[GitHub] [arrow-adbc] kou commented on a diff in pull request #707: feat(c/driver/postgresql): handle non-SELECT statements

["kou (via GitHub)" <gi...@apache.org>]

kou commented on code in PR #707:
URL: https://github.com/apache/arrow-adbc/pull/707#discussion_r1204933000


##########
c/driver/postgresql/statement.cc:
##########
@@ -649,17 +649,27 @@ AdbcStatusCode PostgresStatement::ExecuteQuery(struct ArrowArrayStream* stream,
     return ADBC_STATUS_INVALID_STATE;
   }
 
-  // 1. Execute the query with LIMIT 0 to get the schema
+  // 1. Prepare the query to get the schema
   {
     // TODO: we should pipeline here and assume this will succeed
-    std::string schema_query = "SELECT * FROM (" + query_ + ") AS ignored LIMIT 0";
-    PGresult* result =
-        PQexecParams(connection_->conn(), query_.c_str(), /*nParams=*/0,
-                     /*paramTypes=*/nullptr, /*paramValues=*/nullptr,
-                     /*paramLengths=*/nullptr, /*paramFormats=*/nullptr, kPgBinaryFormat);
-    if (PQresultStatus(result) != PGRES_TUPLES_OK) {
-      SetError(error, "%s%s", "[libpq] Query was: ", schema_query.c_str());
-      SetError(error, "%s%s", "[libpq] Failed to execute query: could not infer schema: ",
+    PGresult* result = PQprepare(connection_->conn(), /*stmtName=*/"", query_.c_str(),
+                                 /*nParams=*/0, nullptr);
+    if (PQresultStatus(result) != PGRES_COMMAND_OK) {
+      SetError(error, "%s%s", "[libpq] Query was: ", query_.c_str());
+      SetError(error, "%s%s",
+               "[libpq] Failed to execute query: could not infer schema: failed to "
+               "prepare query: ",
+               PQerrorMessage(connection_->conn()));

Review Comment:
   How about unifying these `SetError()`s to one `SetError()`?
   It seems that the current `SetError()` implementation discards the previous error message. (`[libpq] Query was: ${query}` is discarded.)



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