Memory exhaustion on large query

Hello,

I am using postgresql 13, on debian server. I use libpq to interface my DB.
On some queries, my program is using so much memory that the application
crashes.

I use libpq in asynchronous mode to avoid this problem, but the memory is
consumed in the PQsendQueryPrepared call, before I invoke the first time
PQgetResult

I copy-pasted the query that causes the trouble here under. My table has an
index that should be usable for the query 'including the "ORDER BY" clause:
CREATE INDEX "ConfigurableWindowTableDataBuffer_last_modified_index" ON
"ConfigurableWindowTableDataBufferInternal" USING btree
("DbSyncLastModifiedBackupVersion", "DbSyncLastModifiedTimeStamp")

And an EXPLAIN on the query shows that the planner uses this index:

Index Scan using
"ConfigurableWindowTableDataBuffer_last_modified_index" on
"ConfigurableWindowTableDataBufferInternal" (cost=0.54..2870.78
rows=14059 width=510)

Index Cond: (("DbSyncLastModifiedBackupVersion" =
"DbSyncGetBackupVersion"()) AND ("DbSyncLastModifiedTimeStamp" > 0))

The content of the table is so huge that, if PQsendQueryPrepared retrieves
all data, or if postgresql engine is creating temp file with all data, the
query cannot succeed (I have no enough RAM or disk space to copy the whole
data). But I was expecting that using PQsendQueryPrepared and PQgetResult
would avoid this by returning one result at a time.

Any idea of what is going wrong ? Or on how I could correct my query to
avoid this ?

Note that basically, what I want to do is retrieve each record at a time,
and send its content on an open TCP connection. I am hoping to be allowed
to do that without requiring to have RAM or disk space to temporarily store
all data that needs to be sent.

Many thanks,
Brice

SELECT
"DbSyncID","DbSyncInsertedBackupVersion","DbSyncInsertedClientUniqueId","DbSyncInsertedClientEntryId","DbSyncDeleted","DbSyncRemovedFromServer","DbSyncLastModifiedBackupVersion","DbSyncLastModifiedTimeStamp","DbSyncFKBackupVersion","DbSyncFKClientUniqueId","DbSyncFKClientEntryId","TableDataID_BV","TableDataID_CID","TableDataID_CEID","TableId","SubTableId","BufferEntries_0"
FROM "ConfigurableWindowTableDataBufferSync" WHERE
"DbSyncLastModifiedBackupVersion"="DbSyncGetBackupVersion"() AND
"DbSyncLastModifiedTimeStamp" > 0 ORDER BY "DbSyncLastModifiedTimeStamp" ASC