Match fields by position via option

We add an option for `COPY FROM` called `match_by_position` which matches Parquet file fields to PostgreSQL table columns
`by their position` in the schema rather than `by their names`. By default, the option is `false`. The option is useful
when field names differ between the Parquet file and the table, but their order aligns.

Closes #39.

README.md

@@ -193,6 +193,7 @@ Alternatively, you can use the following environment variables when starting pos
 
 `pg_parquet` supports the following options in the `COPY FROM` command:
 - `format parquet`: you need to specify this option to read or write Parquet files which does not end with `.parquet[.<compression>]` extension,
+- `match_by_position <bool>`: matches Parquet file fields to PostgreSQL table columns by their position in the schema rather than by their names. By default, the option is `false`. The option is useful when field names differ between the Parquet file and the table, but their order aligns.
 
 ## Configuration
 There is currently only one GUC parameter to enable/disable the `pg_parquet`:

src/arrow_parquet/parquet_reader.rs

@@ -38,10 +38,11 @@ pub(crate) struct ParquetReaderContext {
     parquet_reader: ParquetRecordBatchStream<ParquetObjectReader>,
     attribute_contexts: Vec<ArrowToPgAttributeContext>,
     binary_out_funcs: Vec<PgBox<FmgrInfo>>,
+    match_by_position: bool,
 }
 
 impl ParquetReaderContext {
-    pub(crate) fn new(uri: Url, tupledesc: &PgTupleDesc) -> Self {
+    pub(crate) fn new(uri: Url, match_by_position: bool, tupledesc: &PgTupleDesc) -> Self {
         // Postgis and Map contexts are used throughout reading the parquet file.
         // We need to reset them to avoid reading the stale data. (e.g. extension could be dropped)
         reset_postgis_context();
@@ -69,6 +70,7 @@ impl ParquetReaderContext {
             parquet_file_schema.clone(),
             tupledesc_schema.clone(),
             &attributes,
+            match_by_position,
         );
 
         let attribute_contexts = collect_arrow_to_pg_attribute_contexts(
@@ -85,6 +87,7 @@ impl ParquetReaderContext {
             attribute_contexts,
             parquet_reader,
             binary_out_funcs,
+            match_by_position,
             started: false,
             finished: false,
         }
@@ -116,15 +119,23 @@ impl ParquetReaderContext {
     fn record_batch_to_tuple_datums(
         record_batch: RecordBatch,
         attribute_contexts: &[ArrowToPgAttributeContext],
+        match_by_position: bool,
     ) -> Vec<Option<Datum>> {
         let mut datums = vec![];
 
-        for attribute_context in attribute_contexts {
+        for (attribute_idx, attribute_context) in attribute_contexts.iter().enumerate() {
             let name = attribute_context.name();
 
-            let column_array = record_batch
-                .column_by_name(name)
-                .unwrap_or_else(|| panic!("column {} not found", name));
+            let column_array = if match_by_position {
+                record_batch
+                    .columns()
+                    .get(attribute_idx)
+                    .unwrap_or_else(|| panic!("column {} not found", name))
+            } else {
+                record_batch
+                    .column_by_name(name)
+                    .unwrap_or_else(|| panic!("column {} not found", name))
+            };
 
             let datum = if attribute_context.needs_cast() {
                 // should fail instead of returning None if the cast fails at runtime
@@ -181,8 +192,11 @@ impl ParquetReaderContext {
                 self.buffer.extend_from_slice(&attnum_len_bytes);
 
                 // convert the columnar arrays in record batch to tuple datums
-                let tuple_datums =
-                    Self::record_batch_to_tuple_datums(record_batch, &self.attribute_contexts);
+                let tuple_datums = Self::record_batch_to_tuple_datums(
+                    record_batch,
+                    &self.attribute_contexts,
+                    self.match_by_position,
+                );
 
                 // write the tuple datums to the ParquetReader's internal buffer in PG copy format
                 for (datum, out_func) in tuple_datums.into_iter().zip(self.binary_out_funcs.iter())

src/arrow_parquet/schema_parser.rs

@@ -349,21 +349,38 @@ pub(crate) fn ensure_file_schema_match_tupledesc_schema(
     file_schema: Arc<Schema>,
     tupledesc_schema: Arc<Schema>,
     attributes: &[FormData_pg_attribute],
+    match_by_position: bool,
 ) -> Vec<Option<DataType>> {
     let mut cast_to_types = Vec::new();
 
+    if match_by_position && tupledesc_schema.fields().len() != file_schema.fields().len() {
+        panic!(
+            "column count mismatch between table and parquet file. \
+             parquet file has {} columns, but table has {} columns",
+            file_schema.fields().len(),
+            tupledesc_schema.fields().len()
+        );
+    }
+
     for (tupledesc_schema_field, attribute) in
         tupledesc_schema.fields().iter().zip(attributes.iter())
     {
         let field_name = tupledesc_schema_field.name();
 
-        let file_schema_field = file_schema.column_with_name(field_name);
+        let file_schema_field = if match_by_position {
+            file_schema.field(attribute.attnum as usize - 1)
+        } else {
+            let file_schema_field = file_schema.column_with_name(field_name);
 
-        if file_schema_field.is_none() {
-            panic!("column \"{}\" is not found in parquet file", field_name);
-        }
+            if file_schema_field.is_none() {
+                panic!("column \"{}\" is not found in parquet file", field_name);
+            }
+
+            let (_, file_schema_field) = file_schema_field.unwrap();
+
+            file_schema_field
+        };
 
-        let (_, file_schema_field) = file_schema_field.unwrap();
         let file_schema_field = Arc::new(file_schema_field.clone());
 
         let from_type = file_schema_field.data_type();
@@ -378,7 +395,7 @@ pub(crate) fn ensure_file_schema_match_tupledesc_schema(
         if !is_coercible(from_type, to_type, attribute.atttypid, attribute.atttypmod) {
             panic!(
                 "type mismatch for column \"{}\" between table and parquet file.\n\n\
-                table has \"{}\"\n\nparquet file has \"{}\"",
+                 table has \"{}\"\n\nparquet file has \"{}\"",
                 field_name, to_type, from_type
             );
         }

src/parquet_copy_hook/copy_from.rs

@@ -20,8 +20,8 @@ use crate::{
 };
 
 use super::copy_utils::{
-    copy_stmt_attribute_list, copy_stmt_create_namespace_item, copy_stmt_create_parse_state,
-    create_filtered_tupledesc_for_relation,
+    copy_from_stmt_match_by_position, copy_stmt_attribute_list, copy_stmt_create_namespace_item,
+    copy_stmt_create_parse_state, create_filtered_tupledesc_for_relation,
 };
 
 // stack to store parquet reader contexts for COPY FROM.
@@ -131,9 +131,11 @@ pub(crate) fn execute_copy_from(
 
     let tupledesc = create_filtered_tupledesc_for_relation(p_stmt, &relation);
 
+    let match_by_position = copy_from_stmt_match_by_position(p_stmt);
+
     unsafe {
         // parquet reader context is used throughout the COPY FROM operation.
-        let parquet_reader_context = ParquetReaderContext::new(uri, &tupledesc);
+        let parquet_reader_context = ParquetReaderContext::new(uri, match_by_position, &tupledesc);
         push_parquet_reader_context(parquet_reader_context);
 
         // makes sure to set binary format

src/parquet_copy_hook/copy_utils.rs

@@ -3,11 +3,12 @@ use std::{ffi::CStr, str::FromStr};
 use pgrx::{
     is_a,
     pg_sys::{
-        addRangeTableEntryForRelation, defGetInt32, defGetInt64, defGetString, get_namespace_name,
-        get_rel_namespace, makeDefElem, makeString, make_parsestate, quote_qualified_identifier,
-        AccessShareLock, AsPgCStr, CopyStmt, CreateTemplateTupleDesc, DefElem, List, NoLock, Node,
-        NodeTag::T_CopyStmt, Oid, ParseNamespaceItem, ParseState, PlannedStmt, QueryEnvironment,
-        RangeVar, RangeVarGetRelidExtended, RowExclusiveLock, TupleDescInitEntry,
+        addRangeTableEntryForRelation, defGetBoolean, defGetInt32, defGetInt64, defGetString,
+        get_namespace_name, get_rel_namespace, makeDefElem, makeString, make_parsestate,
+        quote_qualified_identifier, AccessShareLock, AsPgCStr, CopyStmt, CreateTemplateTupleDesc,
+        DefElem, List, NoLock, Node, NodeTag::T_CopyStmt, Oid, ParseNamespaceItem, ParseState,
+        PlannedStmt, QueryEnvironment, RangeVar, RangeVarGetRelidExtended, RowExclusiveLock,
+        TupleDescInitEntry,
     },
     PgBox, PgList, PgRelation, PgTupleDesc,
 };
@@ -109,7 +110,7 @@ pub(crate) fn validate_copy_to_options(p_stmt: &PgBox<PlannedStmt>, uri: &Url) {
 }
 
 pub(crate) fn validate_copy_from_options(p_stmt: &PgBox<PlannedStmt>) {
-    validate_copy_option_names(p_stmt, &["format", "freeze"]);
+    validate_copy_option_names(p_stmt, &["format", "match_by_position", "freeze"]);
 
     let format_option = copy_stmt_get_option(p_stmt, "format");
 
@@ -253,6 +254,16 @@ pub(crate) fn copy_from_stmt_create_option_list(p_stmt: &PgBox<PlannedStmt>) ->
     new_copy_options
 }
 
+pub(crate) fn copy_from_stmt_match_by_position(p_stmt: &PgBox<PlannedStmt>) -> bool {
+    let match_by_position_option = copy_stmt_get_option(p_stmt, "match_by_position");
+
+    if match_by_position_option.is_null() {
+        false
+    } else {
+        unsafe { defGetBoolean(match_by_position_option.as_ptr()) }
+    }
+}
+
 pub(crate) fn copy_stmt_get_option(
     p_stmt: &PgBox<PlannedStmt>,
     option_name: &str,

src/pgrx_tests/copy_from_coerce.rs

@@ -966,7 +966,7 @@ mod tests {
     }
 
     #[pg_test]
-    fn test_table_with_different_field_position() {
+    fn test_table_with_different_position_match_by_name() {
         let copy_to = format!(
             "COPY (SELECT 1 as x, 'hello' as y) TO '{}'",
             LOCAL_TEST_FILE_PATH
@@ -983,6 +983,34 @@ mod tests {
         assert_eq!(result, (Some("hello"), Some(1)));
     }
 
+    #[pg_test]
+    fn test_table_with_different_name_match_by_position() {
+        let copy_to = "COPY (SELECT 1 as a, 'hello' as b) TO '/tmp/test.parquet'";
+        Spi::run(copy_to).unwrap();
+
+        let create_table = "CREATE TABLE test_table (x bigint, y varchar)";
+        Spi::run(create_table).unwrap();
+
+        let copy_from = "COPY test_table FROM '/tmp/test.parquet' WITH (match_by_position true)";
+        Spi::run(copy_from).unwrap();
+
+        let result = Spi::get_two::<i64, &str>("SELECT x, y FROM test_table LIMIT 1").unwrap();
+        assert_eq!(result, (Some(1), Some("hello")));
+    }
+
+    #[pg_test]
+    #[should_panic(expected = "column count mismatch between table and parquet file")]
+    fn test_table_with_different_name_match_by_position_fail() {
+        let copy_to = "COPY (SELECT 1 as a, 'hello' as b) TO '/tmp/test.parquet'";
+        Spi::run(copy_to).unwrap();
+
+        let create_table = "CREATE TABLE test_table (x bigint, y varchar, z int)";
+        Spi::run(create_table).unwrap();
+
+        let copy_from = "COPY test_table FROM '/tmp/test.parquet' WITH (match_by_position true)";
+        Spi::run(copy_from).unwrap();
+    }
+
     #[pg_test]
     #[should_panic(expected = "column \"name\" is not found in parquet file")]
     fn test_missing_column_in_parquet() {