json_read out of memory

[taochangjun]

When read large json file , it will cause oom sometimes.

[taochangjun]

testjson=# select count(*) from testnestedjson_2;

count

5200000
(1 row)

Time: 7393.992 ms
testjson=# select count(*) from testnestedjson_2;

count

5200000
(1 row)

Time: 7306.728 ms
testjson=# select count() from testnestedjson_2;
ERROR: Could not parse JSON string (json_formatter.c:206) (seg0 slice1 seg1:40009 pid=20741) (cdbdisp.c:1326)
DETAIL: External table testnestedjson_2
testjson=# select count() from testnestedjson_2;
ERROR: Out of memory (seg0 slice1 seg1:40009 pid=20741)
DETAIL:
VM protect failed to allocate 8192 bytes from system, VM Protect 8191 MB available
External table testnestedjson_2