Allow Oracle to correctly explain queries (#1354)

Fixes #1353

NAMESPACE

@@ -55,6 +55,8 @@ S3method(db_copy_to,DBIConnection)
 S3method(db_create_index,DBIConnection)
 S3method(db_desc,DBIConnection)
 S3method(db_explain,DBIConnection)
+S3method(db_explain,OraConnection)
+S3method(db_explain,Oracle)
 S3method(db_query_fields,DBIConnection)
 S3method(db_query_fields,PostgreSQLConnection)
 S3method(db_save_query,DBIConnection)

NEWS.md

@@ -1,5 +1,7 @@
 # dbplyr (development version)
 
+* `db_explain()` now works for Oracle (@thomashulst, #1353).
+
 * Database errors now show the generated SQL, which hopefully will make it
   faster to track down problems (#1401).
 

R/backend-oracle.R

@@ -144,10 +144,11 @@ sql_translation.Oracle <- function(con) {
 
 #' @export
 sql_query_explain.Oracle <- function(con, sql, ...) {
-  glue_sql2(
-    con,
-    "EXPLAIN PLAN FOR {sql};\n",
-    "SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY()));",
+
+  # https://docs.oracle.com/en/database/oracle/oracle-database/19/tgsql/generating-and-displaying-execution-plans.html
+  c(
+    glue_sql2(con, "EXPLAIN PLAN FOR {sql}"),
+    glue_sql2(con, "SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY())")
   )
 }
 
@@ -182,6 +183,18 @@ sql_expr_matches.Oracle <- function(con, x, y, ...) {
   glue_sql2(con, "decode({x}, {y}, 0, 1) = 0")
 }
 
+#' @export
+db_explain.Oracle <- function(con, sql, ...) {
+  sql <- sql_query_explain(con, sql, ...)
+
+  msg <- "Can't explain query."
+  db_execute(con, sql[[1]], msg) # EXPLAIN PLAN
+  expl <- db_get_query(con, sql[[2]], msg) # DBMS_XPLAN.DISPLAY
+
+  out <- utils::capture.output(print(expl))
+  paste(out, collapse = "\n")
+}
+
 #' @export
 db_supports_table_alias_with_as.Oracle <- function(con) {
   FALSE
@@ -219,6 +232,9 @@ setdiff.OraConnection <- setdiff.tbl_Oracle
 #' @export
 sql_expr_matches.OraConnection <- sql_expr_matches.Oracle
 
+#' @export
+db_explain.OraConnection <- db_explain.Oracle
+
 #' @export
 db_supports_table_alias_with_as.OraConnection <- db_supports_table_alias_with_as.Oracle
 

tests/testthat/_snaps/backend-oracle.md

@@ -41,8 +41,8 @@
     Code
       sql_query_explain(con, sql("SELECT * FROM foo"))
     Output
-      <SQL> EXPLAIN PLAN FOR SELECT * FROM foo;
-      SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY()));
+      <SQL> EXPLAIN PLAN FOR SELECT * FROM foo
+      <SQL> SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY())
 
 ---