Changes between Initial Version and Version 1 of rfc21_ogrsqlcast

Timestamp:

Jan 24, 2008, 2:25:53 PM (16 years ago)

Author:

tamas

Comment:

--

rfc21_ogrsqlcast

@@ +1 @@
+= RFC 21: OGR SQL type cast and field name alias =
+
+Author: Tamas Szekeres[[BR]]
+Contact: szekerest@gmail.com[[BR]]
+Status: Proposed
+
+== Summary ==
+
+This proposal provides support for changing the column name and the column type
+in the OGR SQL select list.
+
+The main motivation of this change is to provide better control when transferring
+the OGR_STYLE special field from each data source to the other.
+For example we can use ogr2ogr for this purpose using the fillowing command line:
+
+{{{
+ogr2ogr -f "ESRI Shapefile" -sql "select *, OGR_STYLE from rivers" rivers.shp rivers.tab
+}}}
+
+The shape driver will truncate the OGR_STYLE field to 80 characters by default in length
+that may not be enough to hold the actual value.
+So as to fix this issue we might want to specify the desired length in the select list, like:
+
+{{{
+ogr2ogr -f "ESRI Shapefile" -sql "select *, CAST(OGR_STYLE AS character(255)) from rivers" rivers.shp rivers.tab
+}}}
+
+In some cases it would also be useful to change the name of the field in the target data source:
+
+{{{
+ogr2ogr -f "ESRI Shapefile" -sql "select *, CAST(OGR_STYLE AS character(255)) AS 'STYLE' from rivers" rivers.shp rivers.tab
+}}}
+
+== Main concepts ==
+
+To support these new features we will extend the current OGR SQL syntax. The proposed addition will keep the syntax
+conform to the SQL92 specification:
+
+{{{
+SELECT <field-list> FROM <table_def>
+     [LEFT JOIN <table_def> 
+      ON [<table_ref>.]<key_field> = [<table_ref>.].<key_field>]*
+     [WHERE <where-expr>] 
+     [ORDER BY <sort specification list>]
+
+<field-list> ::= <column-spec> [ { , <column-spec> }... ]
+
+<column-spec> ::= <field-spec> [ <as clause> ]
+                 | CAST ( <field-spec> AS <data type> ) [ <as clause> ]
+
+<field-spec> ::= [DISTINCT] <field_ref>
+                 | <field_func> ( [DISTINCT] <field-ref> )
+                 | Count(*)
+
+<as clause> ::= [ AS ] <column_name>
+
+<data type> ::= character [ ( field_length ) ]
+                | float [ ( field_length ) ]
+                | integer [ ( field_length ) ]
+
+<field-func> ::= AVG | MAX | MIN | SUM | COUNT
+
+<field_ref>  ::= [<table_ref>.]field_name
+
+<sort specification list> ::=
+              <sort specification> [ { <comma> <sort specification> }... ]
+
+<sort specification> ::= <sort key> [ <ordering specification> ]
+
+<sort key> ::=  <field_ref>
+
+<ordering specification> ::= ASC | DESC
+
+<table_def> ::= ['<datasource name>'.]table_name [table_alias]
+
+<table_ref> ::= table_name | table_alias
+}}}
+
+== Implementation ==
+
+To implement the addition the following changes should be made in the OGR codebase:
+
+1. In swq.h I'll have to add to 3 fields to swq_col_def to hold the field_alias the target_type
+and the field_length.
+
+2. In swq.c I'll have to change swq_select_preparse to take care of the field alias and the
+CAST specification.
+
+3. A new function (swq_parse_typename) will be added to parse the supported typenames.
+
+4. In ogr_gensql.cpp the .ctor of OGRGenSQLResultsLayer will be changed to set up the field name and the
+field length to the target data source 
+
+5. In ogr_gensql.cpp TranslateFeature will be modified to take care of the type change if specified.
+
+== Backward Compatibility ==
+
+The backward compatibility for the current SQL syntax will be retained. This addition
+doesn't requre changes in the OGR C and SWIG API. 
+
+== Documentation ==
+
+The OGR SQL document will be updated to reflect the new features.
+I'll take care of updating the OGR Style Documentation with the support of transferring the
+styles between the data sources.
+
+== Implementation Staffing ==
+
+Tamas Szekeres will implement the RFC in the development version.
+
+== References ==
+
+ * Tracking bug for this feature (containing all of the proposed code changes): #2171
+
+== Voting History ==
+
+None