mongoc_collection_estimated_document_count

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);

• 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.

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 are propagated via the error parameter.

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

#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);
   }
}

mongoc_collection_count_documents()

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