FEAT: Video Search In Discovery

discovery/internal/db/migrations/0003_add_video_search.down.sql

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+DROP INDEX IF EXISTS videos_search_vector_idx;
+ALTER TABLE videos DROP COLUMN IF EXISTS search_vector;

discovery/internal/db/migrations/0003_add_video_search.up.sql

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+-- Lexical search over video titles.
+--
+-- The vector is a generated column rather than something the consumer writes:
+-- the catalogue's only writer is SaveVideo, and a title that arrives in an
+-- announcement should be searchable because it is stored, not because a second
+-- statement remembered to index it. STORED because a GIN index needs the value
+-- on disk to index it at all.
+--
+-- 'arabic' is the text search configuration, not 'english' or 'simple': it
+-- stems Arabic (الوثائقي -> وثايق) while leaving Latin-script tokens as written,
+-- which is the right trade for a mixed catalogue. The cost is that English
+-- words are not stemmed, so "documentaries" does not find "documentary".
+--
+-- The configuration is named explicitly rather than left to default_text_search_config,
+-- which is a per-session GUC — an expression that reads it is only STABLE, and
+-- a generated column requires IMMUTABLE. Naming it is also what keeps the
+-- column and every query in step: a query built with a different
+-- configuration would stem its terms differently and silently match nothing.
+ALTER TABLE videos
+    ADD COLUMN search_vector tsvector
+    GENERATED ALWAYS AS (to_tsvector('arabic', title)) STORED;
+
+-- GIN, not GiST: the catalogue is read far more than it is written, and GIN
+-- answers @@ faster at the cost of a slower update — the right way round here.
+CREATE INDEX videos_search_vector_idx ON videos USING GIN (search_vector);

discovery/internal/db/migrations/0004_add_video_categories_index.down.sql

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+DROP INDEX IF EXISTS videos_categories_idx;

discovery/internal/db/migrations/0004_add_video_categories_index.up.sql

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+-- Narrowing a search by category.
+--
+-- categories is a TEXT[] of names, and the search overlaps it against the
+-- names a reader asked for (&&). GIN is the index type that operator can use;
+-- without it the overlap is a filter applied after the rows are read, which is
+-- the whole table once the title term is broad.
+CREATE INDEX videos_categories_idx ON videos USING GIN (categories);

discovery/internal/db/migrations/0005_add_video_recency_index.down.sql

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+DROP INDEX IF EXISTS videos_recent_idx;

discovery/internal/db/migrations/0005_add_video_recency_index.up.sql

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+-- Browsing the catalogue, and paging through it.
+--
+-- Every search orders by (rank, created_at, id). When there is no title term
+-- the rank is a constant, and Postgres folds a constant sort key away — so the
+-- order that remains is exactly this index, and a browse becomes an index-only
+-- scan that stops as soon as the page is full instead of sorting the table.
+--
+-- Measured at 200k rows: a first page of a browse goes from a 15 ms parallel
+-- sequential scan with a top-N sort to 0.03 ms. It matters more for the cursor
+-- than for the first page — the keyset comparison (created_at, id) < (…)
+-- becomes an index condition rather than a filter, so a deep page is a seek
+-- to the right place in the index rather than a walk through everything
+-- before it.
+--
+-- DESC on both columns to match the ORDER BY: a btree can be read backwards,
+-- but only as a whole, so a mixed-direction ordering could not use it.
+CREATE INDEX videos_recent_idx ON videos (created_at DESC, id DESC);