Question 1

Why push data from Snowflake into Postgres?

Accepted Answer

Because the systems that need the answer usually cannot query a data warehouse. An internal admin tool, a customer facing dashboard, or a service that gates a feature on account tier all read from the application database, and pointing them at Snowflake is either too slow or not permitted. Reverse ETL lands the modelled result in Postgres where those systems already look, and keeps it current on a schedule rather than through a nightly script somebody maintains.

Question 2

How are rows matched in the destination?

Accepted Answer

You nominate an identity field on the Snowflake side and map it to the corresponding column in Postgres, and updates run against that match. The destination table does not need a declared primary key for this to work, but the field you choose should be unique. If it is not, a single source row can match several destination rows and update all of them, which is rarely what anyone intends.

Question 3

How does the write actually perform?

Accepted Answer

Inserts are batched, up to 250 rows per statement, and if a batch fails it is retried row by row so you get record level errors rather than one opaque failure. Updates are issued as one statement per row, which is worth knowing when a sync updates a very large number of rows on every run. Filtering the model so it emits only rows that genuinely changed is the usual way to keep that in hand.

Question 4

Does every run read the whole Snowflake table?

Accepted Answer

Not once a tracking column is configured. Snowflake is read with ordinary SQL against your compute warehouse, and an incremental sync adds a predicate on a monotonically increasing column such as an updated timestamp, so later runs scan far less. The first run is a full read. Because this is watermark based rather than a change feed, hard deletes in Snowflake are not detected, so a periodic full run is worth scheduling if parity matters.

Connect Snowflake with PostgreSQL

Connect Snowflake and PostgreSQL with Polytomic

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