# mongoc_collection_estimated_document_count()

## Synopsis

    int64_t
    mongoc_collection_estimated_document_count (mongoc_collection_t *collection,
                                                const bson_t *opts,
                                                const mongoc_read_prefs_t *read_prefs,
                                                bson_t *reply,
                                                bson_error_t *error);

## Parameters

- `collection`: A mongoc_collection_t.
- `opts`: A `bson_t`, `NULL` to ignore.
- `read_prefs`: A mongoc_read_prefs_t or `NULL`.
- `reply`: A location for an uninitialized `bson_t` to store the command reply, `NULL` to ignore. If not `NULL`, `reply` will be initialized.
- `error`: An optional location for a bson_error_t or `NULL`.

`opts` may be NULL or a BSON document with additional command options:

- `readConcern`: Construct a mongoc_read_concern_t and use mongoc_read_concern_append() to add the read concern to `opts`. See the example code for mongoc_client_read_command_with_opts(). Read concern requires MongoDB 3.2 or later, otherwise an error is returned.
- `sessionId`: First, construct a mongoc_client_session_t with mongoc_client_start_session(). You can begin a transaction with mongoc_client_session_start_transaction(), optionally with a mongoc_transaction_opt_t that overrides the options inherited from `collection`, and use mongoc_client_session_append() to add the session to `opts`. See the example code for mongoc_client_session_t.
- `collation`: Configure textual comparisons. See Setting Collation Order, and the MongoDB Manual entry on Collation. Collation requires MongoDB 3.2 or later, otherwise an error is returned.
- `serverId`: To target a specific server, include an int32 “serverId” field. Obtain the id by calling mongoc_client_select_server(), then mongoc_server_description_id() on its return value.
- `skip`: An int specifying how many documents matching the `query` should be skipped before counting.
- `limit`: An int specifying the maximum number of documents to count.

## Description

This functions executes a count query on `collection`. In contrast with mongoc_collection_count_documents(), the count returned is not guaranteed to be accurate.

This function is considered a retryable read operation. Upon a transient error (a network error, errors due to replica set failover, etc.) the operation is safely retried once. If `retryreads` is false in the URI (see mongoc_uri_t) the retry behavior does not apply.

## Errors

Errors are propagated via the `error` parameter.

## Returns

-1 on failure, otherwise the number of documents counted.

## Example

    #include <bson/bson.h>
    #include <mongoc/mongoc.h>
    #include <stdio.h>

    static void
    print_count (mongoc_collection_t *collection, bson_t *query)
    {
       bson_error_t error;
       int64_t count;
       bson_t* opts = BCON_NEW ("skip", BCON_INT64(5));

       count = mongoc_collection_estimated_document_count (
          collection, opts, NULL, NULL, &error);
       bson_destroy (opts);

       if (count < 0) {
          fprintf (stderr, "Count failed: %s\n", error.message);
       } else {
          printf ("%" PRId64 " documents counted.\n", count);
       }
    }

## See Also

mongoc_collection_count_documents()