This shrinks the index by implementing a "synthetic" _source field. Instead of saving the field to disk we reconstruct it on the fly using our column store, doc values.

Before removing the feature flag

• Initial implementation Synthetic source #85649
• Figure out how much performance we'd get from using synthetic source for recovery - thus removing the `_recovery_source field. Synthetic source #85649 (comment)
• Figure out final API to turn this on (@qhoxie)
• If it stays synthetic: true then we'll have to fail mappings that contain enabled: false, synthetic: true Synthetic source: don't allow disabling source #87270
• Flip API from synthetic: true to synthetic: strict so we have room to add more later. We totally will. @romseygeek
• Rally track Support synthetic source in tsdb and nyc_taxis tracks rally-tracks#268 + Add fetch operations for synthetic source rally-tracks#270
• Resolve remaining round trip tests Synethetic Source: Fix scaled float #86760
• Realtime GET should synthesize on load to return consistent results. (Tests for synthetic _source from translog #87578)
• Add an option to _search to simulate synthetic source (Add an option to _search to force synthetic source #87068)
• Support the simulate option on GET and MGET (Add force_synthetic_source to GET #87536 + Add force_synthetic_source to mget #87574)
• Make sure we throw an error if you try to enable or disable synthetic source on an index Synthetic source: paranoid tests for configuration #87182
• Support "subobjects" Add support for dots in field names for metrics usecases #86166 (Synthetic source: tests for disabling subobjects #87261)
• Docs Docs for synthetic source #87416
• Figure out highlighting Fixup highlighting with synthetic source #87667

Later

• Make sure there is a nice error message when scripts try to access synethic source - it won't be there. They should use doc values or the fancy new fields API. @tmgordeeva Synthetic source error on script loads #88334
• Add support for more field types
  • aggregate_metric_double field type Add synthetic_source support to aggregate_metric_double fields #88909
  • constant_keyword Enable synthetic source support on constant keyword fields #88603
  • dense_vector Synthetic _source: support dense_vector #89840
  • histogram Synthetic _source: support histogram field #89833
  • keyword fields with ignore_above (Synthetic source: load text from stored fields #87480 + Synthetic _source: support ignore_above #89466)
  • match_only_text Synthetic _source: support match_only_text #89516
  • version Synthetic _source: support version field type #89706
  • _doc_count Support synthetic _source for _doc_count field #91465
• Support fields in runtime fields scripts
  • Numbers REST tests fetching fields with synthetic _source #89888
  • ip REST tests fetching fields with synthetic _source #89888
  • text (Synthetic _source: support field in many cases #89950 + more)
  • keyword (Synthetic _source: support field in many cases #89950)
  • match_only_text (Synthetic _source: support field in many cases #89950)
• Support loading from stored fields (text would love it!) Synthetic source: load text from stored fields #87480
• Rally tests for random fetch
• Look into the enrich processor (More tests for enrich processor #89554)
• Improve performance of synthesis
  • General Speed up synthetic source #87882
  • Load column-wise Speed up synthetic keyword, ip, and text fields #87930 Synthetic source numbers in columns #88025
  • Parallel loading?
• Make fields API aware of synthetic-ness and go to doc values rather than rebuilding _source if _source isn't separately needed.
• Document best practices for load over synthetic source
• Support for ignore_malformed Synthetic _source: support ignore_malformed #90007
  • ip Synthetic _source: ignore_malformed for ip #90038
  • numeric Support malformed numbers in synthetic _source #90428
  • scaled_float Support synthetic source for scaled_float and unsigned_long when ignore_malformed is used #109506
  • date/date_nanos Support synthetic source for date fields when ignore_malformed is used #109410
  • geo_point Support synthetic source for geo_point when ignore_malformed is used #109651
  • histogram Support synthetic source together with ignore_malformed in histogram fields #109882
  • aggregate_double_metric Support synthetic source for aggregate_metric_double when ignore_malformed is used #108746 Simplify ignore_malformed handling for synthetic souce in aggregate_metric_double #109888
  • text type family Text fields are stored by default in TSDB indices #106338
  • keyword Synthetic _source: support ignore_above #89466

Much later

• Synthesize instead of using _recovery_source - we find that it'd improve write performance by ~11%. We'd have to synthesize on load instead. That's pretty slow. We'd love the 11% but we have to be careful here.