Revert #1292

If ROWNUM is used, we need to wrap it in a subquery, and I don't think that's worth it, given that the version of Oracle that supports FETCH FIRST is now 10 years old.

Fixes #1436

NEWS.md

@@ -1,5 +1,9 @@
 # dbplyr (development version)
 
+* Oracle once again translates `head()` to `FETCH FIRST`. This does require
+  Oracle 12c, but it actually works, compared to the approach using `ROWNUM`
+  from #1292 (#1436).
+
 * The databricks backend now supports creating non-temporary tables too (#1418).
 
 * Clearer error if you attempt to embed non-atomic vectors inside of a generated 

R/backend-oracle.R

@@ -48,11 +48,6 @@ sql_query_select.Oracle <- function(con,
                                     subquery = FALSE,
                                     lvl = 0) {
 
-  if (!is.null(limit)) {
-    limit <- as.integer(limit)
-    where = c(paste0("ROWNUM <= ", limit), where)
-  }
-
   sql_select_clauses(con,
     select    = sql_clause_select(con, select, distinct),
     from      = sql_clause_from(from),
@@ -61,6 +56,11 @@ sql_query_select.Oracle <- function(con,
     having    = sql_clause_having(having),
     window    = sql_clause_window(window),
     order_by  = sql_clause_order_by(order_by, subquery, limit),
+    # Requires Oracle 12c, released in 2013
+    limit =   if (!is.null(limit)) {
+      limit <- format(as.integer(limit))
+      glue_sql2(con, "FETCH FIRST {limit} ROWS ONLY")
+    },
     lvl = lvl
   )
 }

tests/testthat/_snaps/backend-oracle.md

@@ -17,7 +17,7 @@
       <SQL>
       SELECT `df`.*
       FROM `df`
-      WHERE (ROWNUM <= 6)
+      FETCH FIRST 6 ROWS ONLY
 
 # `sql_query_upsert()` is correct