- Drilldown the transaction wait event

A query graph consists of nodes linked to each other in various ways. The

execution starts at que_run_threads() which takes a que_thr_t parameter.

que_thr_t contains two fields that control query graph execution: run_node

and prev_node. run_node is the next node to execute and prev_node is the

last node executed.

Each node has a pointer to a 'next' statement, i.e., its brother, and a

pointer to its parent node. The next pointer is NULL in the last statement

of a block.

Loop nodes contain a link to the first statement of the enclosed statement

list. While the loop runs, que_thr_step() checks if execution to the loop

node came from its parent or from one of the statement nodes in the loop. If

it came from the parent of the loop node it starts executing the first

statement node in the loop. If it came from one of the statement nodes in

the loop, then it checks if the statement node has another statement node

following it, and runs it if so.

To signify loop ending, the loop statements (see e.g. while_step()) set

que_thr_t->run_node to the loop node's parent node. This is noticed on the

the loop node's 'next' pointer.