I'll note there is also a check for encoding effectiveness [1] that could come into play but I'd guess that isn't the case here.
[1] https://github.com/apache/parquet-mr/blob/9b5a962df3007009a227ef421600197531f970a5/parquet-column/src/main/java/org/apache/parquet/column/values/dictionary/DictionaryValuesWriter.java#L124 On Fri, Sep 15, 2023 at 9:51 AM Micah Kornfield <emkornfi...@gmail.com> wrote: > I'm glad I was looking at the right setting for dictionary size. I just >> tried it out with 10x, 50x, and even total file size, though, and still am >> not seeing a dictionary get created. Is it possible it's bounded by file >> page size or some other layout option that I need to bump as well? > > > Sorry I'm less familiar with parquet-mr, hopefully someone else to chime > in. If I had to guess, maybe somehow the config value isn't making it to > the writer (but there could also be something else at play). > > On Fri, Sep 15, 2023 at 9:33 AM Claire McGinty <claire.d.mcgi...@gmail.com> > wrote: > >> Thanks so much, Micah! >> >> I think you are using the right setting, but maybe it is possible the >> > strings are still exceeding the threshold (perhaps increasing it by 50x >> or >> > more to verify) >> >> >> I'm glad I was looking at the right setting for dictionary size. I just >> tried it out with 10x, 50x, and even total file size, though, and still am >> not seeing a dictionary get created. Is it possible it's bounded by file >> page size or some other layout option that I need to bump as well? >> >> I haven't seen my discussion during my time in the community but maybe it >> > was discussed in the past. I think the main challenge here is that >> pages >> > are either dictionary encoded or not. I'd guess to make this practical >> > there would need to be a new hybrid page type, which I think it might >> be an >> > interesting idea but quite a bit of work. Additionally, one would >> likely >> > need heuristics for when to potentially use the new mode versus a >> complete >> > fallback. >> > >> >> Got it, thanks for the explanation! It does seem like a huge amount of >> work >> >> >> Best, >> Claire >> >> >> >> On Thu, Sep 14, 2023 at 5:16 PM Micah Kornfield <emkornfi...@gmail.com> >> wrote: >> >> > > >> > > - What's the heuristic for Parquet dictionary writing to succeed for a >> > > given column? >> > >> > >> > >> > >> https://github.com/apache/parquet-mr/blob/9b5a962df3007009a227ef421600197531f970a5/parquet-column/src/main/java/org/apache/parquet/column/values/dictionary/DictionaryValuesWriter.java#L117 >> > >> > >> > > - Is that heuristic configurable at all? >> > >> > >> > I think you are using the right setting, but maybe it is possible the >> > strings are still exceeding the threshold (perhaps increasing it by 50x >> or >> > more to verify) >> > >> > >> > > - For high-cardinality datasets, has the idea of a frequency-based >> > > dictionary encoding been explored? Say, if the data follows a certain >> > > statistical distribution, we can create a dictionary of the most >> frequent >> > > values only? >> > >> > I haven't seen my discussion during my time in the community but maybe >> it >> > was discussed in the past. I think the main challenge here is that >> pages >> > are either dictionary encoded or not. I'd guess to make this practical >> > there would need to be a new hybrid page type, which I think it might >> be an >> > interesting idea but quite a bit of work. Additionally, one would >> likely >> > need heuristics for when to potentially use the new mode versus a >> complete >> > fallback. >> > >> > Cheers, >> > Micah >> > >> > On Thu, Sep 14, 2023 at 12:07 PM Claire McGinty < >> > claire.d.mcgi...@gmail.com> >> > wrote: >> > >> > > Hi dev@, >> > > >> > > I'm running some benchmarking on Parquet read/write performance and >> have >> > a >> > > few questions about how dictionary encoding works under the hood. Let >> me >> > > know if there's a better channel for this :) >> > > >> > > My test case uses parquet-avro, where I'm writing a single file >> > containing >> > > 5 million records. Each record has a single column, an Avro String >> field >> > > (Parquet binary field). I ran two configurations of base setup: in the >> > > first case, the string field has 5,000 possible unique values. In the >> > > second case, it has 50,000 unique values. >> > > >> > > In the first case (5k unique values), I used parquet-tools to inspect >> the >> > > file metadata and found that a dictionary had been written: >> > > >> > > % parquet-tools meta testdata-case1.parquet >> > > > file schema: testdata.TestRecord >> > > > >> > > > >> > > >> > >> -------------------------------------------------------------------------------- >> > > > stringField: REQUIRED BINARY L:STRING R:0 D:0 >> > > > row group 1: RC:5000001 TS:18262874 OFFSET:4 >> > > > >> > > > >> > > >> > >> -------------------------------------------------------------------------------- >> > > > stringField: BINARY UNCOMPRESSED DO:4 FPO:38918 >> > SZ:8181452/8181452/1.00 >> > > > VC:5000001 ENC:BIT_PACKED,PLAIN_DICTIONARY ST:[min: 0, max: 999, >> > > num_nulls: >> > > > 0] >> > > >> > > >> > > But in the second case (50k unique values), parquet-tools shows that >> no >> > > dictionary gets created, and the file size is *much* bigger: >> > > >> > > % parquet-tools meta testdata-case2.parquet >> > > > file schema: testdata.TestRecord >> > > > >> > > > >> > > >> > >> -------------------------------------------------------------------------------- >> > > > stringField: REQUIRED BINARY L:STRING R:0 D:0 >> > > > row group 1: RC:5000001 TS:18262874 OFFSET:4 >> > > > >> > > > >> > > >> > >> -------------------------------------------------------------------------------- >> > > > stringField: BINARY UNCOMPRESSED DO:0 FPO:4 >> SZ:43896278/43896278/1.00 >> > > > VC:5000001 ENC:PLAIN,BIT_PACKED ST:[min: 0, max: 9999, num_nulls: 0] >> > > >> > > >> > > (I created a gist of my test reproduction here >> > > < >> https://gist.github.com/clairemcginty/c3c0be85f51bc23db45a75e8d8a18806 >> > >.) >> > > >> > > Based on this, I'm guessing there's some tip-over point after which >> > Parquet >> > > will give up on writing a dictionary for a given column? After reading >> > > the Configuration >> > > docs >> > > < >> > >> https://github.com/apache/parquet-mr/blob/master/parquet-hadoop/README.md >> > > >, >> > > I tried increasing the dictionary page size configuration 5x, with the >> > same >> > > result (no dictionary created). >> > > >> > > So to summarize, my questions are: >> > > >> > > - What's the heuristic for Parquet dictionary writing to succeed for a >> > > given column? >> > > - Is that heuristic configurable at all? >> > > - For high-cardinality datasets, has the idea of a frequency-based >> > > dictionary encoding been explored? Say, if the data follows a certain >> > > statistical distribution, we can create a dictionary of the most >> frequent >> > > values only? >> > > >> > > Thanks for your time! >> > > - Claire >> > > >> > >> >
