How do you handle vector dimensionality mismatches in pgvector?

-- FIXED-dimension column (recommended when all vectors have same size)
CREATE TABLE docs (embedding VECTOR(1536));  -- enforces exactly 1536 dims

-- ERROR: dimension mismatch
INSERT INTO docs (embedding) VALUES ('[1,2,3]');   -- ERROR: expected 1536 dims
INSERT INTO docs (embedding) VALUES (ARRAY_FILL(0.0::float4, ARRAY[1024])::vector);
-- ERROR: expected 1536 dimensions, got 1024

-- VARIABLE-dimension column (no constraint)
CREATE TABLE docs_flex (embedding VECTOR);   -- accepts any dimension
INSERT INTO docs_flex VALUES ('[1,2,3]');     -- OK: 3 dims
INSERT INTO docs_flex VALUES ('[1,2,3,4,5]');-- OK: 5 dims

-- CAVEAT: you cannot create a standard index on variable-dim columns!
-- Because all rows in the index must have the same dimensionality
-- Use a PARTIAL INDEX or EXPRESSION INDEX on a specific dimension:
CREATE INDEX ON docs_flex USING hnsw ((embedding::vector(1536)) vector_cosine_ops)
WHERE vector_dims(embedding) = 1536;

-- Check dimensions of a stored vector:
SELECT vector_dims(embedding) FROM docs_flex LIMIT 5;

-- Find rows with unexpected dimension count:
SELECT id, vector_dims(embedding) AS dims
FROM docs_flex
WHERE vector_dims(embedding) != 1536;