Which SQL grammar is more popular in OLAP System, MySQL or PostgreSQL?

Nearly every analytics database uses some dialect of ANSI SQL. ClickHouse now uses the PostgreSQL dialect, as does Amazon Redshift (based on a Postgres fork). Microsoft Synapse, of course, uses the MS SQL Server dialect.

A third standard, more common than either MySQL or PostgreSQL, is Apache Calcite, which is the SQL dialect used by a wide range of Apache projects (Apex, Beam, Drill, Druid, Flink, Hive, Kylin, Phoenix, Pinot, Samza, Storm), Alibaba MaxCompute, Dremio, OmniSci, Qubole, SQLl-Gremlin, Tencent Big Data Suite, and others.

Teradata and Google BigQuery use their own SQL dialects.