Fielddata for the _parent field

Elastic Stack Elasticsearch
1

The amount of heap used by fielddata can be checked by：
_stats/fielddata?fields=*
the response is:
...
"total": {
"fielddata": {
"memory_size_in_bytes": 2592,
"evictions": 0,
"fields": {
"_parent": {
"memory_size_in_bytes": 1296
},
"_parent#main_type": {
"memory_size_in_bytes": 1296
}
}
}
}
}
I know parent/child in 5.x need global ordinals to speed up joins,but what confuse me is the relationship between _parent#main_type and _parent. It seems that Fielddata was calculated twice。

BR
zeqi

2

these stats are per node, and you could have more than one parent/child mapping across your indices, so that _parent contains the size of all your ordinals, where as parent#main_type only contains the size of the ordinals for this specific type.

3

Thanks for your reply.I haven known that _parent contains the all ordinals,but I still don't understand why the total memory usage is 2592.Does both _parent and _parent#main_type require memory usage?

4

No, there is no doubled memory consumption. The number is only the same because you only have one parent/child field across all of your indices. If you had more indices using parent/child and those had different names, then you would have another counter for that field plus the summation of these fields in the _parent field.

hope that makes sense!

5

Thanks for your reply again. Now I use two indices using parent/child for testing,so it can be explain clearly. The response is:
....
"fielddata": {
"memory_size_in_bytes": 2144, // 2144=1072+424+648 //
"evictions": 0,
"fields": {
"_parent": {
"memory_size_in_bytes": 1072, //1072=424+648 //
},
"_parent#main_type_2": {
"memory_size_in_bytes": 424
},
"_parent#main_type": {
"memory_size_in_bytes": 648
....
I know the _parent is the summation of _parent#main_type_2 and _parent#main_type. Although the total consumption statistics are 2144, the actual memory consumption is only 1072,is that right?