Re: Emitting JSON to file using COPY TO

hi.

There are two ways we can use to represent the new copy format: json.
1.
typedef struct CopyFormatOptions
{
    bool        binary;            /* binary format? */
    bool        freeze;            /* freeze rows on loading? */
    bool        csv_mode;        /* Comma Separated Value format? */
    bool        json_mode;        /* JSON format? */
    ...
}

2.
typedef struct CopyFormatOptions
{
    CopyFormat    format;            /* format of the COPY operation */
    .....
}

typedef enum CopyFormat
{
    COPY_FORMAT_TEXT = 0,
    COPY_FORMAT_BINARY,
    COPY_FORMAT_CSV,
    COPY_FORMAT_JSON,
} CopyFormat;

both the sizeof(cstate->opts) (CopyToStateData.CopyFormatOptions) is 184.
so the struct size will not influence the performance.

I also did some benchmarks when using CopyFormat.
the following are the benchmarks info:
-------------------------------------------------------------------------------------------------------
create unlogged table t as select g from generate_series(1, 1_000_000) g;

build_type=release patch:
copy t to '/dev/null' json \watch i=0.1 c=10
last execution Time: 108.741 ms

copy t to '/dev/null' (format text)  \watch i=0.1 c=10
last execution Time: 42.600 ms

build_type=release master:
copy t to '/dev/null' (format text)  \watch i=0.1 c=10
last execution Time Time: 42.948 ms

---------------------------------------------------------
so a new version is attached, using the struct CopyFormatOptions.

changes mainly in CopyOneRowTo.
now it is:
""""
        if(!cstate->rel)
        {
            memcpy(TupleDescAttr(slot->tts_tupleDescriptor, 0),
                   TupleDescAttr(cstate->queryDesc->tupDesc, 0),
                   cstate->queryDesc->tupDesc->natts *
sizeof(FormData_pg_attribute));
            for (int i = 0; i < cstate->queryDesc->tupDesc->natts; i++)
                populate_compact_attribute(slot->tts_tupleDescriptor, i);
            BlessTupleDesc(slot->tts_tupleDescriptor);
        }
""""
reasoning for change:
for composite_to_json to construct json key, we only need
FormData_pg_attribute.attname
but code path
composite_to_json->fastgetattr->TupleDescCompactAttr->verify_compact_attribute
means we also need to call populate_compact_attribute to populate
other attributes.

v14-0001-Introduce-CopyFormat-and-replace-csv_mode-and-bi.patch,
author is by Joel Jacobson.
As I mentioned in above,
replacing 3 bool fields by an enum didn't change the struct CopyFormatOptions.
but consolidated 3 bool fields into one enum to make code more lean.
I think the refactoring (v14-0001) is worth it.