ES|QL: PoC - Add UNTABLE command

[luigidellaquila]

Introduce UNPIVOT UNTABLE command, that creates multiple rows starting from columns, eg.

ROW id=1, name1="foo", name2="bar", name3="baz" | UNTABLE fieldName FOR fieldValue IN (name*)

id:integer | fieldName:keyword | fieldValue:keyword
1          | name1             | foo
1          | name2             | bar
1          | name3             | baz

The command syntax is:

UNTABLE <fieldNameColumn> FOR <fieldValueColumn> IN (<fieldPatterns>*)

Untabled columns have to be all the same type.

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LIMIT pushdown is similar to MV_EXPAND, but it also takes into consideration how many columns are untabled (that is deterministic and easy to get at planning time), eg.

| UNTABLE fieldName FOR fieldValue IN (col1, col2, col3) | LIMIT 30 

is optimized as

| LIMIT 10 | UNTABLE fieldName FOR fieldValue IN (col1, col2, col3) | LIMIT 30 

(the lower limit becomes 10, that is 30 / <number of untabled fields>

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TODO

• Filter pushdown:
  • Filters for columns that are not untabled can be pushed down.
  • Filters on both pivoted field name and value can be pushed down as well, eg.

      | UNTABLE fieldName FOR fieldValue IN (col1, col2, col3) 
      | WHERE fieldName == "col1" AND fieldValue == 10
    

    can be pushed down as

      | WHERE col1 == 10
      | UNTABLE fieldName FOR fieldValue IN (col1, col2, col3) 
      | WHERE fieldName == "col1" AND fieldValue == 10
    

• More tests
• Better memory management (ie. paginate the output, same as MV_EXPAND)
• Docs