We continue our series on aggregation pushdown and turn our attention to our Elasticsearch FDW. Implementation details, performance considerations as well as a few Postgres tidbits are shared.

PostgreSQL FDW aggregation pushdown part III: Elasticsearch edition


![Comparing the speed of a COUNT(*) query on Snowflake with and without aggregation pushdown](/static-v2/images/blog/agg-pushdown-part-2/splitgraph-snowflake-tpch-groupby-speedups.png)

## Introduction

[Recently](/blog/postgresql-fdw-aggregation-pushdown-multicorn-part-1) we talked about new capabilities which we have
added to the Multicorn FDW framework, namely pushdown of aggregation and grouping queries to the remote data source.
We've discussed the clear performance benefits of such a mechanism, for the most part significantly faster query execution times,
and also gave a brief overview of the FDW fundamentals in PostgreSQL.
Lastly, we demonstrated a working toy model utilizing these features: an [FDW](https://github.com/splitgraph/Multicorn/blob/master/python/multicorn/pandasfdw.py)
for querying Pandas dataframes via SQL.

In this blog post we will show an actual implementation building on that work, one centering on the Snowflake FDW. We start off
by giving some insights on how this FDW works. Subsequently, we measure the resulting query speedup due to aggregation
pushdown in some real circumstances. Finally, we revisit the subject of data filtering and qualifier pushdown in the context of aggregations.

## SQLAlchemy

To facilitate querying Snowflake tables through Splitgraph we rely on [SqlAlchemyFdw](https://github.com/splitgraph/Multicorn/blob/master/python/multicorn/sqlalchemyfdw.py), one of many
Multicorn-based FDWs that exist. Under the hood it employs SQLAlchemy ORM to talk with any remote data source with a supported [dialect](https://docs.sqlalchemy.org/en/14/dialects/), one of which is Snowflake.

[Recall](/blog/postgresql-fdw-aggregation-pushdown-multicorn-part-1) that in order to utilize Multicorn's aggregation pushdown capabilities, the corresponding Python FDW implementation
must first declare which aggregation functions are supported, in order for the aggregation pushdown plan to be chosen by
Postgres in the first place.

Additionally, it must accept and act on two new keyword arguments in the `execute` method: `aggs` and `group_clauses`.
The former represents a mapping between each aggregation target and its constituent elements (i.e. column and function name),
while the latter is simply a list of columns referenced in the `GROUP BY` clauses.

#### Prototype

Let us prototype how (re-)constructing an aggregation query would look like in this case:

```python
from sqlalchemy.schema import Table, Column, MetaData
from sqlalchemy.types import Integer, String
from sqlalchemy.sql import select, func


_PG_AGG_FUNC_MAPPING = {
    "avg": func.avg,
    "min": func.min,
    "max": func.max,
    "sum": func.sum,
    "count": func.count,
    "count.*": func.count
}


def aggregation_query(table, aggs=None, group_clauses=None):
    target_list = []

    if group_clauses is not None:
        target_list = [table.c[col] for col in group_clauses]

    if aggs is not None:
        for agg_name, agg_props in aggs.items():
            agg_func = _PG_AGG_FUNC_MAPPING[agg_props["function"]]

            if agg_props["column"] == "*":
                agg_target = agg_func()
            else:
                agg_target = agg_func(table.c[agg_props["column"]])

            target_list.append(agg_target.label(agg_name))

    statement = select(*target_list).select_from(table)

    if group_clauses is None:
        return statement

    return statement.group_by(*[table.c[col] for col in group_clauses])
```

Note that targets passed to the `select` method correspond to plain columns if originating from `group_clauses`, or
functions acting on a column for each aggregation target. `_PG_AGG_FUNC_MAPPING` defines a mapping of PostgreSQL
functions to their SQLAlchemy counterparts.

The special case of `count(*)` is also supported; the corresponding `column` value is `*` and refers to all columns.
In such case we don't supply an argument to the underlying `count` function. It is worth pointing out that there is
a subtle difference between `count(*)` and `count(some_column)`—the latter doesn't count `NULL`-valued rows, while
the former takes them into account. Therefore we can't "cheat" and simplify our implementation by using the same remote
expression for both.

Testing this prototype out with a simple input:

```python
In [1]: table = Table("test", MetaData(), Column("number", Integer), Column("parity", String(4)))

In [2]: aggs = {
   ...:     "count.*": {
   ...:         "column": "*",
   ...:         "function": "count"
   ...:     },
   ...:     "max.number": {
   ...:         "column": "number",
   ...:         "function": "max"
   ...:     }
   ...: }

In [3]: group_clauses = ["parity"]

In [4]: query = aggregation_query(table, aggs=aggs, group_clauses=group_clauses)

In [5]: print(query)
SELECT test.parity, count(*) AS "count.*", max(test.number) AS "max.number"
FROM test GROUP BY test.parity
```

we can see the resulting output query is what we expect. Needless to say, the fully fledged [SqlAlchemyFdw](https://github.com/splitgraph/Multicorn/blob/master/python/multicorn/sqlalchemyfdw.py))
implementation handles non-aggregation queries too, and must adequately process the output of all queries, including result-batching support.

## Performance impact

Having sketched out the mechanism of our Snowflake FDW, we turn our attention to some actual performance insights. To that
end, we use our command line client `sgr` to [mount](/docs/sgr-advanced/ingesting-data/foreign-data-wrappers/load-snowflake-tables)
one of the larger schemas (`TPCH_SF100`) in the [sample Snowflake database](https://docs.snowflake.com/en/user-guide/sample-data-tpch.html)
available in each trial account.

```shell
$ sgr mount snowflake sf100 -o@- <<EOF
{
	"username": "user",
	"secret": {
		"secret_type": "password",
		"password": "pass"
	},
	"account": "bw54298.eu-central-1",
	"database": "SNOWFLAKE_SAMPLE_DATA",
	"schema": "TPCH_SF100"
}
EOF
```

By running `pgcli $(sgr config -n)` we can open a DB shell and execute our queries against the local Splitgraph engine.
Our target will be the table `supplier` with exactly one million rows. To set a baseline, we will first run our query
**without** aggregation pushdown enabled, using `EXPLAIN` to verify that full row fetch will occur beforehand:

```sql
sgr@localhost:splitgraph> EXPLAIN SELECT count(*) FROM sf100.supplier
+---------------------------------------------------------------------------------+
| QUERY PLAN                                                                      |
|---------------------------------------------------------------------------------|
| Aggregate  (cost=250000.00..250000.01 rows=1 width=8)                           |
|   ->  Foreign Scan on supplier  (cost=20.00..0.00 rows=100000000 width=0)       |
|         Multicorn:                                                              |
| SELECT "TPCH_SF100".supplier.s_suppkey                                          |
| FROM "TPCH_SF100".supplier                                                      |
|                                                                                 |
| JIT:                                                                            |
|   Functions: 2                                                                  |
|   Options: Inlining false, Optimization false, Expressions true, Deforming true |
+---------------------------------------------------------------------------------+
EXPLAIN
Time: 0.079s
sgr@localhost:splitgraph> SELECT count(*) FROM sf100.supplier
+---------+
| count   |
|---------|
| 1000000 |
+---------+
SELECT 1
Time: 27.699s (27 seconds), executed in: 27.694s (27 seconds)
```

Trying the same **with** aggregation pushdown enabled:

```sql
sgr@localhost:splitgraph> EXPLAIN SELECT count(*) FROM sf100.supplier
+------------------------------------------------+
| QUERY PLAN                                     |
|------------------------------------------------|
| Foreign Scan  (cost=1.00..1.00 rows=1 width=1) |
|   Multicorn:                                   |
| SELECT count(*) AS "count.*"                   |
| FROM "TPCH_SF100".supplier                     |
|                                                |
+------------------------------------------------+
EXPLAIN
Time: 0.080s
sgr@localhost:splitgraph> SELECT count(*) FROM sf100.supplier
+---------+
| count   |
|---------|
| 1000000 |
+---------+
SELECT 1
Time: 0.214s
sgr@localhost:splitgraph>
```

Thus, we see a factor of improvement of more than 100X in the speed of execution. It is worth pointing out that this factor of
improvement scales with the size of the data, which is expected given the behavior in the absence of aggregation pushdown.
For instance, running the same query on `TPCH_SF10` (100,000 rows in the `supplier` table) we see an approximately tenfold speedup,
while running it on `TPCH_SF1000` (10,000,000 rows in the `supplier` table) results in an improvement of effectively three orders of magnitude.
These results are summarized in the graph at the beginning of the article.

## Data filtering

Let us now revisit the subject of qualifier pushdown, i.e. handling of `WHERE` clauses for aggregation queries. Multicorn
already supports qualifier pushdown via the `quals` argument of the `execute` method in the Python FDW implementation. However,
before we can safely use it in case of aggregations, we must declare to Multicorn exactly which operators can be pushed down.
This is needed so that Postgres avoids aggregation pushdown when an operator in the `WHERE` clause does not have an analogue
on the remote data source.

Extending `can_pushdown_upperrel` which we introduced in the [first blog](/blog/postgresql-fdw-aggregation-pushdown-multicorn-part-1)
post, we end up with:

```python
def can_pushdown_upperrel(self):
    return {
        "groupby_supported": True,
        "agg_functions": ["max", "min", "sum", "avg", "count", "count.*"],
        "operators_supported": [">", "<"]
    }
```

For simplicity we only specify two supported operators. Revisiting our prototype from earlier, we can now implement the
`WHERE` clause pushdown alongside the aggregations:

```diff
@@ -1,6 +1,8 @@
+import operator
 from sqlalchemy.schema import Table, Column, MetaData
 from sqlalchemy.types import Integer, String
-from sqlalchemy.sql import select, func
+from sqlalchemy.sql import select, func, and_


 _PG_AGG_FUNC_MAPPING = {
@@ -13,7 +15,24 @@ _PG_AGG_FUNC_MAPPING = {
 }


-def aggregation_query(table, aggs=None, group_clauses=None):
+OPERATORS = {
+    "<": operator.lt,
+    ">": operator.gt
+}
+
+
+def aggregation_query(table, quals=[], aggs=None, group_clauses=None):
     target_list = []

     if group_clauses is not None:
@@ -32,6 +51,16 @@ def aggregation_query(table, aggs=None, group_clauses=None):

     statement = select(*target_list).select_from(table)

+    clauses = []
+    for qual in quals:
+        operator = OPERATORS.get(qual.operator, None)
+        if operator:
+            clauses.append(operator(table.c[qual.field_name], qual.value))
+        else:
+            raise Exception(f"Qual {qual} not pushed to foreign db!")
+    if clauses:
+        statement = statement.where(and_(*clauses))
+
     if group_clauses is None:
         return statement

```

Indeed, testing it out in the console on the same input as before, we get the desired output:

```python
In [1]: from multicorn import Qual

In [2]: quals = [
   ...:     Qual("number", ">", 2),
   ...:     Qual("number", "<", 8)
   ...: ]
   ...:

In [3]: q = aggregation_query(table, quals=quals, aggs=aggs, group_clauses=group_clauses)

In [4]: print(q.compile(compile_kwargs={"literal_binds": True}))
SELECT test.parity, count(*) AS "count.*", max(test.number) AS "max.number"
FROM test
WHERE test.number > 2 AND test.number < 8 GROUP BY test.parity
```

Finally, we can achieve pushdown of some more involved queries:

```sql
sgr@localhost:splitgraph> EXPLAIN SELECT o_orderstatus, count(*), avg(o_totalprice)
FROM sf.orders
WHERE o_totalprice > 300000 AND o_orderstatus != 'P'
GROUP BY o_orderstatus
+-------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                                |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| Foreign Scan  (cost=1.00..1.00 rows=1 width=1)                                                                                            |
|   Multicorn:                                                                                                                              |
| SELECT "TPCH_SF100".orders.o_orderstatus, count(*) AS "count.*", avg("TPCH_SF100".orders.o_totalprice) AS "avg.o_totalprice"              |
| FROM "TPCH_SF100".orders                                                                                                                  |
| WHERE "TPCH_SF100".orders.o_totalprice > 300000.0 AND "TPCH_SF100".orders.o_orderstatus != 'P' GROUP BY "TPCH_SF100".orders.o_orderstatus |
|                                                                                                                                           |
+-------------------------------------------------------------------------------------------------------------------------------------------+
sgr@localhost:splitgraph> SELECT o_orderstatus, count(*), avg(o_totalprice)
 FROM sf.orders
 WHERE o_totalprice > 300000 AND o_orderstatus != 'P'
 GROUP BY o_orderstatus
+---------------+---------+-----------------+
| o_orderstatus | count   | avg             |
|---------------+---------+-----------------|
| F             | 4116025 | 335437.66336111 |
| O             | 4118775 | 335472.48006934 |
+---------------+---------+-----------------+
SELECT 2
Time: 0.152s
```

#### `WHERE` vs `HAVING`

In case of aggregation queries, data filtering can be applied on the input via `WHERE` clauses, or the output via
`HAVING` clauses. At present, our current Multicorn implementation doesn't support pushdown of queries with `HAVING` clauses.

There is one insightful "exception" to that thanks to a clever PostgreSQL trick—when a `HAVING` clause references an output target
that is equivalent to an input target, it gets [internally converted](https://github.com/postgres/postgres/blob/4d5f651f1d651c6fa79f9188e7b9a04654c7125a/src/backend/optimizer/plan/planner.c#L974-L1001)
to a `WHERE` clause. An example is a `HAVING` clause on a column that is also a part of the `GROUP BY` clause. This is
easily verifiable in our Snowflake FDW, by running `EXPLAIN` on such a query to see the re-constructed remote query:

```sql
sgr@localhost:splitgraph> EXPLAIN SELECT o_orderstatus, max(o_totalprice)
FROM sf.orders
GROUP BY o_orderstatus
HAVING o_orderstatus in ('P', 'F')
+--------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                             |
|--------------------------------------------------------------------------------------------------------|
| Foreign Scan  (cost=1.00..1.00 rows=1 width=1)                                                         |
|   Multicorn:                                                                                           |
| SELECT "TPCH_SF100".orders.o_orderstatus, max("TPCH_SF100".orders.o_totalprice) AS "max.o_totalprice"  |
| FROM "TPCH_SF100".orders                                                                               |
| WHERE "TPCH_SF100".orders.o_orderstatus IN ('P', 'F') GROUP BY "TPCH_SF100".orders.o_orderstatus       |
|                                                                                                        |
+--------------------------------------------------------------------------------------------------------+
```

Given that input and output filtering in this case are identical approaches in terms of correctness, Postgres chooses to
discard the redundant rows ahead of time, rather then to do it after having performed aggregation processing.

## Conclusion

This was a second post in our series on PostgreSQL FDW aggregation pushdown, where we described a practical example
utilizing the features we have implemented in the Multicorn framework. In particular, we demonstrated how our SQLAlchemy-based
Snowflake FDW makes use of the new capabilities, allowing aggregation and grouping queries to be computed by the Snowflake
engine itself, thus circumventing the naive approach of fetching all the rows and only then doing the computation locally.

We provided actual measurements confirming that the naive approach grows prohibitively expensive even for medium-sized tables,
while at the same time the aggregation pushdown technique ensures orders-of-magnitude faster execution times.
Finally, we discussed the importance of detecting early in the planning phase the presence of unsupported operators in
`WHERE` clauses, so that aggregation pushdown occurs only if the remote data source can apply all the input filtering
qualifiers.


We demonstrate a concrete application of aggregation pushdown mechanism in the form of our Snowflake FDW. Actual performance benefits are quantified for a selection of real-life examples.

PostgreSQL FDW aggregation pushdown part II: Snowflake speedup

We recently implemented support for aggregation and grouping pushdown in the Multicorn FDW. In this post, we'll demonstrate it on a simple toy example and discuss how PostgreSQL aggregation pushdown works in general.

Splitgraph has been acquired by EDB! Read the blog post.

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