Looking on the internet deeply has found these results:

at the poolside is the most popular phrase on the web.

by the poolside 1492,000 results on the web	at the poolside 13,900,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Our tail said Brad was poolside. Sekiuchi was in a poolside chat with Kato and a guy from another family. I'll come watch you poolside, or... I'd much rather be poolside in South Beach right now. Just wait for our poolside spa treatments. But sitting poolside as others splash around can tempt even the most timid. The victim wasn't shot poolside. At one point my girlfriend worked in hotel bars, like the poolside bars, she was a young...	But sitting poolside as others splash around can tempt even the most timid. Just wait for our poolside spa treatments. Sekiuchi was in a poolside chat with Kato and a guy from another family. I'd much rather be poolside in South Beach right now. The victim wasn't shot poolside. Our tail said Brad was poolside. At one point my girlfriend worked in hotel bars, like the poolside bars, she was a young... He's going to look even greater lounging poolside at the new Blake resort.

by the poolside

1492,000 results on the web

at the poolside

13,900,000 results on the web

More popular!

Our tail said Brad was poolside.
Sekiuchi was in a poolside chat with Kato and a guy from another family.
I'll come watch you poolside, or...
I'd much rather be poolside in South Beach right now.
Just wait for our poolside spa treatments.
But sitting poolside as others splash around can tempt even the most timid.
The victim wasn't shot poolside.
At one point my girlfriend worked in hotel bars, like the poolside bars, she was a young...

But sitting poolside as others splash around can tempt even the most timid.
Just wait for our poolside spa treatments.
Sekiuchi was in a poolside chat with Kato and a guy from another family.
I'd much rather be poolside in South Beach right now.
The victim wasn't shot poolside.
Our tail said Brad was poolside.
At one point my girlfriend worked in hotel bars, like the poolside bars, she was a young...
He's going to look even greater lounging poolside at the new Blake resort.