arangodb - 使用 AQL 变量，例如用于计数(LET sum = sum + 1)

Question

根据 https://www.arangodb.com/2014/07/13/arangodb-2-2-0-released应该可以使用这样的语句:

LET sum = 0
FOR v IN values
  SORT v.year
  LET sum = sum + v.value
  RETURN { year: v.year, value: v.value, sum: sum }

我目前使用 2.4 版，但无法使用它，例如在这样的声明中:

LET sum = 0
FOR i in memoryColl
    LET sum = sum + 1
    // sum = sum + 1
    RETURN { "i": i, "sum": sum }

我得到了错误
[1511] 变量 'sum' 被分配多次(在解析时)

有人能告诉我这样的statemtn原则上是否应该有效，以及具体如何？

Answer 1

如 upgrading docs 中所述对于 2.3，不再可能更新查询中的变量:

Previous versions of ArangoDB allowed the modification of variables inside AQL queries [...]

While this is admittedly a convenient feature, the new query optimizer design did not allow to keep it.

Additionally, updating variables inside a query would prevent a lot of optimizations to queries that we would like the optimizer to make. Additionally, updating variables in queries that run on different nodes in a cluster would like cause non-deterministic behavior because queries are not executed linearly.

要枚举文档，你可以这样做

LET range = 0..LENGTH(memoryColl)-1

FOR i IN range
    RETURN {i: i+1, doc: memoryColl[i]}

但对我来说这似乎是一个非常糟糕的主意。最好返回文档并让客户枚举它们。

如果你真的想计算文档的数量，你可以使用子查询:

LET result = (
    FOR doc IN memoryColl
        FILTER True // add some condition here for instance
        RETURN doc
)

RETURN LENGTH(result)

在2.4中，还可以更高效地计数:
http://jsteemann.github.io/blog/2014/12/12/aql-improvements-for-24/