include <iostream>
#include <fstream>
#include <sstream>
#include <iterator> // For the ostream_iterator
#include <chrono> // For some code benchmarking

// External projects c++ libraries compiled and linked on running 'make'.
#include <seal/seal.h>
#include <thread>
#include <cppcodec/base64_rfc4648.hpp>

The code example above is minimal to help with the question. All the includes are used in the long version of the code.

I can't seem to figure out how to achieve my goal, I prefer to return text[] with the 2 strings as its elements.

TalG · Accepted Answer · 2020-04-09 08:03:13Z

This code worked for me.

extern "C" { 
Datum text_array(PG_FUNCTION_ARGS){ 

        // Get the inputs of TEXT type variables 
        text *t1 = PG_GETARG_TEXT_PP(0); 
        text *t2 = PG_GETARG_TEXT_PP(1); 
        // Cast into cpp strings
        std::string localT1 = TextDatumGetCString(t1); 
        std::string localT2 = TextDatumGetCString(t2); 

        // Return a text array with 2 elemsnts
        ArrayType *array;
        Datum elements[2];
        int16 typlen;
        bool typbyval;
        char typalign;

        // Cast the cpp strings back to pgSQL Datum type
        elements[0] = CStringGetTextDatum(localT1.c_str());
        elements[1] = CStringGetTextDatum(localT2.c_str());
        // Create the return array for pgSQL
        get_typlenbyvalalign(TEXTOID, &typlen, &typbyval, &typalign);
        array = construct_array(elements, 2, TEXTOID, typlen, typbyval, typalign);

        PG_RETURN_ARRAYTYPE_P(array);

}; 
PG_FUNCTION_INFO_V1(text_array); 
}

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