  Blue straggler stars (BSSs) are the most massive stars in a cluster formed
via binary or higher-order stellar interactions. Though the exact nature of
such formation scenarios is difficult to pin down, we provide observational
constraints on the different possible mechanism. In this quest, we first
produce a catalogue of BSSs using Gaia DR2 data. Among the 670 clusters older
than 300 Myr, we identified 868 BSSs in 228 clusters and 500 BSS candidates in
208 clusters. In general, all clusters older than 1 Gyr and massive than 1000
Msun have BSSs. The average number of BSSs increases with cluster age and mass,
and there is a power-law relation between the cluster mass and the maximum
number of BSSs in the cluster. We introduce the term fractional mass excess
(Me) for BSSs. We find that at least 54\% of BSSs have Me $<$ 0.5 (likely to
have gained mass through a binary mass transfer (MT)), 30\% in the $1.0 <$ Me
$< 0.5$ range (likely to have gained mass through a merger) and up to 16\% with
Me $>$ 1.0 (likely from multiple mergers/MT). We also find that the percentage
of low Me BSSs increases with age, beyond 1--2 Gyr, suggesting an increase in
formation through MT in older clusters. The BSSs are radially segregated, and
the extent of segregation depends on the dynamical relaxation of the cluster.
The statistics and trends presented here are expected to constrain the BSS
formation models in open clusters.
