Should functions work with vectorization

I've got pretty simple table;

``` sql
CREATE FOREIGN TABLE samples_cstore
(
id integer,
sampled_at timestamp without time zone
)
```

Trying run datetrunc with grouping as

``` sql
select date_trunc('day', sampled_at) from samples_cstore group by 1;
```

kills psql, I get

```
The connection to the server was lost. Attempting reset: Failed.
```

I'm on osx 10.9 with homebrewed postgres 9.3

首先，在回報的 PostgreSQL 9.3 設定上重現所提供的 CREATE FOREIGN TABLE 和 date_trunc 查詢，然後追蹤用於分組和函式求值的向量化執行器路徑。完成的標誌是該查詢不再終止連線，並且可以成功擷取其分組結果。

由索引模型根據 Issue 內容生成。