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What this error means

PARTIAL_KEY_IN_UNIQUE_KEY means a primary key uses a prefix length, such as name(10). A common error message is: 
table "customers" contains partial index on column "email" in the primary key.

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Why PlanetScale rejects it

A prefix index only indexes part of a column value. PlanetScale needs a unique key that can identify the full row during online schema changes. A partial key is not accepted as that identity.

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How to fix it

Use a full column in the primary key, add a surrogate primary key, or add a separate full unique key over NOT NULL columns. Invalid: 
CREATE TABLE customers (
  email varchar(255) NOT NULL,
  PRIMARY KEY (email(32))
);
Recommended: 
CREATE TABLE customers (
  id bigint unsigned NOT NULL,
  email varchar(255) NOT NULL,
  PRIMARY KEY (id),
  UNIQUE KEY customers_email_idx (email)
);
If the full column is too large for your indexing strategy, use a shorter bounded column type or a separate stable identifier.

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