[GitHub] [arrow-adbc] kou commented on issue #581: c/driver/sqlite: Column type is always int64 with empty table

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

kou commented on issue #581:
URL: https://github.com/apache/arrow-adbc/issues/581#issuecomment-1501267541

   Ah, I thought that `sqlite3_column_type()` can work with an empty table. So I thought that we can improve the current implementation by:
   
   ```diff
   diff --git a/c/driver/sqlite/statement_reader.c b/c/driver/sqlite/statement_reader.c
   index 694b8cc..21fb20f 100644
   --- a/c/driver/sqlite/statement_reader.c
   +++ b/c/driver/sqlite/statement_reader.c
   @@ -495,7 +495,7 @@ void StatementReaderRelease(struct ArrowArrayStream* self) {
    
    /// Initialize buffers for the first (type-inferred) batch of data.
    /// Use raw buffers since the types may change.
   -AdbcStatusCode StatementReaderInitializeInfer(int num_columns, size_t infer_rows,
   +AdbcStatusCode StatementReaderInitializeInfer(sqlite3_stmt *stmt, int num_columns, size_t infer_rows,
                                                  struct ArrowBitmap* validity,
                                                  struct ArrowBuffer* data,
                                                  struct ArrowBuffer* binary,
   @@ -507,7 +507,21 @@ AdbcStatusCode StatementReaderInitializeInfer(int num_columns, size_t infer_rows
        ArrowBufferInit(&data[i]);
        CHECK_NA(INTERNAL, ArrowBufferReserve(&data[i], infer_rows * sizeof(int64_t)), error);
        memset(&binary[i], 0, sizeof(struct ArrowBuffer));
   -    current_type[i] = NANOARROW_TYPE_INT64;
   +    int sqlite_type = sqlite3_column_type(stmt, i);
   +    switch (sqlite_type) {
   +    case SQLITE_FLOAT:
   +      current_type[i] = NANOARROW_TYPE_DOUBLE;
   +      break;
   +    case SQLITE_TEXT:
   +      current_type[i] = NANOARROW_TYPE_STRING;
   +      break;
   +    case SQLITE_BLOB:
   +      current_type[i] = NANOARROW_TYPE_BINARY;
   +      break;
   +    default:
   +      current_type[i] = NANOARROW_TYPE_INT64;
   +      break;
   +    }
      }
      return ADBC_STATUS_OK;
    }  // NOLINT(whitespace/indent)
   @@ -835,7 +849,7 @@ AdbcStatusCode AdbcSqliteExportReader(sqlite3* db, sqlite3_stmt* stmt,
      enum ArrowType* current_type = malloc(num_columns * sizeof(enum ArrowType));
    
      AdbcStatusCode status = StatementReaderInitializeInfer(
   -      num_columns, batch_size, validity, data, binary, current_type, error);
   +      stmt, num_columns, batch_size, validity, data, binary, current_type, error);
      if (status == ADBC_STATUS_OK) {
        int64_t num_rows = 0;
        while (num_rows < batch_size) {
   ```
   
   But `sqlite3_column_type()` always returns `SQLITE_NULL` with the case. :<
   
   We may want to use `sqlite3_column_decltype()`...


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