[BUG] specifying fetch_size in REST API causes issue with aliases

[pcoccoli]

What is the bug?
If I select the same column twice, with 2 different aliases, and specify fetch_size, then first alias is ignored. It works fine if I don't include fetch_size.

How can one reproduce the bug?
Steps to reproduce the behavior:

POST /_plugins/_sql
{
    "query": "SELECT `process.executable` AS `colA`, `process.executable` AS `colB` FROM my_index LIMIT 3",
    "fetch_size": 1000
}

What is the expected behavior?
fetch_size should not effect SQL interpretation.

What is your host/environment?

• OS: Unsure
• Version: 2.12.0
• Plugins: sql

Do you have any screenshots?
With fetch_size:

curl -u "password:password" https://example.com:9200/_plugins/_sql/ -X POST -d '{"query": "SELECT `process.executable` AS `colA`, `process.executable` AS `colB` FROM my_index LIMIT 1", "fetch_size": 10}' -H "Content-Type: application/json"
{
  "schema": [
    {
      "name": "process.executable",
      "alias": "colB",
      "type": "keyword"
    },
    {
      "name": "process.executable",
      "alias": "colB",
      "type": "keyword"
    }
  ],
  "total": 10000,
    "datarows": [[
    "C:\\Windows\\explorer.exe",
    "C:\\Windows\\explorer.exe"
  ]],
  "size": 1,
  "status": 200
}

Notice in the schema of the response, both columns have alias "colB"

Without fetch_size:

curl -u "username:password" https://example.com:9200/_plugins/_sql/ -X POST -d '{"query": "SELECT `process.executable` AS `colA`, `process.executable` AS `colB` FROM my_index LIMIT 1"}' -H "Content-Type: application/json"
{
  "schema": [
    {
      "name": "process.executable",
      "alias": "colA",
      "type": "keyword"
    },
    {
      "name": "process.executable",
      "alias": "colB",
      "type": "keyword"
    }
  ],
  "datarows": [
    [
      "C:\\Windows\\explorer.exe",
      "C:\\Windows\\explorer.exe"
    ]
  ],
  "total": 1,
  "size": 1,
  "status": 200
}

Here the alias fields in the result schema match the query.

Do you have any additional context?
The value of fetch_size doesn't seem to matter; its presence triggers the problem.

[penghuo]

it is bug in legacy engine(v1)
add fetch_size, query execution fallback to legacy engine (v1),