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Im using the debugger on apacheAGE to understand better the source code. Ive come across multiple times the List struct and the ListeCell struct, since they are very common in holding and fetching information.

Im having trouble what the ListCell struct holds as information. Everytime I access the values of a ListCell struct through gdb, I get some random values/numbers as a result. I understand that since ListCell is a union, it may be possible that some of the fields dont make any sense since the memory is overwritten by another field, but still none of the 3 fields make any sense.

What is a way to decode those numbers to understand what information is being used each time?

Panagiotis Foliadis
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1 Answers1

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It appears that you are already familiar with the List struct and ListCell union of postgreSQL.

While you can view the ListCell content using GDB, please note that for Apache AGE, some of it may not appear meaningful as it is transformed and decoded.

I am sharing my GDB session below, which I hope could be of use to you.

Query:

SELECT * FROM cypher('test', $$
  WITH [1, 2, 3] AS c
  RETURN length(c)
$$) AS (res agtype);

GDB:

Breakpoint 1, list_length (l=0x5650776cecc0)
        at ../../../src/include/nodes/pg_list.h:91
    91      return l ? l->length : 0;

List:

(gdb) p l
$1 = (const List *) 0x5650776cecc0

(gdb) p *l
$2 = {type = T_List, length = 2, head = 0x5650776cecf8, tail = 0x5650776cec98}

// head: A pointer to an array of ListCell that store the actual data of elements of the list.

(gdb) p l->length
$3 = 2

// Because our list has 3 elements [1, 2, 3].

ListCell:

(gdb) p l->head
$4 = (ListCell *) 0x5650776cecf8

(gdb) p *l->head
$5 = {data = {ptr_value = 0x5650776cec08, int_value = 2003627016, oid_value = 2003627016}, next = 0x5650776cec98}

(gdb) p l->head->data->ptr_value
$6 = (void *) 0x5650776cec08

// ptr_value: A void pointer which holds our agtype data. So, we need to typecaste ptr_value to agtype pointer to see its content.

(gdb) p *(agtype*)l->head->data->ptr_value
$7 = {vl_len_ = 344, root = {header = 0, children = 0x5650776cec10}}

(gdb) p (*(agtype*)l->head->data->ptr_value)->vl_len_
$8 = 344

// _vl_len is varlena header (int32)

(gdb) p (*(agtype*)l->head->data->ptr_value)->root
$9 = {header = 0, children = 0x5650776cec10}

// root is agtype_container

(gdb) p *((agtype*)l->head->data->ptr_value)->root->children
$10 = 2003626560

// children is 28-bit agtentry
Mohayu Din
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