4d516bece8
Describe the overview of changes. 1. add function element_at; 2. support element_subscript([]) to get element of array, col_array[N] <==> element_at(col_array, N); 3. return error message instead of BE crash while array function execute failed; element_at(array, index) desc: > Returns element of array at given **(1-based)** index. If **index < 0**, accesses elements from the last to the first. Returns NULL if the index exceeds the length of the array or the array is NULL. Usage example: 1. create table with ARRAY type column and insert some data: ``` +------+------+--------+ | k1 | k2 | k3 | +------+------+--------+ | 1 | 2 | [1, 2] | | 2 | 3 | NULL | | 4 | NULL | [] | | 3 | NULL | NULL | +------+------+--------+ ``` 2. enable vectorized: ``` set enable_vectorized_engine=true; ``` 3. element_subscript([]) usage example: ``` > select k1,k3,k3[1] from array_test; +------+--------+----------------------------+ | k1 | k3 | %element_extract%(`k3`, 1) | +------+--------+----------------------------+ | 3 | NULL | NULL | | 1 | [1, 2] | 1 | | 2 | NULL | NULL | | 4 | [] | NULL | +------+--------+----------------------------+ ``` 4. element_at function usage example: ``` > select k1,k3 from array_test where element_at(k3, -1) = 2; +------+--------+ | k1 | k3 | +------+--------+ | 1 | [1, 2] | +------+--------+ ```
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