Database / Google Spanner Database Interview questions
What is the difference between GoogleSQL and PostgreSQL dialects in Spanner?
| GoogleSQL dialect | PostgreSQL dialect |
| Native Spanner types like STRUCT and ARRAY are first-class. | Uses Postgres types: bigint, numeric, text, timestamptz. |
| DDL uses Spanner-specific clauses (e.g. INTERLEAVE IN PARENT). | DDL closely mirrors standard Postgres DDL syntax. |
| Works with Spanner client libraries and GoogleSQL-aware tooling. | Works with many existing PostgreSQL drivers via the PGAdapter proxy. |
The two dialects are not interchangeable within one database - the choice is fixed at creation - and query behavior can differ subtly, for instance in how NULLs sort or how implicit type coercion works. Teams migrating an existing Postgres application typically pick the PostgreSQL dialect specifically to reduce query rewrite effort and to reuse existing Postgres-compatible tooling through PGAdapter, while greenfield Spanner projects more often default to GoogleSQL.
What tool lets standard PostgreSQL drivers connect to Spanner's PostgreSQL dialect?
Can a single Spanner database mix both dialects?
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