# ALTER TABLE ALTER COLUMN SET PARQUET: encoding, compression, and bloom filter options

ALTER TABLE ALTER COLUMN SET PARQUET SQL keyword reference documentation.

Sets or removes per-column Parquet encoding, compression, and bloom filter
configuration on existing tables. Encoding and compression control how column
data is stored in Parquet files, affecting file size and read performance.
Bloom filters allow QuestDB to skip row groups that do not contain matching
values, significantly speeding up equality and `IN` queries on large Parquet
partitions.

These settings only affect
[Parquet partitions](/docs/query/export-parquet/#in-place-conversion) and are
ignored for native partitions.

## Syntax

```questdb-sql
ALTER TABLE tableName ALTER COLUMN columnName
    SET PARQUET ( encoding [, compression [( level )] ] [, BLOOM_FILTER] );
```

Where `encoding` is `default` or a
[supported encoding](#supported-encodings-and-codecs), and `compression` is a
[supported codec](#supported-encodings-and-codecs) with an optional `level`.

## Supported encodings and codecs

See the [CREATE TABLE](/docs/query/sql/create-table/#supported-encodings)
reference for the full list of supported encodings, compression codecs, and
their valid column types.

## Examples

Use `default` for the encoding when specifying compression only.

```questdb-sql title="Set encoding only"
ALTER TABLE trades ALTER COLUMN price SET PARQUET(rle_dictionary);
```

```questdb-sql title="Set compression only (with optional level)"
ALTER TABLE trades ALTER COLUMN price SET PARQUET(default, zstd(3));
```

```questdb-sql title="Set both encoding and compression"
ALTER TABLE trades ALTER COLUMN price SET PARQUET(rle_dictionary, zstd(3));
```

Reset per-column overrides back to the server defaults:

```questdb-sql title="Reset to defaults"
ALTER TABLE trades ALTER COLUMN price SET PARQUET(default);
```

### Bloom filter

The optional `BLOOM_FILTER` keyword can be combined with encoding and
compression, and always appears as the last argument.

```questdb-sql title="Enable bloom filter with default encoding"
ALTER TABLE trades ALTER COLUMN symbol SET PARQUET(default, BLOOM_FILTER);
```

```questdb-sql title="Enable bloom filter with encoding and compression"
ALTER TABLE trades ALTER COLUMN symbol SET PARQUET(rle_dictionary, zstd(3), BLOOM_FILTER);
```

To **remove** the bloom filter from a column, re-issue `SET PARQUET` without
the `BLOOM_FILTER` keyword:

```questdb-sql title="Clear bloom filter, keep encoding"
ALTER TABLE trades ALTER COLUMN symbol SET PARQUET(rle_dictionary);
```

The bloom filter false positive probability (FPP) is a global setting and
cannot be configured per column. See
the [Configuration reference](/docs/configuration/overview/) for details.