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README.md

Virtual Schema API Documentation

Table of Contents

Introduction

There are the following request and response types:

Type Called ...
Create Virtual Schema ... for each CREATE VIRTUAL SCHEMA ... statement
Refresh ... for each ALTER VIRTUAL SCHEMA ... REFRESH ... statement.
Set Properties ... for each ALTER VIRTUAL SCHEMA ... SET ... statement.
Drop Virtual Schema ... for each DROP VIRTUAL SCHEMA ... statement.
Get Capabilities ... whenever a virtual table is queried in a SELECT statement.
Pushdown ... whenever a virtual table is queried in a SELECT statement.

We describe each of the types in the following sections.

Please note: To keep the documentation concise we defined the elements which are commonly in separate sections below, e.g. schemaMetadataInfo and schemaMetadata.

Requests and Responses

Create Virtual Schema

Informs the Adapter about the request to create a Virtual Schema, and asks the Adapter for the metadata (tables and columns).

The Adapter is allowed to throw an Exception if the user missed to provide mandatory properties or in case of any other problems (e.g. connectivity).

Request:

{
    "type": "createVirtualSchema",
    "schemaMetadataInfo": {
        ...
    }
}

Response:

{
    "type": "createVirtualSchema",
    "schemaMetadata": {
        ...
    }
}

Notes

  • schemaMetadata is mandatory. However, it is allowed to contain no tables.

Refresh

Request to refresh the metadata for the whole Virtual Schema, or for specified tables.

Request:

{
    "type": "refresh",
    "schemaMetadataInfo": {
        ...
    },
    "requestedTables": ["T1", "T2"]
}

Notes

  • requestedTables is optional. If existing, only the specified tables shall be refreshed. The specified tables do not have to exist, it just tell Adapter to update these tables (which might be changed, deleted, added, or non-existing).

Response:

{
    "type": "refresh",
    "schemaMetadata": {
        ...
    },
    "requestedTables": ["T1", "T2"]
}

Notes

  • schemaMetadata is optional. It can be skipped if the adapter does not want to refresh (e.g. because he detected that there is no change).
  • requestedTables must exist if and only if the element existed in the request. The values must be the same as in the request (to make sure that Adapter only refreshed these tables).

Set Properties

Request to set properties. The Adapter can decide whether he needs to send back new metadata. The Adapter is allowed to throw an Exception if the user provided invalid properties or in case of any other problems (e.g. connectivity).

Request:

{
    "type": "setProperties",
    "schemaMetadataInfo": {
        ...
    },
    "properties": {
        "JDBC_CONNECTION_STRING": "new-jdbc-connection-string",
        "NEW_PROPERTY": "value of a not yet existing property"
        "DELETED_PROPERTY": null
    }
}

Response:

{
    "type": "setProperties",
    "schemaMetadata": {
        ...
    }
}

Notes

  • Request: A property set to null means that this property was asked to be deleted. Properties set to null might also not have existed before.
  • Response: schemaMetadata is optional. It only exists if the adapter wants to send back new metadata. The existing metadata are overwritten completely.

Drop Virtual Schema

Inform the Adapter that a Virtual Schema is about to be dropped. The Adapter can update external dependencies if he has such. The Adapter is not expected to throw an exception, and if he does, it will be ignored.

Request:

{
    "type": "dropVirtualSchema",
    "schemaMetadataInfo": {
        ...
    }
}

Response:

{
    "type": "dropVirtualSchema"
}

Get Capabilities

Request the list of capabilities supported by the Adapter. Based on these capabilities, the database will collect everything that can be pushed down in the current query and sends a pushdown request afterwards.

Request:

{
    "type": "getCapabilities",
    "schemaMetadataInfo": {
        ...
    }
}

Response:

{
    "type": "getCapabilities",
    "capabilities": [
        "ORDER_BY_COLUMN",
        "AGGREGATE_SINGLE_GROUP",
        "LIMIT",
        "AGGREGATE_GROUP_BY_TUPLE",
        "FILTER_EXPRESSIONS",
        "SELECTLIST_EXPRESSIONS",
        "SELECTLIST_PROJECTION",
        "AGGREGATE_HAVING",
        "ORDER_BY_EXPRESSION",
        "AGGREGATE_GROUP_BY_EXPRESSION",
        "LIMIT_WITH_OFFSET",
        "AGGREGATE_GROUP_BY_COLUMN",
        "FN_PRED_LESSEQUALS",
        "FN_AGG_COUNT",
        "LITERAL_EXACTNUMERIC",
        "LITERAL_DATE",
        "LITERAL_INTERVAL",
        "LITERAL_TIMESTAMP_UTC",
        "LITERAL_TIMESTAMP",
        "LITERAL_NULL",
        "LITERAL_STRING",
        "LITERAL_DOUBLE",
        "LITERAL_BOOL"
    ]
}

The set of capabilities in the example above would be sufficient to pushdown all aspects of the following query:

SELECT user_id, count(url) FROM VS.clicks
 WHERE user_id>1
 GROUP BY user_id
 HAVING count(url)>1
 ORDER BY user_id
 LIMIT 10;

The whole set of capabilities is a lot longer. The current list of supported Capabilities can be found in the sources of the JDBC Adapter:

Pushdown

Contains an abstract specification of what to be pushed down, and requests an pushdown SQL statement from the Adapter which can be used to retrieve the requested data.

Request:

Running the following query

SELECT user_id, count(url) FROM VS.clicks
 WHERE user_id>1
 GROUP BY user_id
 HAVING count(url)>1
 ORDER BY user_id
 LIMIT 10;

will produce the following Request, assuming that the Adapter has all required capabilities.

{
    "type": "pushdown",
    "pushdownRequest": {
        "type" : "select",
        "aggregationType" : "group_by",
        "from" :
        {
            "type" : "table",
            "name" : "CLICKS"
        },
        "selectList" :
        [
            {
                "type" : "column",
                "name" : "USER_ID",
                "columnNr" : 1,
                "tableName" : "CLICKS"
            },
            {
                "type" : "function_aggregate",
                "name" : "count",
                "arguments" :
                [
                    {
                        "type" : "column",
                        "name" : "URL",
                        "columnNr" : 2,
                        "tableName" : "CLICKS"
                    }
                ]
            }
        ],
        "filter" :
        {
            "type" : "predicate_less",
            "left" :
            {
                "type" : "literal_exactnumeric",
                "value" : "1"
            },
            "right" :
            {
                "type" : "column",
                "name" : "USER_ID",
                "columnNr" : 1,
                "tableName" : "CLICKS"
            }
        },
        "groupBy" :
        [
            {
                "type" : "column",
                "name" : "USER_ID",
                "columnNr" : 1,
                "tableName" : "CLICKS"
            }
        ],
        "having" :
        {
            "type" : "predicate_less",
            "left" :
            {
                "type" : "literal_exactnumeric",
                "value" : "1"
            },
            "right" :
            {
                "type" : "function_aggregate",
                "name" : "count",
                "arguments" :
                [
                    {
                        "type" : "column",
                        "name" : "URL",
                        "columnNr" : 2,
                        "tableName" : "CLICKS"
                    }
                ]
            }
        },
        "orderBy" :
        [
            {
                "type" : "order_by_element",
                "expression" :
                {
                    "type" : "column",
                    "columnNr" : 1,
                    "name" : "USER_ID",
                    "tableName" : "CLICKS"
                },
                "isAscending" : true,
                "nullsLast" : true
            }
        ],
        "limit" :
        {
            "numElements" : 10
        }
    },
    "involvedTables": [
    {
        "name" : "CLICKS",
        "columns" :
        [
            {
                "name" : "ID",
                "dataType" :
                {
                    "type" : "DECIMAL",
                    "precision" : 18,
                    "scale" : 0
                }
            },
            {
                "name" : "USER_ID",
                "dataType" :
                {
                   "type" : "DECIMAL",
                   "precision" : 18,
                    "scale" : 0
                }
            },
            {
                "name" : "URL",
                "dataType" :
                {
                   "type" : "VARCHAR",
                   "size" : 1000
                }
            },
            {
                "name" : "REQUEST_TIME",
                "dataType" :
                {
                    "type" : "TIMESTAMP"
                }
            }
        ]
    }
    ],
    "schemaMetadataInfo": {
        ...
    }
}

Notes

  • pushdownRequest: Specification what needs to be pushed down. You can think of it like a parsed SQL statement.
    • from: The requested from clause. Currently only tables are supported, joins might be supported in future.
    • selectList: The requested select list elements, a list of expression. The order of the selectlist elements matters. If the select list is an empty list, we request at least a single column/expression, which could also be constant TRUE.
    • selectList.columnNr: Position of the column in the virtual table, starting with 0
    • filter: The requested filter (where clause), a single expression.
    • aggregationType: Optional element, set if an aggregation is requested. Either "group_by" or "single_group", if a aggregate function is used but no group by.
    • groupBy: The requested group by clause, a list of expressions.
    • having: The requested having clause, a single expression.
    • orderBy: The requested order-by clause, a list of order_by_element elements. The field expression contains the expression to order by.
    • limit The requested limit of the result set, with an optional offset.
  • involvedTables: Metadata of the involved tables, encoded like in schemaMetadata.

Response:

Following the example above, a valid result could look like this:

{
    "type": "pushdown",
    "sql": "IMPORT FROM JDBC AT 'jdbc:exa:remote-db:8563;schema=native' USER 'sys' IDENTIFIED BY 'exasol' STATEMENT 'SELECT USER_ID, count(URL) FROM NATIVE.CLICKS WHERE 1 < USER_ID GROUP BY USER_ID HAVING 1 < count(URL) ORDER BY USER_ID LIMIT 10'"
}

Notes

  • sql: The pushdown SQL statement. It must be either an SELECT or IMPORT statement.

Embedded Commonly Used Json Elements

The following Json objects can be embedded in a request or response. They have a fixed structure.

Schema Metadata Info

This document contains the most important metadata of the virtual schema and is sent to the adapter just "for information" with each request. It is the value of an element called schemaMetadataInfo.

{"schemaMetadataInfo":{
    "name": "MY_HIVE_VSCHEMA",
    "adapterNotes": {
        "lastRefreshed": "2015-03-01 12:10:01",
        "key": "Any custom schema state here"
    },
    "properties": {
        "HIVE_SERVER": "my-hive-server",
        "HIVE_DB": "my-hive-db",
        "HIVE_USER": "my-hive-user"
    }
}}

Schema Metadata

This document is usually embedded in responses from the Adapter and informs the database about all metadata of the Virtual Schema, especially the contained Virtual Tables and it's columns. The Adapter can store so called adapterNotes on each level (schema, table, column), to remember information which might be relevant for the Adapter in future. In the example below, the Adapter remembers the table partitioning and the data type of a column which is not directly supported in EXASOL. The Adapter has these information during pushdown and can consider the table partitioning during pushdown or can add an appropriate cast for the column.

{"schemaMetadata":{
    "adapterNotes": {
        "lastRefreshed": "2015-03-01 12:10:01",
        "key": "Any custom schema state here"
    },
    "tables": [
    {
        "type": "table",
        "name": "EXASOL_CUSTOMERS",
        "adapterNotes": {
            "hivePartitionColumns": ["CREATED", "COUNTRY_ISO"]
        },
        "columns": [
        {
            "name": "ID",
            "dataType": {
                "type": "DECIMAL",
                "precision": 18,
                "scale": 0
            },
            "isIdentity": true
        },
        {
            "name": "COMPANY_NAME",
            "dataType": {
                "type": "VARCHAR",
                "size": 1000,
                "characterSet": "UTF8"
            },
            "default": "foo",
            "isNullable": false,
            "comment": "The official name of the company",
            "adapterNotes": {
                "hiveType": {
                    "dataType": "List<String>"
                }
            }
        },
        {
            "name": "DISCOUNT_RATE",
            "dataType": {
                "type": "DOUBLE"
            }
        }
        ]
    },
    {
        "type": "table",
        "name": "TABLE_2",
        "columns": [
        {
            "name": "COL1",
            "dataType": {
                "type": "DECIMAL",
                "precision": 18,
                "scale": 0
            }
        },
        {
            "name": "COL2",
            "dataType": {
                "type": "VARCHAR",
                "size": 1000
            }
        }
        ]
    }
    ]
}}

Notes

  • adapterNotes is an optional field which can be attached to the schema, a table or a column. It can be an arbitrarily nested Json document.

The following EXASOL data types are supported:

Decimal:

{
    "name": "C_DECIMAL",
    "dataType": {
        "type": "DECIMAL",
        "precision": 18,
        "scale": 2
    }
}

Double:

{
    "name": "C_DOUBLE",
    "dataType": {
        "type": "DOUBLE"
    }
}

Varchar:

{
    "name": "C_VARCHAR_UTF8_1",
    "dataType": {
        "type": "VARCHAR",
        "size": 10000,
        "characterSet": "UTF8"
    }
}
{
    "name": "C_VARCHAR_UTF8_2",
    "dataType": {
        "type": "VARCHAR",
        "size": 10000
    }
}
{
    "name": "C_VARCHAR_ASCII",
    "dataType": {
        "type": "VARCHAR",
        "size": 10000,
        "characterSet": "ASCII"
    }
}

Char:

{
    "name": "C_CHAR_UTF8_1",
    "dataType": {
        "type": "CHAR",
        "size": 3
    }
}
{
    "name": "C_CHAR_UTF8_2",
    "dataType": {
        "type": "CHAR",
        "size": 3,
        "characterSet": "UTF8"
    }
}
{
    "name": "C_CHAR_ASCII",
    "dataType": {
        "type": "CHAR",
        "size": 3,
        "characterSet": "ASCII"
    }
}

Date:

{
    "name": "C_DATE",
    "dataType": {
        "type": "DATE"
    }
}

Timestamp:

{
    "name": "C_TIMESTAMP_1",
    "dataType": {
        "type": "TIMESTAMP"
    }
}
{
    "name": "C_TIMESTAMP_2",
    "dataType": {
        "type": "TIMESTAMP",
        "withLocalTimeZone": false
    }
}
{
    "name": "C_TIMESTAMP_3",
    "dataType": {
        "type": "TIMESTAMP",
        "withLocalTimeZone": true
    }
}

Boolean:

{
    "name": "C_BOOLEAN",
    "dataType": {
        "type": "BOOLEAN"
    }
}

Geometry:

{
    "name": "C_GEOMETRY",
    "dataType": {
        "type": "GEOMETRY",
        "srid": 1
    }
}

Interval:

{
    "name": "C_INTERVAL_DS_1",
    "dataType": {
        "type": "INTERVAL",
        "fromTo": "DAY TO SECONDS"
    }
}
{
    "name": "C_INTERVAL_DS_2",
    "dataType": {
        "type": "INTERVAL",
        "fromTo": "DAY TO SECONDS",
        "precision": 3,
        "fraction": 4
    }
}
{
    "name": "C_INTERVAL_YM_1",
    "dataType": {
        "type": "INTERVAL",
        "fromTo": "YEAR TO MONTH"
    }
}
{
    "name": "C_INTERVAL_YM_2",
    "dataType": {
        "type": "INTERVAL",
        "fromTo": "YEAR TO MONTH",
        "precision": 3
    }
}

Expressions

This section handles the expressions that can occur in a pushdown request. Expressions are consistently encoded in the following way. This allows easy and consisting parsing and serialization.

{
    "type": "<type-of-expression>",
    ...
}

Each expression-type can have any number of additional fields of arbitrary type. In the following sections we define the known expressions.

Table

This element currently only occurs in from clause

{
    "type": "table",
    "name": "CLICKS"
}

Column Lookup

{
    "type": "column",
    "tableName": "T",
    "tablePosFromClause": 0,
    "columnNr": 0,
    "name": "ID"
}

Notes

  • tablePosFromClause: Position of the table in the from clause, starting with 0. Required for joins where same table occurs several times.
  • columnNr: column number in the virtual table, starting with 0

Literal

{
    "type": "literal_null"
}
{
    "type": "literal_string",
    "value": "my string"
}
{
    "type": "literal_double",
    "value": "1.234"
}
{
    "type": "literal_exactnumeric",
    "value": "12345"
}
{
    "type": "literal_bool",
    "value": true
}
{
    "type": "literal_date",
    "value": "2015-12-01"
}
{
    "type": "literal_timestamp",
    "value": "2015-12-01 12:01:01.1234"
}
{
    "type": "literal_timestamputc",
    "value": "2015-12-01 12:01:01.1234"
}

Predicates

Whenever there is ... this is a shortcut for an arbitrary expression.

{
    "type": "predicate_and",
    "expressions": [
        ...
    ]
}

The same can be used for "predicate_or".

{
    "type": "predicate_not",
    "expression": {
        ...
    }
}
{
    "type": "predicate_equals",
    "left": {
        ...
    },
    "right": {
        ...
    }
}

The same can be used for "predicate_notequals", "predicate_less" and "predicate_lessequals".

{
    "type": "predicate_like",
    "expression": {
        ...
    },
    "pattern": {
        ...
    },
    "escapeChar": "%"
}

The same can be used for predicate_like_regexp

Notes

  • escapeChar is optional
{
    "type": "predicate_between",
    "expression": {
        ...
    },
    "left": {
        ...
    },
    "right": {
        ...
    }
}

IN (, )

{
    "type": "predicate_in_constlist",
    "expression": {
        ...
    }
    "arguments": [
        ...
    ]
}

Scalar Functions

Single argument (consistent with multiple argument version)

{
    "type": "function_scalar",
    "numArgs": 1,
    "name": "ABS",
    "arguments": [
    {
        ...
    }
    ]
}

Multiple arguments

{
    "type": "function_scalar",
    "numArgs": 2,
    "name": "POWER",
    "arguments": [
    {
        ...
    },
    {
        ...
    }
    ]
}
{
    "type": "function_scalar",
    "variableInputArgs": true,
    "name": "CONCAT",
    "arguments": [
    {
        ...
    },
    {
        ...
    },
    {
        ...
    }
    ]
}

Notes

  • variableInputArgs: default value is false. If true, numArgs is not defined.

Arithmetic operators have following names: ADD, SUB, MULT, FLOAT_DIV. They are defined as infix (just a hint, not necessary)

{
    "type": "function_scalar",
    "numArgs": 2,
    "name": "ADD",
    "infix": true,
    "arguments": [
    {
        ...
    },
    {
        ...
    }
    ]
}

Special cases

EXTRACT(toExtract FROM exp1) (requires scalar-function capability EXTRACT)

{
    "type": "function_scalar_extract",
    "name": "EXTRACT",
    "toExtract": "MINUTE",
    "arguments": [
    {
        ...
    }
    ],
}

CAST(exp1 AS dataType) (requires scalar-function capability CAST)

{
    "type": "function_scalar_cast",
    "name": "CAST",
    "dataType": 
    {
        "type" : "VARCHAR",
        "size" : 10000
    },
    "arguments": [
    {
        ...
    }
    ],
}

CASE (requires scalar-function capability CAST)

CASE basis WHEN exp1 THEN result1
           WHEN exp2 THEN result2
           ELSE result3
           END
{
    "type": "function_scalar_case",
    "name": "CASE",
    "basis" :
    {
        "type" : "column",
        "columnNr" : 0,
        "name" : "NUMERIC_GRADES",
        "tableName" : "GRADES"
    },
    "arguments": [
    {        
        "type" : "literal_exactnumeric",
        "value" : "1"
    },       
    {        
        "type" : "literal_exactnumeric",
        "value" : "2"
    }
    ],
    "results": [
    {        
        "type" : "literal_string",
        "value" : "VERY GOOD"
    },       
    {        
        "type" : "literal_string",
        "value" : "GOOD"
    },
    {        
        "type" : "literal_string",
        "value" : "INVALID"
    }
    ]
}

Notes:

  • arguments: The different cases.
  • results: The different results in the same order as the arguments. If present, the ELSE result is the last entry in the results array.

Aggregate Functions

Consistent with scalar functions. To be detailed: star-operator, distinct, ...

{
    "type": "function_aggregate",
    "name": "SUM",
    "arguments": [
    {
        ...
    }
    ]
}
{
    "type": "function_aggregate",
    "name": "CORR",
    "arguments": [
    {
        ...
    },
    {
        ...
    }
    ]
}

Special cases

COUNT(exp) (requires set-function capability COUNT)

{
    "type": "function_aggregate",
    "name": "COUNT",
    "arguments": [
    {
        ...
    }
    ]
}

COUNT(*) (requires set-function capability COUNT and COUNT_STAR)

{
    "type": "function_aggregate",
    "name": "COUNT"
}

COUNT(DISTINCT exp) (requires set-function capability COUNT and COUNT_DISTINCT)

{
    "type": "function_aggregate",
    "name": "COUNT",
    "distinct": true,
    "arguments": [
    {
        ...
    }
    ]
}

COUNT((exp1, exp2)) (requires set-function capability COUNT and COUNT_TUPLE)

{
    "type": "function_aggregate",
    "name": "COUNT",
    "distinct": true,
    "arguments": [
    {
        ...
    },
    {
        ...
    }
    ]
}

AVG(exp) (requires set-function capability AVG)

{
    "type": "function_aggregate",
    "name": "AVG",
    "arguments": [
    {
        ...
    }
    ]
}

AVG(DISTINCT exp) (requires set-function capability AVG and AVG_DISTINCT)

{
    "type": "function_aggregate",
    "name": "AVG",
    "distinct": true,
    "arguments": [
    {
        ...
    }
    ]
}

GROUP_CONCAT(DISTINCT exp1 orderBy SEPARATOR ', ') (requires set-function capability GROUP_CONCAT)

{
    "type": "function_aggregate_group_concat",
    "name": "GROUP_CONCAT",
    "distinct": true,
    "arguments": [
    {
        ...
    }
    ],
    "orderBy" : [
        {
            "type" : "order_by_element",
            "expression" :
            {
              "type" : "column",
               "columnNr" : 1,
                "name" : "USER_ID",
                "tableName" : "CLICKS"
            },
            "isAscending" : true,
            "nullsLast" : true
        }
    ],
    "separator": ", "
}

Notes:

  • distinct: Optional. Requires set-function capability GROUP_CONCAT_DISTINCT.
  • orderBy: Optional. The requested order-by clause, a list of order_by_element elements. The field expression contains the expression to order by. The group-by clause of a SELECT query uses the same order_by_element element type. The clause requires the set-function capability GROUP_CONCAT_ORDER_BY.
  • separator: Optional. Requires set-function capability GROUP_CONCAT_SEPARATOR.