refactor: add origin information to Column

sqlx-core/src/column.rs

@@ -2,6 +2,7 @@ use crate::database::Database;
 use crate::error::Error;
 
 use std::fmt::Debug;
+use std::sync::Arc;
 
 pub trait Column: 'static + Send + Sync + Debug {
     type Database: Database<Column = Self>;
@@ -20,6 +21,54 @@ pub trait Column: 'static + Send + Sync + Debug {
 
     /// Gets the type information for the column.
     fn type_info(&self) -> &<Self::Database as Database>::TypeInfo;
+
+    /// If this column comes from a table, return the table and original column name.
+    /// 
+    /// Returns [`ColumnOrigin::Expression`] if the column is the result of an expression
+    /// or else the source table could not be determined.
+    /// 
+    /// Returns [`ColumnOrigin::Unknown`] if the database driver does not have that information,
+    /// or has not overridden this method. 
+    fn origin(&self) -> ColumnOrigin { ColumnOrigin::Unknown }
+}
+
+/// A [`Column`] that originates from a table.
+#[derive(Debug, Clone)]
+pub struct TableColumn {
+    /// The name of the table (optionally schema-qualified) that the column comes from.
+    pub table: Arc<str>,
+    /// The original name of the column.
+    pub name: Arc<str>,
+}
+
+/// The possible statuses for our knowledge of the origin of a [`Column`]. 
+#[derive(Debug, Clone)]
+pub enum ColumnOrigin {
+    /// The column is known to originate from a table. 
+    /// 
+    /// Included is the table name and original column name. 
+    Table(TableColumn),
+    /// The column originates from an expression, or else its origin could not be determined.
+    Expression,
+    /// The database driver does not know the column origin at this time.
+    /// 
+    /// This may happen if:
+    /// * The connection is in the middle of executing a query, 
+    ///   and cannot query the catalog to fetch this information.
+    /// * The connection does not have access to the database catalog.
+    /// * The implementation of [`Column`] did not override [`Column::origin()`].
+    Unknown,
+}
+
+impl ColumnOrigin {
+    /// Returns the true column origin, if known.
+    pub fn table_column(&self) -> Option<&TableColumn> {
+        if let Self::Table(table_column) = self {
+            Some(table_column)
+        } else {
+            None
+        }
+    }
 }
 
 /// A type that can be used to index into a [`Row`] or [`Statement`].

sqlx-postgres/src/column.rs

@@ -2,6 +2,7 @@ use crate::ext::ustr::UStr;
 use crate::{PgTypeInfo, Postgres};
 
 pub(crate) use sqlx_core::column::{Column, ColumnIndex};
+use sqlx_core::column::ColumnOrigin;
 
 #[derive(Debug, Clone)]
 #[cfg_attr(feature = "offline", derive(serde::Serialize, serde::Deserialize))]
@@ -13,6 +14,8 @@ pub struct PgColumn {
     pub(crate) relation_id: Option<crate::types::Oid>,
     #[cfg_attr(feature = "offline", serde(skip))]
     pub(crate) relation_attribute_no: Option<i16>,
+    #[cfg_attr(feature = "offline", serde(skip))]
+    pub(crate) origin: ColumnOrigin,
 }
 
 impl PgColumn {
@@ -51,4 +54,8 @@ impl Column for PgColumn {
     fn type_info(&self) -> &PgTypeInfo {
         &self.type_info
     }
+
+    fn origin(&self) -> ColumnOrigin {
+        self.origin.clone()
+    }
 }

sqlx-postgres/src/connection/describe.rs

@@ -1,3 +1,4 @@
+use std::collections::btree_map;
 use crate::error::Error;
 use crate::ext::ustr::UStr;
 use crate::io::StatementId;
@@ -14,6 +15,9 @@ use futures_core::future::BoxFuture;
 use smallvec::SmallVec;
 use sqlx_core::query_builder::QueryBuilder;
 use std::sync::Arc;
+use sqlx_core::column::{ColumnOrigin, TableColumn};
+use sqlx_core::hash_map;
+use crate::connection::TableColumns;
 
 /// Describes the type of the `pg_type.typtype` column
 ///
@@ -122,13 +126,20 @@ impl PgConnection {
             let type_info = self
                 .maybe_fetch_type_info_by_oid(field.data_type_id, should_fetch)
                 .await?;
+
+            let origin = if let (Some(relation_oid), Some(attribute_no)) = (field.relation_id, field.relation_attribute_no) {
+                self.maybe_fetch_column_origin(relation_oid, attribute_no, should_fetch).await?
+            } else {
+                ColumnOrigin::Expression
+            };
 
             let column = PgColumn {
                 ordinal: index,
                 name: name.clone(),
                 type_info,
                 relation_id: field.relation_id,
                 relation_attribute_no: field.relation_attribute_no,
+                origin,
             };
 
             columns.push(column);
@@ -188,6 +199,54 @@ impl PgConnection {
             Ok(PgTypeInfo(PgType::DeclareWithOid(oid)))
         }
     }
+
+    async fn maybe_fetch_column_origin(
+        &mut self, 
+        relation_id: Oid, 
+        attribute_no: i16,
+        should_fetch: bool,
+    ) -> Result<ColumnOrigin, Error> {
+        let mut table_columns = match self.cache_table_to_column_names.entry(relation_id) {
+            hash_map::Entry::Occupied(table_columns) => {
+                table_columns.into_mut()
+            },
+            hash_map::Entry::Vacant(vacant) => {
+                if !should_fetch { return Ok(ColumnOrigin::Unknown); }
+
+                let table_name: String = query_scalar("SELECT $1::oid::regclass::text")
+                    .bind(relation_id)
+                    .fetch_one(&mut *self)
+                    .await?;
+
+                vacant.insert(TableColumns {
+                    table_name: table_name.into(),
+                    columns: Default::default(),
+                })
+            }
+        };
+
+        let column_name = match table_columns.columns.entry(attribute_no) {
+            btree_map::Entry::Occupied(occupied) => Arc::clone(occupied.get()),
+            btree_map::Entry::Vacant(vacant) => {
+                if !should_fetch { return Ok(ColumnOrigin::Unknown); }
+
+                let column_name: String = query_scalar(
+                    "SELECT attname FROM pg_attribute WHERE attrelid = $1 AND attnum = $2"
+                )
+                    .bind(relation_id)
+                    .bind(attribute_no)
+                    .fetch_one(&mut *self)
+                    .await?;
+
+                Arc::clone(vacant.insert(column_name.into()))
+            }
+        };
+
+        Ok(ColumnOrigin::Table(TableColumn {
+            table: table_columns.table_name.clone(),
+            name: column_name
+        }))
+    }
 
     fn fetch_type_by_oid(&mut self, oid: Oid) -> BoxFuture<'_, Result<PgTypeInfo, Error>> {
         Box::pin(async move {

sqlx-postgres/src/connection/mod.rs

@@ -1,3 +1,4 @@
+use std::collections::BTreeMap;
 use std::fmt::{self, Debug, Formatter};
 use std::sync::Arc;
 
@@ -57,6 +58,7 @@ pub struct PgConnection {
     cache_type_info: HashMap<Oid, PgTypeInfo>,
     cache_type_oid: HashMap<UStr, Oid>,
     cache_elem_type_to_array: HashMap<Oid, Oid>,
+    cache_table_to_column_names: HashMap<Oid, TableColumns>,
 
     // number of ReadyForQuery messages that we are currently expecting
     pub(crate) pending_ready_for_query_count: usize,
@@ -68,6 +70,12 @@ pub struct PgConnection {
     log_settings: LogSettings,
 }
 
+pub(crate) struct TableColumns {
+    table_name: Arc<str>,
+    /// Attribute number -> name.
+    columns: BTreeMap<i16, Arc<str>>,
+}
+
 impl PgConnection {
     /// the version number of the server in `libpq` format
     pub fn server_version_num(&self) -> Option<u32> {

src/lib.rs

@@ -5,6 +5,7 @@ pub use sqlx_core::acquire::Acquire;
 pub use sqlx_core::arguments::{Arguments, IntoArguments};
 pub use sqlx_core::column::Column;
 pub use sqlx_core::column::ColumnIndex;
+pub use sqlx_core::column::ColumnOrigin;
 pub use sqlx_core::connection::{ConnectOptions, Connection};
 pub use sqlx_core::database::{self, Database};
 pub use sqlx_core::describe::Describe;